WO2006006121A1

WO2006006121A1 - Display device

Info

Publication number: WO2006006121A1
Application number: PCT/IB2005/052220
Authority: WO
Inventors: Theodorus J. P. Van Den Biggelaar; Roy Van Dijk; Hubertus W. J. J. Van De Laar
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2004-07-08
Filing date: 2005-07-04
Publication date: 2006-01-19
Anticipated expiration: 2007-01-08

Abstract

A display based on the Roll Blind principle having a driver for selecting a set of picture elements by supplying patterns of selection voltages which display driver comprises two switches (15, 26, 27) in series between a first voltage (VCC1) and a second voltage (VCC2) at the common point of which a part of the voltages of the patterns of selection voltages is supplied by the driver, a third voltage (VCC3) being applicable to said common point, via a further switch (15) or by means of a resistor (16) for providing a further selection voltage.

Description

Display device

The invention relates to a display device having picture elements, each of the picture elements comprising on a substrate, a first electrode, a dielectric layer between said first electrode and a second electrode, the second electrode being movable in response to an electric field between a first position corresponding to an edge region of the picture element and a second position in which the second electrode at least partly covers the further surface region of the picture element.

Wherever in this Patent Application reference is made to a picture element (pixel) it may either be a full picture element or a sub-pixel such as the red, green or blue sub-pixel in a picture element. Wherever in this Patent Application reference is made to a dielectric layer a layer is meant having such a high resistance that the mobility of the second electrode to move between the two positions, which positions, in the case of a display device, are related to electro-optical states of the display device (fully transmissive, fully reflecting or fully opaque (back)) is not influenced. The invention also relates to a display driver for driving such a display device.

The display device can be used, dependent on the pixel size in micro-projector applications, large screen applications such as wallpaper but also in window applications.

A display device of the kind mentioned above is known from e.g. USP

5,519,565. The second electrode here is rollable in response to an electric field between a first position in which the rolled electrode is present at the edge region of the picture element and a second position in which the second electrode is unrolled and covers the further surface region of the picture element. One of the problems encountered in driving such a display is that due to the bistability (the difference between the voltages for switching on and off respectively) in such display pixels that are switched on will stay on and all pixel that are switched off stay switched off unless addressed a second time. With currently available display drivers providing voltages in the order of 10 V to 200 V, this can only be obtained with high voltage drivers leading to high driver costs.

The invention has as its purpose to overcome at least partly the above- mentioned problems. To this end in a display device according to the invention has a display driver for selecting a set of picture elements by supplying patterns of selection voltages which display driver comprises two switches in series between a first voltage and a second voltage at the common point of which a part of the voltages of the patterns of selection voltages is supplied by the driver, a third voltage being applicable to said common point for providing a further selection voltage.

By introducing a further selection voltage, being applicable to said common point a high- ohmic current path or even an open path is introduced during non-selection, leading to substantially no dissipation. During the non-selection, in the embodiment using a high- ohmic current path the row is connected to the hold voltage through a resistor. Since the display element is capacitive there is no power dissipation. The hold voltage keeps the pixel elements in its bistable area and the pixels will remain in their states.

In a first embodiment of the invention the further switch is provided between the common point for providing a further selection voltage and the third voltage. The driver now does not need external resistors. A further device according to the invention has a resistor provided between the common point for providing a further selection voltage and the third voltage, the driver comprising means to bring the two switches in series simultaneously in an open state. This device can easily be realized by modifying existing, commercially available, two-level drivers such as PDP - drivers into three-level drivers. A driver according to the invention can also be used in other (high-) voltage displays such as foil-displays and other mainly capacitive display elements.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings:

Figure 1 schematically shows a part of a device according to the invention, Figure 2 shows a plan view of the part of the device of Figure 1, Figure 3 shows transmission voltage characteristics of the device of Figure 1, Figure 4 schematically shows a matrix of display elements, Figure 5 show driving forms for the device of Figure 4 Figure 6 shows an embodiment of a device according to the invention, Figure 7 shows driving forms for the device of Figure 6, Figure 8 shows a further embodiment of a device according to the invention, while

Figure 9 shows driving forms for the device of Figure 8 and Figure 10 shows a possible realization of a driver according to the invention. The Figures are diagrammatic and not to scale; corresponding components are generally denoted by the same reference numerals.

Figures 1 and 2 schematically show a part of a device 1 according to the invention, in this particular embodiment a transparent display device. A transparent substrate 2 is covered with transparent first electrodes 3 e.g. ITO -electrodes. The electrodes 3 are covered with a thin dielectric layer 4. A foil 6, which is covered with a conductive electrode part 5, forms together with said conductive electrode part 5 a second, reliable electrode. The thin dielectric layer 4 electrically isolates the electrodes 3 (e.g. parts of column electrodes) from the reliable electrode parts 5 (e.g. parts of column electrodes). Figure 1 shows three (sub) picture elements, two of which are in an open, transparent state (reliable electrode 5, 6 rolled up to a first position), the other one being in a closed, opaque (black) state (reliable electrode 5, 6 unrolled to a second position).

