HK1258165B - Methods for driving electro-optic displays - Google Patents

Description

Method for driving electro-optic display

RELATED APPLICATIONS

This application claims the benefit of provisional application serial No. 62/305,833 filed on 9/3/2016.

The present application also relates to co-pending application serial No. 14/849,658 filed on 9/10/2015 and claims the benefit of application serial No. 62/048,591 filed on 9/10/2014; the benefit of application serial No. 62/169,221 filed on 1/6/2015; and application serial No. 62/169,710 filed on day 2/6/2015. The entire contents of the above-mentioned application and all U.S. patents and published and co-pending applications referred to below are incorporated herein by reference.

Technical Field

The present invention relates to a method for driving an electro-optic display, and in particular, but not exclusively, to an electrophoretic display capable of rendering more than two colors using a single layer of electrophoretic material comprising a plurality of colored particles.

Background

The term color as used herein includes black and white. The white particles are typically of the light scattering type.

The term gray state is used herein in its conventional sense in the imaging art to refer to a state intermediate two extreme optical states of a pixel, but does not necessarily imply a black-and-white transition between the two extreme states. For example, several of the above referenced imperial patents and published applications describe electrophoretic displays in which the extreme states are white and deep blue, so that the intermediate gray state is effectively pale blue. In fact, as already mentioned, the change in optical state may not be a color change at all. The terms black and white may be used hereinafter to refer to the two extreme optical states of the display and should be understood to generally include extreme optical states (which are not limited to only black and white), such as the white and deep blue states mentioned above.

The terms bistable and bistability are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven to assume its first or second display state using an addressing pulse of finite duration, that state will last at least several times (e.g. at least 4 times) the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. It is shown in U.S. patent No.7,170,670 that some particle-based electrophoretic displays that support gray scale can be stabilized not only in their extreme black and white states, but also in their intermediate gray states, as can some other types of electro-optic displays. This type of display is properly referred to as multi-stable rather than bi-stable, but for convenience the term bi-stable may be used herein to cover both bi-stable and multi-stable displays.

When used to refer to driving an electrophoretic display, the term impulse is used herein to refer to the integral over time of the voltage applied during the period of time in which the display is driven.

Particles that absorb, scatter, or reflect light at broadband or selected wavelengths are referred to herein as colored or pigment particles. Various materials that absorb or reflect light, such as dyes or photonic crystals, in addition to pigments (which term is intended to mean insoluble colored materials in a strict sense), may also be used in the electrophoretic media and displays of the present invention.

Particle-based electrophoretic displays have been the subject of intensive research and development for many years. In such displays, a plurality of charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. Electrophoretic displays may have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays. However, problems with the long-term image quality of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in insufficient lifetime of these displays.

As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but the electrophoretic medium can be produced using a gaseous fluid; see, e.g., Kitamura, T.et al, Electronic Toner movement for Electronic Paper-like display, IDW Japan,2001, Paper HCS1-1, and Yamaguchi, Y.et al, Toner display using insulating substrates charged triboelectric, IDW Japan,2001, Paper AMD 4-4). See also U.S. patent nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media are susceptible to the same problems when used in a direction that allows for particle settling, such as in signs where the media are arranged in a vertical plane, due to the same particle settling as the liquid-based electrophoretic media. In fact, the problem of particle settling in gas-based electrophoretic media is more severe than in liquid-based electrophoretic media, because the viscosity of gaseous suspending fluids is lower compared to liquids, thereby allowing faster settling of the electrophoretic particles.

A number of patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and yingke corporation describe various techniques for encapsulating electrophoretic and other electro-optic media. Such encapsulated media comprise a plurality of microcapsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form a coherent layer between two electrodes. The techniques described in these patents and applications include:

(a) electrophoretic particles, fluids, and fluid additives; see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814;

(b) capsule, adhesive and packaging process; see, e.g., U.S. patent nos. 6,922,276 and 7,411,719;

(c) microcell structures, wall materials, and methods of forming microcells; see, e.g., U.S. patent nos. 7,072,095 and 9,279,906;

(d) a method for filling and sealing a microcell; see, e.g., U.S. patent nos. 7,144,942 and 7,715,088;

(e) films and sub-assemblies comprising electro-optic material; see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564;

(f) backsheets, adhesive layers, and other auxiliary layers and methods for use in displays; see, e.g., U.S. patent nos. 7,116,318 and 7,535,624;

(g) color formation and color adjustment; see, e.g., U.S. Pat. Nos. 6,017,584; 6,545,797, respectively; 6,664,944, respectively; 6,788,452, respectively; 6,864,875, respectively; 6,914,714, respectively; 6,972,893, respectively; 7,038,656, respectively; 7,038,670, respectively; 7,046,228; 7,052,571, respectively; 7,075,502 ×; 7,167,155, respectively; 7,385,751, respectively; 7,492,505, respectively; 7,667,684, respectively; 7,684,108, respectively; 7,791,789, respectively; 7,800,813, respectively; 7,821,702, respectively; 7,839,564 ×; 7,910,175, respectively; 7,952,790, respectively; 7,956,841, respectively; 7,982,941, respectively; 8,040,594, respectively; 8,054,526, respectively; 8,098,418, respectively; 8,159,636, respectively; 8,213,076, respectively; 8,363,299, respectively; 8,422,116, respectively; 8,441,714, respectively; 8,441,716, respectively; 8,466,852, respectively; 8,503,063, respectively; 8,576,470, respectively; 8,576,475, respectively; 8,593,721, respectively; 8,605,354, respectively; 8,649,084, respectively; 8,670,174, respectively; 8,704,756, respectively; 8,717,664, respectively; 8,786,935, respectively; 8,797,634, respectively; 8,810,899, respectively; 8,830,559, respectively; 8,873,129, respectively; 8,902,153, respectively; 8,902,491, respectively; 8,917,439, respectively; 8,964,282, respectively; 9,013,783, respectively; 9,116,412, respectively; 9,146,439, respectively; 9,164,207, respectively; 9,170,467, respectively; 9,170,468, respectively; 9,182,646, respectively; 9,195,111, respectively; 9,199,441, respectively; 9,268,191, respectively; 9,285,649, respectively; 9,293,511, respectively; 9,341,916, respectively; 9,360,733, respectively; 9,361,836, respectively; 9,383,623, respectively; and 9,423,666; and U.S. patent application publication No. 2008/0043318; 2008/0048970, respectively; 2009/0225398, respectively; 2010/0156780, respectively; 2011/0043543, respectively; 2012/0326957, respectively; 2013/0242378, respectively; 2013/0278995, respectively; 2014/0055840, respectively; 2014/0078576, respectively; 2014/0340430, respectively; 2014/0340736, respectively; 2014/0362213, respectively; 2015/0103394, respectively; 2015/0118390, respectively; 2015/0124345, respectively; 2015/0198858, respectively; 2015/0234250, respectively; 2015/0268531, respectively; 2015/0301246, respectively; 2016/0011484, respectively; 2016/0026062, respectively; 2016/0048054, respectively; 2016/0116816, respectively; 2016/0116818, respectively; and 2016/0140909;

(h) a method for driving a display; see, e.g., U.S. Pat. Nos. 5,930,026; 6,445,489, respectively; 6,504,524; 6,512,354, respectively; 6,531,997, respectively; 6,753,999, respectively; 6,825,970, respectively; 6,900,851, respectively; 6,995,550, respectively; 7,012,600; 7,023,420, respectively; 7,034,783, respectively; 7,061,166, respectively; 7,061,662, respectively; 7,116,466, respectively; 7,119,772; 7,177,066, respectively; 7,193,625, respectively; 7,202,847, respectively; 7,242,514, respectively; 7,259,744; 7,304,787, respectively; 7,312,794, respectively; 7,327,511, respectively; 7,408,699, respectively; 7,453,445, respectively; 7,492,339, respectively; 7,528,822, respectively; 7,545,358, respectively; 7,583,251, respectively; 7,602,374, respectively; 7,612,760, respectively; 7,679,599, respectively; 7,679,813, respectively; 7,683,606, respectively; 7,688,297, respectively; 7,729,039, respectively; 7,733,311, respectively; 7,733,335, respectively; 7,787,169, respectively; 7,859,742, respectively; 7,952,557, respectively; 7,956,841, respectively; 7,982,479, respectively; 7,999,787, respectively; 8,077,141, respectively; 8,125,501, respectively; 8,139,050, respectively; 8,174,490, respectively; 8,243,013, respectively; 8,274,472, respectively; 8,289,250, respectively; 8,300,006, respectively; 8,305,341, respectively; 8,314,784, respectively; 8,373,649, respectively; 8,384,658, respectively; 8,456,414, respectively; 8,462,102, respectively; 8,514,168, respectively; 8,537,105, respectively; 8,558,783, respectively; 8,558,785, respectively; 8,558,786, respectively; 8,558,855, respectively; 8,576,164, respectively; 8,576,259, respectively; 8,593,396, respectively; 8,605,032, respectively; 8,643,595, respectively; 8,665,206, respectively; 8,681,191, respectively; 8,730,153, respectively; 8,810,525, respectively; 8,928,562, respectively; 8,928,641, respectively; 8,976,444, respectively; 9,013,394, respectively; 9,019,197, respectively; 9,019,198, respectively; 9,019,318, respectively; 9,082,352, respectively; 9,171,508, respectively; 9,218,773, respectively; 9,224,338, respectively; 9,224,342, respectively; 9,224,344, respectively; 9,230,492, respectively; 9,251,736, respectively; 9,262,973, respectively; 9,269,311, respectively; 9,299,294, respectively; 9,373,289, respectively; 9,390,066, respectively; 9,390,661, respectively; and 9,412,314; and U.S. patent application publication No. 2003/0102858; 2004/0246562, respectively; 2005/0253777, respectively; 2007/0091418, respectively; 2007/0103427, respectively; 2007/0176912, respectively; 2008/0024429, respectively; 2008/0024482, respectively; 2008/0136774, respectively; 2008/0291129, respectively; 2008/0303780, respectively; 2009/0174651, respectively; 2009/0195568, respectively; 2009/0322721, respectively; 2010/0194733, respectively; 2010/0194789, respectively; 2010/0220121, respectively; 2010/0265561, respectively; 2010/0283804, respectively; 2011/0063314, respectively; 2011/0175875, respectively; 2011/0193840, respectively; 2011/0193841, respectively; 2011/0199671, respectively; 2011/0221740, respectively; 2012/0001957, respectively; 2012/0098740, respectively; 2013/0063333, respectively; 2013/0194250, respectively; 2013/0249782, respectively; 2013/0321278, respectively; 2014/0009817, respectively; 2014/0085355, respectively; 2014/0204012, respectively; 2014/0218277, respectively; 2014/0240210, respectively; 2014/0240373, respectively; 2014/0253425, respectively; 2014/0292830, respectively; 2014/0293398, respectively; 2014/0333685, respectively; 2014/0340734, respectively; 2015/0070744, respectively; 2015/0097877, respectively; 2015/0109283, respectively; 2015/0213749, respectively; 2015/0213765, respectively; 2015/0221257, respectively; 2015/0262255, respectively; 2015/0262551, respectively; 2016/0071465, respectively; 2016/0078820, respectively; 2016/0093253, respectively; 2016/0140910, respectively; and 2016/0180777 (these patents and applications may be referred to hereinafter as MEDEOD (method for driving electro-optic displays) applications);

