HK1081712A - Display driver circuits for electroluminescent displays, using constant current generators - Google Patents

Description

Display driver circuit for electroluminescent display using constant current generator

Technical Field

The present invention relates generally to display driver circuits for electro-optic displays, and more particularly to circuits and methods for more efficiently driving organic light emitting diode displays, particularly passive matrix displays.

Background

Organic Light Emitting Diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, multi-colored, fast-switching, provide a wide viewing angle, and are easy and inexpensive to manufacture on a variety of substrates. Organic LEDs (light emitting diodes) can be fabricated with polymers or small molecules in a range of colors (or in multi-color displays), depending on the materials used. Examples of polymer-based organic LEDs are described in WO90/13148, WO 95/06400 and WO 99/48160; an example of a so-called small molecule based device is described in US 4,539,507.

The basic structure 100 of a typical organic LED is shown in fig. 1 a. A glass or plastic substrate 102 supports a transparent anode layer 104, the anode layer 104 comprising, for example, Indium Tin Oxide (ITO), on which ITO layer 104 a hole transport layer 106, an electroluminescent layer 108 and a cathode 110 are deposited. The electroluminescent layer 108 may comprise, for example, PPV (poly p-phenylene vinylene) and the hole transport layer 106, which helps to match the hole levels of the anode layer 104 and the electroluminescent layer 108, may comprise, for example, PEDOT: PSS (poly ethylenedioxythiophene doped with poly p-styrene sulfonic acid). Cathode layer 110 typically comprises a low work function metal (function metal), such as calcium, and may include additional layers, such as an aluminum layer, directly adjacent electroluminescent layer 108 for improved electron energy level matching. Contact wires 114 and 116 to the anode and cathode, respectively, provide connections to a power supply 118. The same basic structure can also be used for small molecule devices.

In the example shown in fig. 1a, light is emitted through the transparent anode 104 and the substrate 102, and this device is referred to as a "bottom emitter". Devices that emit light through a cathode may also be constructed, for example, by keeping the thickness of the cathode layer 110 less than about 50-100nm so that the cathode is substantially transparent.

Organic LEDs may be deposited on a substrate in the form of a matrix of pixels to form a single or multi-colour pixellated display. A multicolor display may be constructed using groups of red, green, and blue emitting pixels. In such displays, the individual elements are typically accessed by actuating row (or column) lines to select pixels, and writing rows (or columns) of pixels to generate the display. So-called active matrix displays have a storage element, typically a storage capacitor and a transistor, associated with each pixel, whereas passive matrix displays have no such storage element and are scanned repeatedly, somewhat like a Television (TV) picture, to give the impression of a stable image.

Fig. 1b shows a cross-section through a passive matrix OLED display 150, where elements identical to those of fig. 1a are denoted by the same reference numerals. In a passive matrix display 150, the electroluminescent layer 108 comprises a plurality of pixels 152 and the cathode layer 110 comprises a plurality of electrically insulated wires 154, the wires 154 extending into the page in figure 1b, and each wire 154 having an associated contact 156. Similarly, the ITO anode layer 104 also includes a plurality of anode lines 158, only one of which is shown in fig. 1b, extending in a direction orthogonal to the cathode lines. A contact (not shown in fig. 1 b) is also provided for each anode line. The electroluminescent pixel 152 at the intersection of the anode and cathode lines is accessed by applying a voltage between the associated anode and cathode lines.

Referring now to fig. 2a, fig. 2a conceptually shows a drive arrangement for a passive matrix OLED display 150 of the type shown in fig. 1 b. A plurality of constant current generators 200 are provided, each connected to a power supply line 202 and to one of a plurality of column lines 204, of which only one column line 204 is shown in fig. 2a for clarity. A plurality of row lines 206 (only one of which is shown in fig. 2 a) are also provided, and each of these row lines 206 may be selectively connected to a ground line 208 by a switch connection 210. As shown, in the presence of a positive supply voltage on line 202, column line 204 includes an anode connection 158 and row line 206 includes a cathode connection 154, although these connections would be reversed if the supply line 202 were negative with respect to the ground line 208.

The display pixels 212 are shown powered and thus illuminated. To generate an image, as each column line is sequentially activated, the connection 210 for a row is maintained until the entire row has been visited, then the next row is selected, and the process is repeated. Alternatively, a row may be selected and all columns written in parallel, i.e., a row selected, and current energized to each column line simultaneously to illuminate each pixel in a row simultaneously with a desired brightness. Although this latter arrangement requires more column drive circuitry, this arrangement is preferred because it allows each pixel to be refreshed more quickly. In a further alternative arrangement, each pixel in a column may be accessed in turn before the next column is accessed, although this arrangement is not preferred because of, inter alia, the effects of column capacitance (column capacitance) as discussed below. It will be appreciated that in the arrangement of figure 2a, the functions of the column driver circuitry and the row driver circuitry may be interchanged.