In this example the foil 6 is glued to the dielectric layer 4 on one side part 7 of every picture element. The reliable electrode 5, 6 are switchable between a transmissive (open) state and an opaque (closed) state, e.g. by choosing aluminum for the electrode parts 5. The device 1 further comprises e.g. driving means and for example a backlighting system. On the other hand, in a reflective device the foil 6 or the Tollable electrode 5, 6 may be covered with a white layer to reflect the ambient light, while the substrate now is opaque by covering it with an opaque layer at one of its sides. A further possibility is making the substrate 2 reflective and the reliable electrode 5, 6 black.

It is assumed that in the device 1 three (or four) forces determine the switching behavior, an elastic force, an electrostatic force, and the "van der Waals" force and to a minor extend the gravitational force force and residual electrostatic force (due to charging). The elastic force is the force present in the Tollable electrode 5, 6 and is the result of e.g. shrinkage during manufacturing and this force is directed at rolling up the rollable electrode 5, 6 of a (sub) picture element or (sub) pixel. The electrostatic force is the attractive force between the conductive electrode part 5 and the (ITO) on the substrate by applying a voltage. The "van der Waals" force is the force between the (sub) pixel foil 6 and the substrate 2. This force depends on the distance between the two media, the roughness of the media and the material properties. The smaller the distance is, the larger the "van der Waals" force is. The electrostatic force depends strongly on the distance, the surface area, dielectric constant of the materials and the voltage difference between the foil and the substrate. The gravitational force acts upon the rolled up electrode 5, 6 which also depends on the orientation of this foil. It is very thin and has therefore a very low mass, so it is probably negligible.

The elastic force is directed at rolling up the rollable electrode 5, 6 , while the electrostatic force and the "van der Waals" force are directed at keeping the rollable electrode 5, 6 closed. To keep a picture element open (the left two picture elements in Figures 1, 2, the elastic force must be larger than the "van der Waals" force, since the picture element (pixel) in the device 1 is open if no or little electrostatic force is present. When a picture element or pixel is closed, the "van der Waals" force and the electrostatic force keep it closed, whereas the elastic force wants to open it.

If no voltage is applied the rollable electrode 5, 6 is in a rolled up state, giving a transparent picture element in transmissive mode or a dark pixel in reflective mode. When applying a certain voltage V₂, in matrix-display devices the difference between the column voltages and the row voltages, the electrostatic forces rolls down the rollable electrode 5, 6 on to the substrate 2, covering the pixel area and creating a dark pixel in transmissive mode or a white pixel in reflective mode.

This switching behavior is shown by means of the transmission voltage characteristic of Figure 3, which shows the transmission T of the device of Figure 1 as a function of the voltage V. At a first threshold voltage Vi (which may be presented as a voltage difference between a row 11 and column 12 in a matrix display 1, see Figure 4) a picture element is opened (the rollable electrode 5, 6 rolls up), if it was not open already. At the second threshold voltage, V₂ a pixel is closed (the rollable electrode 5, 6 becomes flattened), if it was not already closed. The polarity of the voltages is not important, only the absolute value is important. In between these values a pixel that was open, will remain open and a pixel that was closed will remain closed. The threshold voltages are determined by the material parameters, i.e. the elastic forces, thickness of the foil, material properties, and surface properties, etcetera. The (matrix) display 1 of Figure 4, having n rows and m columns is driven by a (schematically shown) row driver 13 and column driver 14, having picture element at the crossings of rows and columns. The row driver 13 selects rows 11, having row numbers j (j = 1...m), while the column driver 13 selects columns 12, having column numbers i (i = 1...n). Selection is not necessarily sequential.

Figure 5 schematically shows driving forms for the device of Figure 4 during four successive row times selecting rows j, j+1, j+2 and j+3. When a row is not being addressed, the voltage of such a row is for example OV, indicated in Figure 5 for row j+3 as Level 3. In that case the column voltages must be between |V1| and |V2|, so that the voltage difference between a non-selected row and a column is larger than |V1| and smaller than |V2|, so all picture elements remain in their state as defined by a previous selection.

When a row is being addressed or selected, all the pixels of such a row are first brought in one state either on or off. A row that is being addressed is first brought to a low voltage (for instance a negative voltage, indicated in Figure 5 for row j+3 as Level 1), so that the voltage difference between the columns and the selected row is larger than |V2|. This results that all pixels of the selected row close. The rows of the unselected rows are at OV (Level 3). This means that the column voltages must be larger than |V1| and smaller than |V2|, since the pixels of the unselected rows mustn't change state.

Then the row is brought at a higher voltage, indicated in Figure 5 for row j+3 as Level 2. Simultaneously, the column voltages are switched; to a(n absolute) voltage above I Vl I for pixels that must be opened, and to a(n absolute) voltage below |V2| for pixels that must remain closed. As a consequence the voltage difference between the selected row and column voltage | Vl] is smaller than |V1], thus a pixel will open. The voltage difference between the selected row and column voltage |V2| is larger than |V1| and smaller than |V2|, thus a pixel will remain at its state, in this case closed. After that the selected row is brought to its unselected row value and a next row can be addressed.