(i) an application for a display; see, e.g., U.S. patent nos. 7,312,784 and 8,009,348; and

(h) non-electrophoretic displays, such as those described in U.S. patent nos. 6,241,921; and U.S. patent application publication No. 2015/0277160; and U.S. patent application publication nos. 2015/0005720 and 2016/0012710.

Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and the discrete droplets of electrophoretic fluid within such polymer-dispersed electrophoretic display can be considered capsules or microcapsules, even if no discrete capsule film is associated with each individual droplet; see, for example, U.S. patent No.6,866,760. Accordingly, for the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.

A related type of electrophoretic display is the so-called microcell electrophoretic display. In microcell electrophoretic displays, the charged particles and fluid are not encapsulated within microcapsules, but are held within a plurality of cavities formed within a carrier medium (typically a polymer film). See, for example, U.S. patent nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called shutter mode in which one display state is substantially opaque and one display state is light-transmissive. See, for example, U.S. patent nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. A dielectrophoretic display similar to an electrophoretic display but relying on a change in electric field strength may operate in a similar mode; see U.S. patent No.4,418,346. Other types of electro-optic displays can also operate in the shutter mode. Electro-optic media operating in shutter mode may be used in the multilayer structure of full color displays; in this configuration, at least one layer adjacent to the viewing surface of the display operates in a shutter mode to expose or hide a second layer further from the viewing surface.

Encapsulated electrophoretic displays are generally not plagued by the aggregation and settling failure modes of conventional electrophoretic devices and provide further benefits such as the ability to print or coat the display on a variety of flexible and rigid substrates. (the use of the word "printing" is intended to include all forms of printing and coating including, but not limited to, pre-metered coating such as slot or extrusion coating, slide or cascade coating, curtain coating, roll coating such as knife coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, electrophoretic deposition (see U.S. patent No.7,339,715), and other similar techniques.) thus, the resulting display may be flexible. In addition, because the display media can be printed (using a variety of methods), the display itself can be inexpensively manufactured.

As mentioned above, most simple prior art electrophoretic media display substantially only two colors. Such electrophoretic media use a single type of electrophoretic particle having a first color in a colored fluid having a second, different color (in which case the first color is displayed when the particle is positioned adjacent the viewing surface of the display and the second color is displayed when the particle is spaced apart from the viewing surface), or first and second types of electrophoretic particles having different first and second colors in a colorless fluid (in which case the first color is displayed when the first type of particle is positioned adjacent the viewing surface of the display and the second color is displayed when the second type of particle is positioned adjacent the viewing surface). Typically, the two colors are black and white. If a full color display is desired, an array of color filters can be deposited on the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on region sharing and color mixing to create color stimuli. The available display area is shared between three or four primary colors, e.g. red/green/blue (RGB) or red/green/blue/white (RGBW), and the filters may be arranged in a one-dimensional (stripe) or two-dimensional (2x2) repeating pattern. Other choices of primary colors or more than three primary colors are also known in the art. The three (in the case of an RGB display) or four (in the case of an RGBW display) sub-pixels are chosen small enough so that they visually blend together into a single pixel with a uniform color stimulus ("color blending") at the intended viewing distance. An inherent disadvantage of area sharing is that colorant is always present and the color can only be modulated by switching the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue and white primaries occupies one quarter of the display area (one subpixel out of four), the white subpixel is as bright as the underlying monochrome display white, and each colored subpixel is no brighter than one third of the monochrome display white. The luminance of the white displayed by the display as a whole cannot exceed half the luminance of the white sub-pixel (the white area of the display is created by displaying one white sub-pixel out of every four sub-pixels, plus each colored sub-pixel in colored form is equal to one third of the white sub-pixel, so that the combined three colored sub-pixels contribute no more than one white sub-pixel). Sharing with the area of the colored pixels switched to black reduces the brightness and saturation of the color. When mixing yellow, region sharing is particularly problematic because it is brighter than any other color of the same brightness, and saturated yellow is almost as bright as white. Switching the blue pixels (one quarter of the display area) to black would make the yellow too dark.

Multilayer stacked electrophoretic displays are known in the art; see, for example, J.Heikenfeld, P.Drzaic, J-S Yeo and T.Koch, Journal of the SID, 19(2), 2011, p.129-. In such displays, ambient light passes through the image in each of the three subtractive primary colors, exactly similar to conventional color printing. Us patent No.6,727,873 describes a stacked electrophoretic display in which three layers of switchable cells are placed on a reflective background. Similar displays are known in which the coloured particles are moved laterally (see international application No. wo 2008/065605), or isolated into microcells using a combination of vertical and lateral motion. In both cases, each layer is provided with electrodes for gathering or dispersing the colored particles pixel by pixel, so that each of the three layers requires a Thin Film Transistor (TFT) (two of the three layers must be substantially transparent) and a light-transmissive counter electrode. Such a complex electrode arrangement is expensive to manufacture and it is difficult in the prior art to provide a pixel electrode plane that is sufficiently transparent, especially because the white state of the display must be observed through several layers of electrodes. Multi-layer displays also suffer from parallax problems as the thickness of the display stack approaches or exceeds the pixel size.

U.S. application publication nos. 2012/0008188 and 2012/0134009 describe a multicolor electrophoretic display having a single backplane that includes independently addressable pixel electrodes and a common light-transmissive front electrode. A plurality of electrophoretic layers are disposed between the back plate and the front electrode. The displays described in these applications are capable of rendering any primary color (red, green, blue, cyan, magenta, yellow, white and black) at any pixel location. However, there are disadvantages to using multiple electrophoretic layers between a single set of addressing electrodes. The electric field experienced by the particles in a particular layer is lower than in the case of a single electrophoretic layer addressed using the same voltage. In addition, optical losses (e.g., caused by light scattering or unwanted absorption) in the electrophoretic layer closest to the viewing surface may affect the appearance of the image formed in the underlying electrophoretic layer.

Attempts have been made to provide full color electrophoretic displays using a single electrophoretic layer. For example, U.S. patent application publication No.2013/0208338 describes a color display that includes an electrophoretic fluid containing one or both types of pigment particles dispersed in a transparent and colorless or colored solvent, the electrophoretic fluid being disposed between a common electrode and a plurality of pixel or drive electrodes. The drive electrode is arranged to expose the background layer. U.S. patent application publication No.2014/0177031 describes a method for driving a display cell filled with an electrophoretic fluid comprising two charged particles carrying opposite charge polarities and two contrasting colors. Both types of pigment particles are dispersed in a colored solvent or a solvent in which uncharged or slightly charged colored particles are dispersed. The method includes driving the display cell to display the color of the solvent or the color of the uncharged or slightly charged colored particles by applying a drive voltage that is about 1 to about 20% of the full drive voltage. U.S. patent application publication nos. 2014/0092465 and 2014/0092466 describe electrophoretic fluids and methods for driving electrophoretic displays. The fluid comprises first, second and third types of pigment particles, all of which are dispersed in a solvent or solvent mixture. The first and second types of pigment particles carry opposite charge polarities, and the charge level of the third type of pigment particles is less than about 50% of the charge level of the first or second type. The three types of pigment particles have different levels of threshold voltage, or different mobility levels, or both. None of these patent applications discloses a full color display in the sense that the term is used below.

U.S. patent application publication No.2007/0031031 describes an image processing apparatus for processing image data to display an image on a display medium, where each pixel is capable of displaying white, black, and another color. U.S. patent application publication No. 2008/0151355; 2010/0188732, respectively; and 2011/0279885 describe a color display in which moving particles move through a porous structure. U.S. patent application publication nos. 2008/0303779 and 2010/0020384 describe a display medium including first, second, and third particles of different colors. The first and second particles may form an aggregate and the smaller third particles may move through the pores left between the aggregated first and second particles. U.S. patent application publication No.2011/0134506 describes a display device including an electrophoretic display element including a plurality of types of particles enclosed between a pair of substrates, at least one of the substrates being translucent, and each of the respective plurality of types of particles carrying charges of the same polarity, differing in optical characteristics, and differing in migration speed and/or electric field threshold for movement, a translucent display-side electrode being provided on a substrate side on which the translucent substrate is provided, a first back-side electrode being provided on a side of another substrate, facing the display-side electrode, and a second back-side electrode being provided on a side of the other substrate, facing the display-side electrode; and a voltage control section that controls voltages applied to the display-side electrode, the first back-side electrode, and the second back-side electrode so that a type of a particle having a fastest migration speed among the plurality of types of particles, or a type of a particle having a lowest threshold among the plurality of types of particles, sequentially moves to the first back-side electrode or the second back-side electrode through each of the different types of particles, and then the particle moving to the first back-side electrode moves to the display-side electrode. U.S. patent application publication No. 2011/0175939; 2011/0298835, respectively; 2012/0327504, respectively; and 2012/0139966 describe color displays that rely on the aggregation and threshold voltage of multiple particles. U.S. patent application publication No.2013/0222884 describes an electrophoretic particle comprising a polymer having a chargeA colored particle of a polymer of the group and a colorant, and a branched silicone-based polymer attached to the colored particle and containing a reactive monomer as a copolymerized component and at least one monomer selected from a specific monomer group. U.S. patent application publication No.2013/0222885 describes a dispersion for an electrophoretic display, which comprises a dispersion medium, a population of colored electrophoretic particles dispersed in the dispersion medium and migrating in an electric field, a population of non-electrophoretic particles not migrating and having a color different from that of the population of electrophoretic particles, and a compound having neutral polar groups and hydrophobic groups, which is contained in the dispersion medium in a proportion of about 0.01 to about 1 mass% based on the entire dispersion. U.S. patent application publication No.2013/0222886 describes a dispersion for a display that includes floating particles comprising: a core particle comprising a colorant and a hydrophilic resin; and a shell covering the surface of each core particle and containing a hydrophobic resin having a difference in solubility parameter of 7.95 (J/cm)³)^1/2Or larger. U.S. patent application publication nos. 2013/0222887 and 2013/0222888 describe electrophoretic particles having specific chemical compositions. Finally, U.S. patent application publication No.2014/0104675 describes a particle dispersion including first and second colored particles that move in response to an electric field, and a dispersion medium, the second colored particles having a larger diameter than the first colored particles and a charging characteristic that is the same as the charging characteristic of the first colored particles, and wherein a ratio (Cs/Cl) of a charge amount Cs of the first colored particles to a charge amount Cl of the second colored particles per unit area of the display is less than or equal to 5. Some of the above displays do provide full color, but at the cost of requiring long and cumbersome addressing methods.

U.S. patent application publication nos. 2012/0314273 and 2014/0002889 describe an electrophoretic device including a plurality of first and second electrophoretic particles contained in an insulating liquid, the first and second particles having different charging characteristics from each other; the device further comprises a porous layer comprised in the insulating liquid and formed by the fibrous structure. These patent applications are not full color displays in the sense that the term is used below.

See also U.S. patent application publication No.2011/0134506 and the aforementioned application serial No. 14/277,107; the latter describes a full color display using three different types of particles in a colored fluid, but the presence of the colored fluid limits the quality of the white state that can be achieved by the display.

In order to obtain a high resolution display, individual pixels of the display must be addressable without interference from adjacent pixels. One way to achieve this goal is to provide an array of non-linear elements, such as transistors or diodes, with at least one non-linear element associated with each pixel to create an "active matrix" display. The addressing or pixel electrode addressing a pixel is connected to a suitable voltage source via an associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to a drain of the transistor, and this arrangement will be adopted in the following description, although it is substantially arbitrary, and the pixel electrode may be connected to a source of the transistor. Traditionally, in high resolution arrays, pixels are arranged in a two-dimensional array of rows and columns such that any particular pixel is uniquely defined by the intersection of one designated row and one designated column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode; likewise, the assignment of sources to rows and gates to columns is conventional, but arbitrary in nature and can be reversed if desired. The row electrodes are connected to a row driver which essentially ensures that only one row is selected at any given time, i.e. a selected voltage is applied to the selected row electrodes, such as to ensure that all transistors in the selected row are conductive, while non-selected voltages are applied to all other rows, such as to ensure that all transistors in these non-selected rows remain non-conductive. The column electrodes are connected to a column driver which places voltages on the respective column electrodes which are selected to drive the pixels in the selected row to their desired optical states. (with respect to a common front electrode typically disposed on the opposite side of the electro-optic medium from the non-linear array and extending across the entire display). After a pre-selected interval called the "line address time", the selected row is deselected, the next row is selected, and the voltage on the column driver is changed in order to write the next row of the display. This process is repeated so that the entire display is written in a row-by-row fashion.

Conventionally, each pixel electrode is associated with a capacitor electrode, such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, international patent application WO 01/07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form the transistor, and the "selected" and "unselected" voltages applied to the gate electrode may be positive and negative, respectively.

Fig. 10 of the accompanying drawings depicts an exemplary equivalent circuit for a single pixel of an electrophoretic display. As shown, the circuit includes a capacitor 10 formed between the pixel electrode and the capacitor electrode. Electrophoretic medium 20 is shown as a capacitor and a resistor in parallel. In some cases, direct or indirect coupling capacitance 30 (commonly referred to as "parasitic capacitance") between the gate electrode of the transistor associated with the pixel and the pixel electrode may generate undesirable noise to the display. Typically, the parasitic capacitance 30 is much smaller than the capacitance of the storage capacitor 10, and when a row of pixels of the display is selected or deselected, the parasitic capacitance 30 may result in a small negative bias voltage, also referred to as a "kickback voltage", to the pixel electrode, typically less than 2 volts. In some embodiments, to compensate for unwanted "jump voltages", the common potential V may be set_comProvided to the top plane electrode and the capacitor electrode associated with each pixel such that when V is selected_comIs set equal to the jump voltage (V)_KB) Each voltage supplied to the display may be offset by the same amount and will not experience a net DC imbalance.

However, when V_comA problem may occur when the voltage is set not to compensate for the jump voltage. This may occur when it is desired to apply a higher voltage to the display than is available from the backplane alone. It is well known in the art, for example, if the backplane is provided with a nominal + V, 0 or-V choice, e.g., when V_comMaximum applied to the display when supplied with-VThe voltage can be doubled. The maximum voltage experienced in this case is +2V (i.e., relative to the top plane at the backplane), while the minimum is zero. If a negative voltage is required, then V_comThe potential must rise to at least zero. Therefore, the waveform for a display using top-plane switching to address positive and negative voltages must have a waveform assigned to more than one V_comA particular frame of each of the voltage settings.

When (as described above) V_comIs intentionally set to V_KBA separate power supply may be used. However, when using top plane switching, the AND V is used_comProviding as many separate power supplies is expensive and inconvenient. Therefore, it is necessary to use for the back plate and V_comTo compensate for DC offset caused by the jump voltage.

Disclosure of Invention

The present invention therefore provides a method of driving an electro-optic display which is DC balanced despite the presence of a kickback voltage and variations in the voltage applied to the front electrode.

Accordingly, in one aspect, the present invention provides a method for driving an electro-optic display having a front electrode, a backplane, and a display medium between the front electrode and the backplane. The method includes applying a first drive phase to the display medium, the first drive phase having a first signal having a first polarity, a first amplitude as a function of time, and a first duration, and a second signal subsequent to the first signal and having a second polarity opposite the first polarity, a second amplitude as a function of time, and a second duration, such that a sum of the first amplitude as a function of time integrated over the first duration and the second amplitude as a function of time integrated over the second duration produces a first impulse offset. The method further includes applying a second drive phase to the display medium, the second drive phase producing a second impulse offset, wherein a sum of the first and second impulse offsets is substantially zero.

In some other aspects, the invention also provides a method for driving an electro-optic display having a front electrode, a backplane, and a display medium between the front electrode and the backplane, the method comprising applying a reset phase and a color transition phase to the display. Wherein the reset phase comprises applying a first signal having a first polarity, a first amplitude as a function of time, and a first duration on the front electrode; applying a second signal having a second polarity opposite the first polarity, a second amplitude as a function of time, and a second duration over the backplane during the first duration; applying a third signal having a second polarity, a third amplitude as a function of time, and a third duration on the front electrode after the first duration; a fourth signal having the first polarity, a fourth amplitude as a function of time, and a fourth duration is applied to the backplane after the second duration. Wherein the sum of the first amplitude as a function of time integrated over a first duration and the second amplitude as a function of time integrated over a second duration and the third amplitude as a function of time integrated over a third duration and the fourth amplitude as a function of time integrated over a fourth duration yields an impulse offset designed to maintain DC balance on the display medium over the reset phase and the color transition phase.

The electrophoretic medium used in the display of the present invention may be any of the electrophoretic media described in the above-mentioned application serial No. 14/849,658. Such a medium comprises light scattering particles, which are usually white, and three substantially non-light scattering particles. The electrophoretic medium of the present invention may be in any of the forms discussed above. Thus, the electrophoretic medium may be unencapsulated, encapsulated in discrete capsules surrounded by a capsule wall, or in the form of a polymer-dispersed or microcell medium.

Drawings

Figure 1 of the accompanying drawings is a schematic cross-section showing the positions of various particles in an electrophoretic medium of the invention when displaying black, white, three subtractive primary colors and three additive primary colors.

Fig. 2 shows in schematic form four pigment particles for use in the present invention.

Fig. 3 shows in schematic form the relative strength of the interaction between pairs of particles of the present invention.

Figure 4 shows in schematic form the behaviour of particles of the invention when subjected to electric fields of different strength and duration.

Fig. 5A and 5B show waveforms for driving the electrophoretic medium shown in fig. 1 to its black and white states, respectively.

Fig. 6A and 6B illustrate waveforms for driving the electrophoretic medium illustrated in fig. 1 to its magenta and blue states.

Fig. 6C and 6D show waveforms for driving the electrophoretic medium shown in fig. 1 to its yellow and green states.

Fig. 7A and 7B show waveforms for driving the electrophoretic medium shown in fig. 1 to its red and cyan states, respectively.

Fig. 8-9 show waveforms that may be used in place of the waveforms shown in fig. 5A-5B, 6A-6D, and 7A-7B to drive the electrophoretic medium shown in fig. 1 to all of its color states.