In the alternative way of driving, all pixels are opened first, so the foils are rolled up first. A row that is being addressed is first brought to a high voltage, so that the voltage difference between the columns and the selected row is smaller than |V1|. This results that all pixels of the selected row open up. The rows of the unselected rows are at ground level. This means that the column voltages must be larger than |V1| and smaller than |V2|, since the pixels of the unselected rows mustn't change state. The voltage of the columns can either be put in a common state, thus at the same voltage, for all columns as also has been discussed in the first situation. Then the row is brought at a lower voltage. Simultaneously, the column voltages are switched to a(n absolute) voltage above |V1| for pixels that must remain open, and to a(n absolute) below |V2| for pixels that must be closed. The voltage difference between the selected row and column voltage |V2| is larger than |V2|, so a pixel will open. The voltage difference between the selected row and column voltage |V2| is larger than |V1| and smaller than |V2|, thus a pixel will remain at its state, in this case closed. Then the selected row is brought to its unselected row value (Level 1) and a next row can be addressed.

The choice either to close all pixels first of an addressed row or first open all pixels in the selected row can depend on whether a panel is driven in reflective or transmissive mode. For reducing the charging up of the panel, all the voltages can also be driven to their opposite values at regular or irregular intervals, for instance every second frame.

According to the invention the three levels, indicated in Figure 5 for row j+3 as Levels 1,2,3 can be obtained by the embodiment of Figure 6, using three switches 15

(Switches 1, 2 and 3). The output is switched between VCCl, VCC2 and VCC3 as shown in the timing diagram of Figure 7. Every time only one switch is turned on. When all three the switches are turned off the output is placed in a high-impedance state, see Figure 7.

Figure 8 shows another embodiment in which an external resistor 16 is added to a configuration of two switches (Switch circuit) and is applied with an external power signal (VCC3). With this external resistor a three level driver can be created using a commercially available two level driver (e.g. for driving plasma displays). The switches 1 and 2 offer the possibility of switching the output between VCCl and VCC2; when both switches are turned off the output is equal to VCC3 . A possible timing diagram to realize this is shown in Figure 9. Only one switch is turned on at one time. The resistor only dissipates during the time that one of the switches is activated. This is only done for one row at a time (during row selection). The resistor value can be adapted but a high resistance value is preferably chosen to keep dissipation as low as possible such as 1 kOmn - 100 MOhm. This is possible since the switching speed can be low Figure 10 shows a schematical realization a part of a driver using this principle. Information 20 to be displayed is stored into a latch 22 via a shift register 21. The switches of Figure 8 have been realized as FETs 26 and 27, which are enabled by logical NAND - circuits 24 and 25 respectively. For selecting a row first FET circuit 26 is activated to switch the row to VCCl during a first phase. Then the second FET circuit 27 is activated to switch the row to VCC2 during the next phase. Then the FET circuits are deactivated via signal HZ and the high ohmic external resistor 16 pulls the row to VDD3.

The invention is not restricted to the embodiments shown. The reliable electrodes 5, 6 have two sides. In a reflective display device the top side is reflective (white, red, green or blue). The bottom side is black. When a pixel is open (Tollable electrodes 5, 6 unrolled) the topside is shown over the full pixel (except for the black matrix, which is obtained by using said black substrate). When a pixel is closed, the pixel rolls up and shows the bottom side of the roll and simultaneously, the black substrate is shown. As mentioned in the introduction a driver according to the invention can also be used in other high-voltage displays such as foil-displays.

The invention resides in each and every novel characteristic feature and each and every combination of features. Reference numerals in the claims do not limit the protective scope of these claims. The use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. The use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A display device (1) having picture elements, each of the picture elements comprising on a substrate (2) a) a first electrode (3) b) a dielectric layer (4) between said first electrode and c) a second electrode (5), the second electrode being movable in response to an electric field between a first position corresponding to an edge region of the picture element and a second position in which the second electrode at least partly covers the further surface region of the picture element which display device has a driver for selecting a set of picture elements by supplying patterns of selection voltages which driver comprises two switches in series between a first voltage and a second voltage at the common point of which a part of the voltages of the patterns of selection voltages is supplied by the driver, a third voltage being applicable to said common point for providing a further selection voltage.

2. A display device according to Claim 1, in which a further switch is provided between the common point for providing a further selection voltage and the third voltage.

3. A display device according to Claim 1 in which a resistor is provided between the common point for providing a further selection voltage and the third voltage, the driver comprising means to bring the two switches in series simultaneously in an open state

4. A display device according to Claims 1 to 3 in which the switches are field effect transistors.

5. A display driver for driving a display device having hysteresis in its picture elements driving characteristics comprising two switches in series between a first voltage and a second voltage at the common point of which a part of the voltages of the patterns of selection voltages is supplied by the driver, said common point for providing a further selection voltage being connectable to a third voltage.

6. A display driver according to Claim 5, in which a further switch is provided between the common point for providing a further selection voltage and the third voltage.

7. A display driver according to Claim 5 in which a resistor is provided between the common point for providing a further selection voltage and the third voltage, the driver comprising means to bring the two switches in series simultaneously in an open state

8. A display driver according to Claims 5 to 7 in which the switches are field effect transistors.