As already mentioned, fig. 10 shows an exemplary equivalent circuit of a single pixel of an electrophoretic display.

Fig. 11 is a voltage versus time diagram showing the change over time of the front and pixel electrodes for generating waveforms for one color in the drive scheme of the present invention, and the resulting voltages on the electrophoretic medium.

Fig. 12 is a voltage versus time diagram showing the change over time of the front and pixel electrodes of the reset phase of the waveform shown in fig. 11, and also showing various parameters used in the DC balance calculation described below.

FIG. 13 is another graph of voltage versus time showing various parameters used in a DC balanced drive waveform.

Detailed Description

As described above, the present invention can be used with an electrophoretic medium comprising one light scattering particle (typically white) and three other particles providing three subtractive primary colors.

The three particles providing the three subtractive primary colors may be substantially non-light scattering ("SNLS"). The use of SNLS particles allows for the mixing of colors and provides more color results than can be obtained using the same number of scattering particles. The above-mentioned US 2012/0327504 uses particles with subtractive primary colors, but requires two different voltage thresholds to independently address the non-white particles (i.e. the display is addressed with three positive and three negative voltages). These thresholds must be sufficiently separated to avoid cross talk and this separation makes it necessary to use high addressing voltages for certain colors. Furthermore, addressing the colored particle with the highest threshold will also move all other colored particles and these other particles must then be switched to their desired position at a lower voltage. This stepwise color addressing scheme produces unwanted color flicker and long transition times. The present invention does not require the use of such a step-wise waveform and, as described below, addressing all colors can be accomplished with only two positive and two negative voltages (i.e., only five different voltages are needed in the display, two positive, two negative and zero voltages, although it may be more preferable to address the display with more different voltages, as described below in some embodiments).

As already mentioned, fig. 1 of the accompanying drawings is a schematic cross-section showing the positions of various particles in an electrophoretic medium of the invention when displaying black, white, three subtractive primary colors and three additive primary colors. In fig. 1, it is assumed that the viewing surface of the display is at the top (as shown), i.e. the user views the display from this direction, and light is incident from this direction. As already noted, in a preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, whereas in fig. 1 the particles are considered to be white pigments. Basically, such light scattering white particles form a white reflector from which any particles above the white particles can be viewed (as shown in fig. 1). Light entering the viewing surface of the display passes through the particles, reflects from the white particles, returns through the particles and exits the display. Thus, the particles above the white particles may absorb various colors, and the color seen by the user is the color produced by the combination of the particles above the white particles. Any particles that are placed below the white particles (from the user's point of view) are masked by the white particles and do not affect the displayed color. Since the second, third and fourth particles are substantially non-light scattering, their order or arrangement with respect to each other is unimportant, but for the reasons already stated their order or arrangement with respect to the white (light scattering) particles is critical. .

More specifically, when cyan, magenta, and yellow particles are located below the white particles (case [ a ] in fig. 1), there are no particles above the white particles, and the pixel displays only white. When a single particle is above a white particle, the color of the single particle is displayed, yellow, magenta and cyan in cases [ B ], [ D ] and [ F ] in fig. 1, respectively. When two particles are above the white particle, the color displayed is a combination of the two particles; in fig. 1, in case [ C ], magenta and yellow particles display red, in case [ E ], cyan and magenta particles display blue, and in case [ G ], yellow and cyan particles display green. Finally, when all three colored particles are above the white particles (case [ H ] in fig. 1), all incident light is absorbed by the three subtractive primary colored particles, and the pixel displays black.

One of the subtractive primary colors may be rendered by particles scattering light such that the display will comprise two types of light scattering particles, one of which is white and the other colored. However, in this case the position of the light scattering colored particles relative to the other colored particles covering the white particles will be important. For example, when rendering the color black (when all three color particles are above the white particle), the scattering color particles cannot be located on the non-scattering color particles (otherwise they will be partially or completely hidden behind the scattering particles and the rendered color will be the color of the scattering color particles, not black).

If more than one type of colored particles scatter light, it is not easy to render the color black.

Fig. 1 shows the ideal case where the color is not contaminated (i.e. the light scattering white particles completely mask any particles behind the white particles). In practice, the masking of white particles may be imperfect, so that particles that ideally can be completely masked may have some small absorption of light. Such contamination typically reduces the brightness and chromaticity of the rendered colors. In the electrophoretic medium of the present invention, such color contamination should be minimized to the extent that the resulting color is commensurate with industry standards for color reproduction. A particularly advantageous criterion is SNAP (a criterion for the production of newspaper advertisements), which specifies the values of L, a and b for each of the above-mentioned eight primary colors. (hereinafter, "primaries" will be used to refer to eight colors, black, white, three subtractive primaries, and three additive primaries, as shown in FIG. 1.)

Methods for electrophoretically arranging a plurality of differently colored particles in a "layer" as shown in fig. 1 have been described in the prior art. The simplest such methods involve "racing" pigments with different electrophoretic mobilities; see, for example, U.S. patent No.8,040,594. This competition is more complex than initially understandable, since the movement of the charged pigments themselves alters the electric field locally experienced in the electrophoretic fluid. For example, when positively charged particles move towards the cathode and negatively charged particles move towards the anode, their charge screens the electric field experienced by the charged particles between the two electrodes. It is believed that although pigment competition is involved in the electrophoresis of the present invention, it is not the only phenomenon that causes the alignment of the particles shown in fig. 1.

A second phenomenon that can be used to control the motion of multiple particles is heterogeneous aggregation between different pigment types; see, for example, US 2014/0092465, supra. This aggregation may be charge-mediated (coulomb) or may occur due to, for example, hydrogen bonding or van der waals interactions. The strength of the interaction can be influenced by the choice of the surface treatment of the pigment particles. For example, coulombic interaction may be reduced when the closest distance of oppositely charged particles is maximized by a steric barrier (typically a polymer grafted or adsorbed onto the surface of one or both particles). In the present invention, as described above, such polymeric barriers are used on the first and second types of particles, and may or may not be used on the third and fourth types of particles.

A third phenomenon that can be used to control the motion of a plurality of particles is voltage or current dependent mobility, as described in detail in the above-mentioned application serial No. 14/277,107.

Fig. 2 shows a schematic cross-sectional representation of four pigment types (1-4) used in a preferred embodiment of the present invention. The polymeric shell adsorbed to the core pigment is indicated by dark shading, while the core pigment itself is shown as unshaded. The core pigment may be used in a variety of forms: spherical, acicular or other anisometric, aggregates of smaller particles (i.e., "grape clusters"), composite particles comprising small pigment particles or dyes dispersed in a binder, and the like, as is well known in the art. The polymer shell may be a covalently bonded polymer prepared by grafting processes or chemisorption as is well known in the art, or may be physisorbed on the particle surface. For example, the polymer may be a block copolymer comprising insoluble and soluble segments. Some methods of securing the polymeric shell to the core pigment are described in the examples below.

The first and second particle types in one embodiment of the invention preferably have a larger polymer shell than the third and fourth particle types. The light scattering white particles are of the first or second type (negatively or positively charged). In the following discussion it is assumed that the white particles are negatively charged (i.e. have type 1), but it will be clear to the person skilled in the art that the general principles described will apply to a set of particles in which the white particles are positively charged.

In the present invention, the electric field required to separate aggregates formed of a mixture of the types 3 and 4 of particles in the suspension solvent containing the charge control agent is larger than the electric field required to separate aggregates formed of any other combination of the two types of particles. On the other hand, the electric field required to separate the aggregates formed between the first and second types of particles is smaller than the electric field required to separate the aggregates formed between the first and fourth particles or the second and third particles (of course smaller than the electric field required to separate the third and fourth particles).

In fig. 2, the core pigments comprising particles are shown to have approximately the same size, and it is assumed that the zeta potential (although not shown) of each particle is approximately the same. Varying is the thickness of the polymeric shell surrounding each core pigment. As shown in fig. 2, the polymer shell is thicker for the types 1 and 2 than for the types 3 and 4-and this is actually the preferred case for certain embodiments of the invention.

To understand how the thickness of the polymer shell affects the electric field required to separate aggregates of oppositely charged particles, it may be helpful to consider the force balance between pairs of particles. In practice, aggregates may be composed of a large number of particles, and the situation will be much more complex than the case of simple pair-wise interactions. However, particle pair analysis does provide some guidance for understanding the present invention.

The force acting on one of a pair of particles in an electric field is given by:

when F is present_AppIs the force exerted on the particle by the applied electric field, F_CIs the coulomb force, F, exerted on the particle by a second particle of opposite charge_VWIs an attractive van der Waals force exerted on one particle by a second particle, and F_DIs the attractive force exerted on the particle pairs by depletion flocculation (flocculation) due to (optionally) inclusion of a stabilizing polymer in the suspending solvent.

Force F exerted on the particle by the applied electric field_AppGiven by:

where q is the charge of the particle, which is related to the zeta potential, as shown in equation (2) (approximately in the Huckel limit), where a is the core pigment radius, s is the thickness of the solvent swollen polymer shell, and other symbols have conventional meanings known in the art.

For particles 1 and 2, the magnitude of the force exerted by the other particle on one particle due to coulombic interaction is roughly given by:

note that F is applied to each particle_AppThe forces are used to separate the particles, while the other three forces are attractive between the particles. According to Newton's third law, if F acts on a particle_AppHigher force than F acting on another particle_AppForce (because the charge on one particle is higher than the charge on the other), the force used to separate the pair is composed of two F_AppThe weaker of the forces is given.

As can be seen from (2) and (3), the magnitude of the difference between the attracting and detaching coulomb terms is given by:

making (a + s) smaller or zeta larger makes the particles more difficult to separate if the particles have equal radii and zeta potentials. Thus, in one embodiment of the invention, it is preferred that the particles of types 1 and 2 are large and have a relatively low zeta potential, while the particles 3 and 4 are small and have a relatively large zeta potential.

However, if the thickness of the polymer shell is increased, the van der waals forces between the particles may also change significantly. The polymer shells on the particles are swollen by the solvent and move to further separate the surfaces of the core pigment by van der waals interactions. For radius (a)₁,a₂) Much greater than the distance(s) between them₁+s₂) The spherical core pigment of (a) is,

where A is the Hamaker constant. As the distance between the core pigments increases, the expression becomes more complex, but the effect remains the same: increase of s₁Or s₂Has a significant impact on reducing attractive van der waals interactions between particles.

In this context, it is possible to understand the basic principle behind the particle types shown in fig. 2. The particles of types 1 and 2 have large polymeric shells swollen by the solvent, the moving core pigment is further separated, and the van der waals interactions between them will be reduced more than the particles of types 3 and 4 having smaller or no polymeric shells. Even if the particles have approximately the same size and magnitude of the electromotive potential, according to the present invention, the strength of the interaction between the paired aggregates can be configured to meet the above requirements.

For more details of preferred particles for use in the display of fig. 2, the reader is referred to the above application, serial No. 14/849,658.

Figure 3 shows in schematic form the electric field strength required to separate the paired aggregates of particle types of the present invention. The interaction between the particles of types 3 and 4 is stronger than the interaction between the particles of types 2 and 3. The interaction between the particles of types 2 and 3 is approximately equal to the interaction between the particles of types 1 and 4, and stronger than the interaction between the particles of types 1 and 2. All interactions between pairs of particles with the same charge sign are equal to or weaker than the interactions between particles of types 1 and 2.

Fig. 4 shows how these interactions can be used to make all primary colors (subtractive, additive, black and white) as generally discussed with reference to fig. 1.

When addressed with a low electric field (fig. 4(a)), the particles 3 and 4 are aggregated and not separated. Particles 1 and 2 are free to move in the field. If the particles 1 are white particles, the color seen from the left side is white and from the right side is black. The polarity of the inversion field is switched between the black and white state. However, the instantaneous color between the black and white states is chromatic. The aggregate of particles 3 and 4 will move very slowly in the field with respect to particles 1 and 2. It can be seen that particle 2 has moved past particle 1 (to the left) without significant movement of the aggregates of particles 3 and 4. In this case, particle 2 will be seen from the left side view, while the aggregate of particles 3 and 4 will be seen from the right side view. As shown in the examples below, in certain embodiments of the invention, the aggregates of particles 3 and 4 carry a weak positive charge and are therefore located near particle 2 at the beginning of this transition.

When addressed with a high electric field (fig. 4(B)), the particles 3 and 4 are separated. Which of the particles 1 and 3 (each particle having a negative charge) is visible when viewed from the left will depend on the waveform (see below). As shown, particle 3 is visible from the left side, and the combination of particles 2 and 4 is visible from the right side.

Starting from the state shown in fig. 4(B), a low voltage of opposite polarity moves the positively charged particles to the left and the negatively charged particles to the right. However, positively charged particles 4 will encounter negatively charged particles 1 and negatively charged particles 3 will encounter positively charged particles 2. The result is that a combination of particles 2 and 3 will be seen from the left side and particle 4 will be seen from the right side.

As mentioned above, preferably, particles 1 are white, particles 2 are cyan, particles 3 are yellow and particles 4 are magenta.

The core pigment used in the white particles is typically a metal oxide of high refractive index, as is well known in the art of electrophoretic displays. Examples of white pigments are described in the examples below.

As mentioned above, the core pigments used to make particles of types 2-4 provide three subtractive primary colors: cyan, magenta, and yellow.

The electrophoretic fluid of the present invention may be used in several ways known in the art to construct a display device. The electrophoretic fluid may be encapsulated in microcapsules or incorporated into a microcell structure and then sealed with a polymer layer. The microcapsule or microcell layer may be coated or embossed onto a plastic substrate or film with a transparent coating of conductive material. The assembly may be laminated to a backplane carrying the pixel electrodes using a conductive adhesive.

A first embodiment of a waveform for realizing each particle arrangement shown in fig. 1 will now be described with reference to fig. 5 to 7. Hereinafter, this driving method will be referred to as "first driving scheme" of the present invention. In this discussion, it is assumed that the first particles are white and negatively charged, the second particles are cyan and positively charged, the third particles are yellow and negatively charged, and the fourth particles are magenta and positively charged. If these assignments of particle colors are changed, the skilled person will understand how the color transition will change, as one of the first and second particles may be provided white. Similarly, the polarity of the charge on all the particles may be reversed, and the electrophoretic medium will still function in the same manner, provided that the polarity of the waveform used to drive the medium (see next paragraph) is similarly reversed.

In the following discussion, the waveform (voltage versus time) applied to the pixel electrode of the backplane of the display of the present invention is described and plotted, while assuming that the front electrode is grounded (i.e., at zero potential). The electric field experienced by the electrophoretic medium is of course determined by the potential difference between the back plate and the front electrode and the distance separating them. The display is typically viewed through its front electrode, so that the particles adjacent to the front electrode control the colour displayed by the pixel and the optical transitions involved if the potential of the front electrode relative to the backplane is considered are sometimes more easily understood; this can be done simply by inverting the waveform discussed below.

These waveforms require that each pixel of the display can be driven at five different addressing voltages, denoted + V_high、+V_low、0、-V_lowand-V_highShown in FIGS. 5-7 as 30V, 15V, 0, -15V, and-30V. In practice, it may be preferable to use a larger number of addressing voltages. If only three voltages are available (i.e. + V)_high0 and-V_high) Then can be obtained by using the voltage V_highBut with addressing at a duty cycle of 1/n, and at a lower voltage (e.g., V)_highN, where n is>Positive integer of 1) is used.

The waveform used in the present invention may include three phases: a DC-balancing phase, in which DC-imbalances due to previous waveforms applied to the pixels are corrected, or in which DC-imbalances to be caused in subsequent color rendering transitions are corrected (as known in the art); a "reset" phase in which the pixels return to at least approximately the same starting configuration regardless of the previous optical state of the pixels; and a "color rendering" phase as described below. The DC balancing and reset phases are optional and may be omitted, depending on the requirements of a particular application. The "reset" phase (if employed) may be the same as the magenta rendering waveform described below, or may comprise driving the maximum possible positive and negative voltages continuously, or may be some other pulse pattern, as long as it returns the display to a state from which a subsequent color can be reproducibly obtained.

Fig. 5A and 5B show in idealized form typical color rendering stages of waveforms for producing black and white states in a display of the present invention. The graphs in fig. 5A and 5B show the voltage applied to the backplane (pixel) electrode of the display, while the transparent common electrode on the top plane is grounded. The x-axis represents time measured in arbitrary units, while the y-axis is applied voltage in volts. Driving the display into a black (fig. 5A) or white (fig. 5B) state, respectively, is effected by a sequence of positive or negative impulses, preferably at a voltage V_lowSince, as mentioned above, in the case of V_lowThe magenta and yellow pigments come together at the field (or current). Thus, the white and cyan pigments move while the magenta and yellow pigments remain stationary (or move at a much slower speed) and the display switches between a white state and a state corresponding to the absorption of the cyan, magenta and yellow pigments (commonly referred to in the art as "composite black"). The length of the pulses driven to black and white can vary between about 10-1000 milliseconds, and the pulses can be separated by a rest (rest) of length (at zero applied voltage) in the range of 10-1000 milliseconds. Although fig. 5 shows positive and negative voltage pulses producing black and white, respectively, these pulses are separated by "rest" periods providing zero voltage, which are sometimes preferredIncluding pulses of opposite polarity to the drive pulses but of lower impulse (i.e., of shorter duration or lower applied voltage than the main drive pulse, or both).

Fig. 6A-6D illustrate exemplary color rendering stages for generating waveforms for magenta and blue (fig. 6A and 6B) and yellow and green (fig. 6C and 6D). In fig. 6A, the waveform oscillates between positive and negative impulses, but the length of the positive impulse (t)_p) Shorter than the length (t) of the negative impulse_n) And a voltage (V) applied under positive shock_p) Greater than the voltage (V) applied under negative impulse_n). When:

V_pt_p＝V_nt_nwhen the temperature of the water is higher than the set temperature,

the waveform as a whole is "DC balanced". The period of one cycle of positive and negative impulses may be in the range of about 30-1000 milliseconds.

At the end of the positive impulse the display is in the blue state, and at the end of the negative impulse the display is in the magenta state. This is consistent with the change in optical density corresponding to movement of the cyan pigment being greater than the change corresponding to the magenta or yellow pigment (relative to the white pigment). This is expected if the interaction between the magenta pigment and the white pigment is stronger than the interaction between the cyan pigment and the white pigment, according to the assumptions presented above. The relative mobility of the yellow and white pigments (both negatively charged) is much lower than the relative mobility of the cyan and white pigments (oppositely charged). Therefore, in generating the preferred waveform for magenta or blue, it is preferable to include V_pt_pAnd following V_nt_nOf at least one cycle of an impulse sequence of, wherein V_p>V_nAnd t is_p<t_n. When blue is required, the sequence is at V_pEnd up and when magenta is required, the sequence is at V_nAnd (6) ending the process.

Fig. 6B shows an alternative waveform that uses only three voltage levels to produce the magenta and blue states. In this alternative waveform, V is preferable_pt_pAnd following V_nt_nAt least one cycle of (2), wherein V_p＝V_n＝V_highAnd t is_n<t_p. This sequence is not DC balanced. When blue is required, the sequence is at V_pEnd up and when magenta is required, the sequence is at V_nAnd (6) ending the process.

The waveforms shown in fig. 6C and 6D are the inverse of the waveforms shown in fig. 6A and 6B, respectively, and produce corresponding complementary yellow and green colors. In one preferred waveform for producing yellow or green, as shown in FIG. 6C, a waveform including V is used_pt_pAnd following V_nt_nOf at least one cycle of an impulse sequence of, wherein V_p<V_nAnd t is_p>t_n. When green is desired, the sequence is at V_pEnd above, and when yellow is desired, the sequence is at V_nAnd (6) ending the process.

Another preferred waveform for producing yellow or green using only three voltage levels is shown in fig. 6D. In this case, V is used_pt_pAnd following V_nt_nAt least one cycle of (2), wherein V_p＝V_n＝V_highAnd t is_n>t_p. This sequence is not DC balanced. When green is desired, the sequence is at V_pEnd above, and when yellow is desired, the sequence is at V_nAnd (6) ending the process.

Fig. 7A and 7B show the color rendering stages for rendering red and cyan waveforms on the display of the present invention. These waveforms also oscillate between positive and negative impulses, but they differ from the waveforms of fig. 6A-6D in that the period of one cycle of positive and negative impulses is typically longer and the addressing voltage used may (but need not) be lower. The red waveform of FIG. 7A consists of a pulse (+ V) that produces black (similar to the waveform shown in FIG. 5A)_low) And a shorter pulse of opposite polarity (-V) followed_low) Composition which removes cyan particles and changes black to red (the complementary color of cyan). The cyan waveform is the inverse of the red waveform, with the effect of producing white (-V)_low) And a following short pulse (V) moving the cyan particles to the vicinity of the viewing surface_low). As in the waveforms shown in fig. 6A-6D, with respect to whiteThe color, cyan, moves faster than the magenta or yellow pigment. However, in contrast to the waveform of FIG. 6, the yellow pigment and magenta particles in the waveform of FIG. 7 remain on the same side of the white particles.

The waveforms described above with reference to fig. 5-7 use a five level drive scheme, i.e., a drive scheme in which the pixel electrode may be at any one of two different positive voltages, two different negative voltages, or zero volts relative to the common front electrode at any given time. In the particular waveforms shown in fig. 5-7, the five levels are 0, ± 15V and ± 30V. However, at least in some cases, it has been found to be advantageous to use a seven-level drive scheme that uses seven different voltages: three positive voltages, three negative voltages and zero voltage. This seven-level driving scheme may be referred to as a "second driving scheme" of the present invention hereinafter. The amount of voltage used to address the display should be selected in consideration of the limitations of the electronics used to drive the display. In general, a larger number of drive voltages will provide greater flexibility in addressing different colors, but complicates the arrangement required to provide such a larger number of drive voltages to conventional device display drivers. The inventors have found that the use of seven different voltages provides a good compromise between the complexity of the display structure and the color gamut.

The general principle of generating eight primary colors (white, black, cyan, magenta, yellow, red, green and blue) using a second drive scheme applied to a display of the invention (e.g. as shown in fig. 1) will now be described. As shown in fig. 5-7, assume that the first pigment is white, the second cyan, the third yellow, and the fourth magenta. It will be clear to one of ordinary skill in the art that if the distribution of the color of the paint is changed, the color presented by the display will change.

The maximum positive and negative voltages (denoted as ± Vmax in fig. 8) applied to the pixel electrode produce a color formed by a mixture of second and fourth particles (cyan and magenta to produce blue, see fig. 1E and 4B viewed from the right) or only third particles (yellow-see fig. 1B and 4B viewed from the left-white pigments scatter light and are located between the color pigments), respectively. These blue and yellow colors are not necessarily the best achievable blue and yellow colors for the display. Intermediate positive and negative voltages (denoted as ± Vmid in fig. 8) applied to the pixel electrodes produce colors of black and white, respectively (although not necessarily the best black and white achievable by the display-see fig. 4A).

From these blue, yellow, black or white optical states, the other four primary colors can be obtained by moving only the second particles (in this case the cyan particles) relative to the first particles (in this case the white particles), using the lowest applied voltage (denoted as ± Vmin in fig. 8). Thus, shifting cyan out of blue (by applying-Vmin to the pixel electrode) produces magenta (see fig. 1E and 1D for blue and magenta, respectively); shifting cyan into yellow (by applying + Vmin to the pixel electrode) provides green (see fig. 1B and 1G for yellow and green, respectively); moving cyan out of black (by applying-Vmin to the pixel electrode) provides red (see fig. 1H and 1C for black and red, respectively), and moving cyan into white (by applying + Vmin to the pixel electrode) provides cyan (see fig. 1A and 1F for white and cyan, respectively).

While these general principles may be used to construct waveforms to produce specific colors in the displays of the present invention, in practice the ideal behavior described above may not be observed, and modifications to the basic scheme are desired.

A modified general waveform embodying the above basic principle is shown in fig. 8, where the abscissa represents time (in arbitrary units) and the ordinate represents the voltage difference between the pixel electrode and the common front electrode. The magnitude of the three positive voltages used in the driving scheme shown in fig. 8 may be between about +3V and +30V, and the magnitude of the three negative voltages between about-3V and-30V. In an empirically preferred embodiment, the highest positive voltage + Vmax is +24V, the intermediate positive voltage + Vmid is 12V, and the lowest positive voltage + Vmin is 5V. In a similar manner, the negative voltages-Vmax, -Vmid, and-Vmin are-24V, -12V, and-9V in the preferred embodiment. The magnitude of the voltage | + V | ═ V | is not necessary for any of the three voltage levels, although it may be preferred in some circumstances.

There are four different stages in the general waveform shown in fig. 8. In the first phase ("a" in fig. 8), pulses are provided at + Vmax and-Vmax (where "pulse" denotes a unipolar square wave, i.e. a constant voltage is applied for a predetermined time), which are used to erase a previous image rendered on the display (i.e. "reset" the display). These pulses (t)₁And t₃) And length of rest (i.e. period of zero voltage (t) between them)₂And t₄) May be selected such that the entire waveform (i.e., the integral of voltage over the entire waveform with respect to time as shown in fig. 8) is dc balanced (i.e., the integral is substantially zero). Dc balancing can be achieved by adjusting the length of the pulses and pauses in phase a so that the net impulse provided in this phase is equal in magnitude and opposite in sign to the net impulse provided in the combination of phases B and C during which the display is switched to a particular desired color as described below.

The waveforms shown in fig. 8 are purely for purposes of illustrating the structure of the general waveforms and are not intended to limit the scope of the present invention in any way. Thus, in fig. 8, the negative pulse is shown before the positive pulse in phase a, but this is not a requirement of the invention. It is also not required that there be only one negative pulse and one positive pulse in phase a.

As noted above, the generic waveform is DC balanced in nature, and this may be preferred in certain embodiments of the invention. Alternatively, the pulses in phase a may provide DC balance to a series of color transitions rather than a single transition in a manner similar to that provided in some black and white displays of the prior art; see, for example, U.S. patent No.7,453,445 and the earlier applications mentioned in column 1 of this patent.

In the second phase of the waveform (phase B in fig. 8), pulses using maximum and intermediate voltage amplitudes are provided. In this stage, white, black, magenta, red and yellow are preferably rendered in the manner previously described with reference to fig. 5-7. More generally, in this phase of the waveform, colors corresponding to particles of type 1 (assuming that the white particles are negatively charged), combinations of particles of types 2,3, and 4 (black), particles of type 4 (magenta), combinations of particles of types 3 and 4 (red), and particles of type 3 (yellow) are formed.

As described above (see fig. 5B and related description), white can be rendered by one or more pulses at-Vmid. However, in some cases, the white color produced in this way may be contaminated with a yellow pigment and appear yellowish. To correct for this color contamination, it may be necessary to introduce some pulses of positive polarity. Thus, for example, white may be obtained by a single instance or repetition of instances of a pulse sequence comprising a pulse having a length T₁And an amplitude of + Vmax or + Vmid, followed by a pulse having a length T₂And a pulse of amplitude-Vmid, where T₂>T₁. The final pulse should be a negative pulse. In fig. 8, the duration t is shown₅Of + Vmax sequence, followed by a duration t₆is-Vmid. During this pulse sequence, the appearance of the display oscillates between magenta (although usually not the ideal magenta) and white (i.e. white will be preceded by a state with lower L and higher a than the final white state). This is similar to the pulse sequence shown in fig. 6A, where oscillation between magenta and blue is observed. The difference here is that the net impulse of the pulse train is more negative than the pulse train shown in fig. 6A, so the oscillation is biased towards the negatively charged white pigment.

As described above (see fig. 5A and related description), black may be obtained by rendering at one or more pulses of + Vmid (separated by periods of zero voltage).

As described above (see FIGS. 6A and 6B and related description), magenta may be obtained by a single instance or repetition of instances of a pulse train comprising pulses having a length T₃And an amplitude of + Vmax or + Vmid, followed by a pulse having a length T₄And a pulse of amplitude-Vmid, where T₄>T₃. To produce magenta, the net impulse for this phase of the waveform should be more positive than the net impulse used to produce white. During the pulse train for generating magenta, the display will oscillate between a state of substantially blue and magenta. Magenta colorWill be states with more negative a and lower L than the final magenta state.

As described above (see FIG. 7A and associated description), red color may be achieved by a single instance or repetition of instances of a pulse sequence comprising a pulse having a length T₅And an amplitude of + Vmax or + Vmid, followed by a pulse having a length T₆And pulses of amplitude-Vmax or-Vmid. To produce red, the net impulse should be more positive than the net impulse used to produce white or yellow. Preferably, to produce red, the positive and negative voltages used are of substantially the same magnitude (both Vmax or both Vmid), the length of the positive pulse is longer than the length of the negative pulse, and the final pulse is a negative pulse. During the pulse sequence for generating the color red, the display will oscillate between the substantially black and red states. The red front will be the state with lower L, lower a and lower b than the final red state.

Yellow (see fig. 6C and 6D and associated description) may be obtained by a single instance or repetition of instances of a pulse sequence comprising a pulse having a length T₇And an amplitude of + Vmax or + Vmid, followed by a pulse having a length T₈And a pulse of amplitude-Vmax. The final pulse should be a negative pulse. Alternatively, as described above, the yellow color may be obtained by a single pulse or multiple pulses at-Vmax.

In the third phase of the waveform (phase C in fig. 8), pulses using intermediate and minimum voltage amplitudes are provided. In this phase of the waveform, blue and cyan are produced after driving toward white in the second phase of the waveform, and green is produced after driving toward yellow in the second phase of the waveform. Thus, when a waveform transient of the inventive display is observed, the front of blue and cyan will be a color with b more positive than the b value of the final cyan or blue, and the front of green will be a more yellow color, where L is higher and a and b more positive than L, a and b of the final green. More generally, when the display of the present invention renders a color corresponding to a colored particle of the first and second particles, the state is preceded by a substantially white state (i.e., having C less than about 5). When the display of the invention renders a color corresponding to a combination of colored ones of the first and second particles and ones of the third and fourth particles having an opposite charge to that of the particles, the display will first substantially render a color of ones of the third and fourth particles having an opposite charge to the colored ones of the first and second particles.

Typically, cyan and green will result from a pulse sequence that must use + Vmin. This is because the cyan pigment can move relative to the white pigment independently of the magenta and yellow pigments only at this minimum positive voltage. This movement of cyan pigment is necessary to render cyan from white or green from yellow.

Finally, in the fourth phase of the waveform (phase D in fig. 8), zero voltage is supplied.

Although the display of the present invention has been described as producing eight primary colors, in practice it is preferred to produce as many colors as possible at the pixel level. A full-color grayscale image can then be rendered by dithering between these colors using techniques well known to those skilled in the imaging arts. For example, in addition to the eight primary colors generated as described above, the display may be configured to render eight additional colors. In one embodiment, these additional colors are: light red, light green, light blue, dark cyan, dark magenta, dark yellow, and two gray levels between black and white. The terms "light" and "dark" as used in this context refer to colors having substantially the same hue angle as the reference color, but a higher or lower L, respectively, in a color space such as CIE L a b.

Normally, light colors are obtained in the same way as dark colors, but waveforms with slightly different net impulses are used in phases B and C. Thus, for example, the light red, light green, and light blue waveforms have net impulses that are more negative than the corresponding red, green, and blue waveforms in phases B and C, while the dark cyan, dark magenta, and dark yellow waveforms have net impulses that are more positive than the corresponding cyan, magenta, and yellow waveforms in phases B and C. The variation in net impulse can be achieved by varying the length of the pulses, the number of pulses or the size of the pulses in phases B and C.

The grey color is typically achieved by a sequence of pulses oscillating between a low voltage or an intermediate voltage.

It will be clear to those of ordinary skill in the art that in displays of the present invention driven using Thin Film Transistor (TFT) arrays, the available time increments on the abscissa of fig. 8 will typically be quantified by the frame rate of the display. Also, it is clear that the display is addressed by varying the potential of the pixel electrode relative to the front electrode, and this can be achieved by varying the potential of the pixel electrode or the front electrode or both. In the prior art, there is typically a matrix of pixel electrodes on the backplane, with the front electrode being common to all pixels. Thus, when the potential of the front electrode is changed, the addressing of all pixels is affected. The basic structure of the waveform described above with reference to fig. 8 is the same regardless of whether a varying voltage is applied to the front electrode.

The general waveform shown in fig. 8 requires that the drive electronics provide up to seven different voltages to the data lines during the updating of the selected row of the display. While it is possible to provide a multi-level source driver capable of providing seven different voltages, many commercially available source drivers for electrophoretic displays only allow three different voltages (typically positive, zero and negative) to be delivered during a single frame. Here, the term "frame" refers to a single update of all rows in the display. The general waveform of fig. 8 can be modified to accommodate a three-level source driver architecture, provided that the three voltages (typically + V, 0, and-V) provided to the panel can be changed from one frame to the next (i.e., so that, for example, in frame n, the voltages (+ Vmax, 0, -Vmin) can be provided, and in frame n +1, the voltages (+ Vmid, 0, -Vmax) can be provided).

Since variations in the voltage supplied to the source driver affect each pixel, the waveforms need to be modified accordingly so that the waveforms used to produce each color must be aligned with the supplied voltage. Figure 9 shows a suitable modification to the general waveform of figure 8. In phase A, no change is required, as only three voltages (+ Vmax, 0, -Vmax) are required. Stage B consists of sub-stages B1 and B2 generations of length defined as L1 and L2, respectivelyInstead, a specific set of three voltages is used in each sub-phase. In FIG. 9, in phase B1, voltages + Vmax, 0, -Vmax are available, and in phase B2, voltages + Vmid, 0, -Vmid are available. As shown in FIG. 9, the waveform requires a duration t in sub-phase B1₅Is measured at a pulse of + Vmax. Sub-phase B1 vs. time t₅Longer (e.g., to accommodate may need to be longer than t)₅The waveform of the other color of the longer pulse), and thus for time L₁-t₅A zero voltage is provided. Length t within sub-phase B1₅And zero pulse or length L₁-t₅May be adjusted as desired (i.e., sub-phase B1 need not be of length t as shown in the figure)₅The start of a pulse). By subdividing phases B and C into sub-phases, where one of the three positive voltages, one of the three negative voltages and zero can be selected, the same optical result can be obtained as obtained using a multi-level source driver, albeit at the expense of a longer waveform (to accommodate the necessary zero pulse).

It may sometimes be necessary to control an electrophoretic display using a so-called "top-plane-switching" driving scheme. In a top-plane switching drive scheme, the top-plane common electrode may be switched between-V, 0 and + V, while the voltage applied to the pixel electrode may also vary between-V, 0 to + V, with pixel transitions in one direction being processed when the common electrode is at 0 and transitions in the other direction being processed when the common electrode is at + V.

When top plane switching is used in conjunction with a three-level source driver, the same general principles apply as described above with reference to fig. 9. Top plane switching may be preferred when the source driver cannot provide as high a voltage as the preferred Vmax. Methods of driving an electrophoretic display using top-plane switching are well known in the art.

Typical waveforms for the second drive scheme according to the present invention are shown in table 3 below, where the numbers in parentheses correspond to the number of frames driven with the indicated backplane voltage (relative to the top plane assumed to be at zero potential).

TABLE 3

In the reset phase pulses of maximum negative and positive voltages are provided to erase the previous state of the display. The number of frames at each voltage is offset by an amount (expressed as Δ for color x)_x) Which compensates for the net impulse in the high/medium voltage and low/medium voltage phases, where the color is rendered. To achieve DC balance, Δ_xIs chosen to be half the net impulse. The reset phase need not be implemented exactly in the manner shown in the table; for example, when using top plane switching, a certain number of frames must be allocated to the negative and positive drivers. In this case, it is preferable to provide the maximum number of high voltage pulses consistent with achieving DC balance (i.e., subtracting 2 Δ from either the negative or positive frames as appropriate)_x)。

In the high/medium voltage phase, a sequence of N repetitions of a pulse sequence suitable for each color is provided, as described above, where N may be 1-20. As shown, the sequence includes 14 frames that are assigned a positive or negative voltage of Vmax or Vmid, or zero. The pulse sequence shown is consistent with the discussion given above. It can be seen that the pulse sequences rendering white, blue and cyan are the same in this phase of the waveform (since in this case blue and cyan are achieved starting from the white state, as described above). Also in this phase, the pulse sequence rendering yellow and green is the same (since green is achieved starting from the yellow state, as described above).

In the low/medium voltage phase, blue and cyan are obtained from white, and green is obtained from yellow.

The preceding discussion of the waveforms shown in fig. 5-9, and particularly the discussion of DC balancing, neglects the problem of trip voltage. In effect, as previously described, the voltage offset provided by each backplane voltage from the power supply is equal to the trip voltage V_KBThe amount of (c). Thus, if the power supply used provides three voltages + V, 0 and-V, the backplane will actually receive the voltage V + V_KB、V_KBand-V + V_KB(Note that in the case of amorphous silicon TFT，V_KBUsually a negative number). However, the same power supply will provide + V, 0 and-V to the front electrode without any jump voltage offset. Thus, for example, when-V is supplied to the front electrode, the display will experience 2V + V_KBMaximum voltage sum V of_KBThe minimum voltage of (c). Instead of using a separate power supply to supply V to the front electrode_KB(which can be expensive and inconvenient), the waveform can be divided into positive, negative and V supplies to the front electrode_KBOf (2) a section of (a).

As described above, in some of the waveforms described in the above application of serial No. 14/849,658, seven different voltages may be applied to the pixel electrodes: three positive voltages, three negative voltages and zero; as shown in the discussion of fig. 8 and 9 above. Preferably, the maximum voltage used in these waveforms is higher than what the prior art amorphous silicon thin film transistors can handle. In this case, a high voltage can be obtained by using top-plane switching, and the drive waveform can be configured to compensate for the kickback voltage and DC-balanced inherently by the method of the present invention. One such waveform for displaying a single color is schematically depicted in fig. 11. As shown in fig. 11, the waveform of each color has the same basic form: that is, the waveform is inherently DC balanced and may include two sections or stages: (1) an initial series of frames for providing a "reset" of the display for a state from which any color can be reproducibly obtained and during which a DC imbalance equal and opposite to that of the remainder of the waveform is provided, and (2) a series of frames specific to the color to be rendered; see, sections a and B of the waveform shown in fig. 8.

During the first "reset" phase, the reset of the display desirably erases any memory of the previous state, including the remnant voltage and pigment configuration specific to the previously displayed color. This erase is most effective when addressing the display with the maximum possible voltage in the "reset/DC balance" phase. In addition, enough frames can be allocated in this stage to allow balancing the most unbalanced color transitions. Since some colors require positive DC balance in the second section of the waveform and others require negative levelBalance, so that the front electrode voltage V is about half the frame of the "reset/DC balance" phase_comIs set to V_pH (allowing the maximum possible negative voltage between the back plate and the front electrode), and in the rest, V_comIs set to V_nH (allowing the maximum possible positive voltage between the back plate and the front electrode). Empirically, it has been found that preference is given to V_com＝V_pH frame leading V_com＝V_nAnd H frames.

The "desired" waveform (i.e., the actual voltage versus time curve desired to be applied to the electrophoretic medium) is shown at the bottom of FIG. 11, and its implementation with top-plane switching is shown above, where application to the front electrode (V) is shown_com) And the potential of the Back Plate (BP). Assume that a five-level column driver is used connected to a power supply capable of providing the following voltages: v_pH、V_nH (highest positive and negative voltage, usually in the range of + -10-15V), V_pL、V_nL (lower positive and negative voltages, typically in the range of + -1-10V) and zero. In addition to these voltages, a jump voltage V can be supplied to the front electrode by an additional power supply_KB(small values specific to the particular backing sheet used, for example as measured as described in U.S. patent No.7,034,783).

As shown in FIG. 11, each backplane voltage is offset by a voltage V from the power supply_KB(shown as a negative number) and the front electrode voltage is not so shifted unless the front electrode is explicitly set to V as described above_KB。

DC balancing can be achieved by:

it is assumed that the color transition of the waveform (the second section or portion or phase as described above, without the reset/DC balance section or portion or phase) has n frames. So that

Is the total impulse of the color transition segment caused by the transition voltage, whereinIs the voltage on the back plate, anIs the front electrode voltage at frame i. The overall impulse of the "reset" phase should be-I_uTo maintain overall DC balance of the entire waveform.

The impulse offset σ can now be chosen, which will be a deviation from the DC balance, so a value of 0 corresponds to an accurate DC balance. The reset duration d may also be selected_r(the overall duration of the reset phase) and two reset voltages of opposite sign, given by:

see fig. 12.

Then, d₁And d₂The duration of the reset phase shown in fig. 12, can be determined by the following equation:

d₂＝d_r-d₁

subsequently, the parameter d may be calculated_2zWhich specifies the period V in the latter half of the reset_B＝V_COMOf a duration such that

Note that 0 ≦ d is required_2z≤d₂. Reset duration d_rAnd a reset voltage V₁、V₂Must be large enough to allow for updatesTotal impulse of (3). If d is_2zBeyond this constraint, it can simply be set to the nearest boundary. For example, if d_2z<0, it is set to 0 if d_2z>d₂Then set it to d₂. In this case, the resulting balance/reset will not effectively DC balance the update, but will be as close as possible within a given voltage/duration of reset.

Once d is calculated_2zThe calculation of the remaining balance parameters can be done such that:

d_1s＝d₁-d_1p

d_2p＝d₂-d_2s

once these parameters are calculated, an updated reset/balance portion is created as shown in fig. 12. V_comIn thatDriven duration d₁Followed byDriven duration d₂. The back plate is arranged atDriven duration d_1pAnd then driven at 0 for a duration d_1zThen is atDriven duration d_2pAnd finally driven at 0 for a duration d_2z。

In some embodiments, a "zero" voltage V for the reset phase may be calculated_jz(i.e. the actual voltage across the electrophoretic layer when the front and back electrodes are nominally at the same voltage) such that:

whereinIs the backplane voltage during the "zero" portion of the reset phase and should be selected to minimize the following voltages:

now also the duration (d) of the sub-phases of the reset phase can be calculated_1p，d_1z)、(d_2p，d_2z) So that each pulse is divided between a drive and a zero sub-phase, wherein

d_2p＝d₂-d_2z

d_1z＝d₁-d_1p

Wherein the content of the first and second substances,

γ＝σ-I_u-V_KBd_r-V_1zd₁-V_2pd₂

note that if the updated impulse is large enough to make d large_2pWill fall within the range [0, d₂]Beyond that, the transition will not be DC balanced, but will be as close as possible within the voltage/duration of the first phase.

Once d is calculated_1p、d_1z、d_2pAnd d_2zIs calculated to thereby calculate d₁And d₂Value of (2), front electrodeIs driven under the following conditions (see FIG. 12)

1.Duration d₁Wherein

2.Duration d₂Wherein

And the back plate is driven under the following conditions:

1.duration d_1pWherein

2.Duration d_1zWherein

3.Duration d_2pWherein

4.Duration d_2zWherein

As described above, the backplane is addressed by scanning the gate lines (rows) during each frame. Thus, each row is refreshed at a slightly different time. However, when using top plane switching, V occurs at a particular time_comReset to a different voltage. At the occurrence of V_comDuring the switched frame, all but one row experiences a slightly incorrect impulse, as shown in fig. 13.

As described above, the backplane is addressed by scanning the gate lines (rows) during each frame. Thus, each row is refreshed at a slightly different time. However, when using top plane switching, V occurs at a particular time_comReset to a different voltage. At the occurrence of V_comDuring the switched frame, all but one row experiences a slightly incorrect impulse, as shown in fig. 13.

FIG. 13 shows V for three frames_comFrom V_KBAdjusted to a negative voltage and then to a positive voltage for three frames, back to V_KBThe case (1). It is desirable to maintain an approximately zero potential throughout the series of transitions. Suppose V_comOccurs at the beginning of the frame (i.e., at backplane row 1, BP)₁At (c). As mentioned above, at V_comIs not set to V_KBThe potential difference across the display is V over time_KB. Reach row BP on scanning backplane_XPreviously, the top plane switches a bit. Thus, for a period of time that may be nearly as long as one frame, some rows of the image may receive impulse offsets from where they are expected. However, it can be seen that with V_comThe settings are again adjusted and the compensation offset occurs in the following frame. Thus, the scanning of the backplane does not affect the net DC balance achieved by the present invention.

At first glance, it appears that the sequential scanning of the rows of an active matrix display may disturb the above-described calculations designed to ensure accurate DC balance of the waveforms and the drive scheme, because as the voltage of the current electrode changes (typically between successive scans of the active matrix), each pixel of the display will experience an "incorrect" voltage until the scan reaches the relevant pixel and the voltage on its pixel electrode is adjusted to compensate for the change in the voltage of the front electrode, and the period of time between the change in the front plane voltage and the time at which the scan reaches the relevant pixel varies according to the row in which the relevant pixel is located. However, further studies will show that the actual "error" in the impulse applied to a pixel is proportional to the change in the front plane voltage times the period between the front plane voltage change and the time at which the scan reaches the relevant pixel. Assuming no change in scan rate, the latter period is fixed, so for any series of changes in the front plane voltage that make the final front plane voltage equal to the initial front plane voltage, the sum of the "errors" in the impulse will be zero and the overall DC balance of the drive scheme will not be affected.

Claims (16)

1. A method for driving an electro-optic display having a front electrode, a backplane, and a display medium between the front electrode and the backplane, the method comprising:

applying a first drive phase to the display medium, the first drive phase having a first signal and a second signal, the first signal having a first polarity, a first amplitude as a function of time, and a first duration, the second signal following the first signal and having a second polarity opposite the first polarity, a second amplitude as a function of time, and a second duration, such that a sum of the first amplitude as a function of time integrated over the first duration and the second amplitude as a function of time integrated over the second duration produces a first impulse offset;

and applying a second drive phase to the display medium, the second drive phase producing a second impulse offset;

wherein the first duration is determined by a ratio between an amount of the second impulse offset and an amplitude difference between the first amplitude and the second amplitude; and

wherein a sum of the first impulse offset and the second impulse offset is zero.

2. The method of claim 1, wherein the first polarity is a negative voltage and the second polarity is a positive voltage.

3. The method of claim 1, wherein the first polarity is a positive voltage and the second polarity is a negative voltage.

4. The method of claim 1, wherein a duration of the first drive phase is different from a duration of the second drive phase.

5. The method of claim 1, wherein the display medium is an electrophoretic medium.

6. The method of claim 5, wherein the display medium is an encapsulated electrophoretic display medium.

7. The method of claim 5, wherein the electrophoretic display medium comprises an electrophoretic medium comprising a liquid and at least one particle disposed within the liquid and capable of moving therethrough upon application of an electric field to the medium.

8. A method for driving an electro-optic display having a front electrode, a backplane, and a display medium between the front electrode and the backplane, the method comprising:

applying a reset phase and a color transition phase to the display, the reset phase comprising:

applying a first signal having a first polarity, a first amplitude as a function of time, and a first duration on the front electrode;

applying a second signal having a second polarity opposite the first polarity, a second amplitude as a function of time, and a second duration over the backplane during the first duration;

applying a third signal having the second polarity, a third amplitude as a function of time, and a third duration on the front electrode after the first duration;

applying a fourth signal having the first polarity, a fourth amplitude as a function of time, and a fourth duration after the second duration on the backplane;

wherein the sum of a first amplitude as a function of time integrated over the first duration and a second amplitude as a function of time integrated over the second duration and a third amplitude as a function of time integrated over the third duration and a fourth amplitude as a function of time integrated over the fourth duration produces an impulse offset designed to maintain DC balance on the display medium over the reset phase and the color transition phase.

9. The method of claim 8, wherein the reset phase erases previous optical properties rendered on the display.

10. The method of claim 8, wherein the color transition phase substantially changes an optical property displayed by the display.

11. The method of claim 8, wherein the first polarity is a negative voltage.

12. The method of claim 8, wherein the first polarity is a positive voltage.

13. The method of claim 8, wherein the impulse offset is proportional to a kickback voltage experienced by the display medium.

14. The method of claim 8, wherein the first duration and the second duration begin simultaneously.

15. The method of claim 8, wherein the fourth duration occurs during the third duration.

16. The method of claim 15, wherein the third duration and the fourth duration begin simultaneously.