Current-controlled driving, rather than voltage-controlled driving, is typically provided to the OLED because the brightness of the OLED is determined by the current flowing through it, which determines the number of photons output by the OLED. In a voltage controlled arrangement, the brightness may vary across the area of the display with time, temperature and age, which makes it difficult to predict how bright a pixel will appear when the pixel is driven with a given voltage. In color displays, the accuracy of the color display is also affected.

Fig. 2b to 2d show the current drive 220 applied to the pixel, the voltage 222 across the pixel and the light output 224 from the pixel over time 226 when the pixel is accessed, respectively. The row containing the pixel is accessed and current is energized onto the column line for the pixel at the time indicated by dashed line 228. The column line (and pixel) has an associated capacitance whereby the voltage gradually rises to a maximum value 230. The pixel does not begin to emit light until a point 232 is reached, at which point 232 the voltage across the pixel is greater than the OLED diode drop. Similarly, when the drive current is turned off at time 234, the light output gradually decays as the column capacitance discharges. In the case of writing all pixels in a row at the same time, i.e. driving the columns in parallel, the time interval between times 228 and 234 corresponds to a line scan period.

For many applications it is desirable, but by no means essential, to be able to provide a display of the grey scale type, i.e. the apparent brightness of individual pixels in the display can be varied, rather than simply being set to on or off. Here, "gray scale" refers to such variable brightness display regardless of whether the pixel is white or colored.

A conventional method of changing the brightness of a pixel is to change the pixel on time using Pulse Width Modulation (PWM). In the case of fig. 2b above, the apparent pixel brightness can be varied by varying the percentage of the interval between times 228 and 234 that the drive current is applied. In the PWM mode, the pixels are either fully on or fully off, but the apparent brightness of the pixels varies due to the temporal integration of the observer's eyes.

Pulse width modulation schemes provide good linear luminance response, but to overcome the effects associated with delayed pixel turn-on, pulse width modulation schemes typically employ a pre-charge current pulse (not shown in fig. 2 b) at the leading edge 236 of the drive current waveform, and sometimes a discharge pulse at the trailing edge 238 of the waveform. As a result, in displays incorporating this type of brightness control, charging (discharging) the column capacitance can take up about half of the total power consumption. Other important factors that have been identified by the applicant as contributing to the power consumption of the display plus driver combination include dissipation within the OLED itself (a function of OLED efficiency), resistive losses in the row and column lines, and, importantly in practical circuits, the effect of limited current driver compliance (compliance), as described in more detail later.

Fig. 3 shows a schematic diagram 300 of a generic driver circuit for a passive matrix OLED display. Dashed line 302 represents an OLED display, comprising: n row lines 304, each row line 304 having a corresponding row electrode contact 306; and m column lines 308, the column lines 308 having a corresponding number of column electrode contacts 310. An OLED is connected between each pair of row and column lines, and in the arrangement shown, the anode of the OLED is connected to the column line. A y-driver 314 drives the column lines 308 with a constant current and an x-driver 316 drives the row lines 304 by selectively grounding the row lines 304. Both the y-driver 314 and the x-driver 316 are typically controlled by a processor 318. The power supply 320 supplies power to the circuitry, and in particular to the y-driver 314.

In US6,014,119, US6,201,520, US6,332,661, EP1,079,361 a and EP1,091,339A, specific examples of OLED display drivers are described; OLED display driver integrated circuits are also sold by the company Clare Micronix of Clare, Mass.A.MA, Beverly, U.S.A. The ClareMicronix driver provides current control drive and realizes gray scale by using a conventional PWM method; US6,014,119 describes a driver circuit in which pulse width modulation is used to control the brightness; US6,201,520 describes a driver circuit in which each column driver has a constant current generator for providing digital (on/off) pixel control; US6,332,661 describes a pixel driver circuit in which a reference current generator sets the current output of a constant current driver for a plurality of columns, but this arrangement is also not suitable for variable brightness displays; and EP1,079,361 a and EP1,091,339a both describe similar drivers for organic electroluminescent display elements in which voltage driving is employed rather than current driving.

In general, it is desirable to reduce the power consumption of the display plus driver combination, particularly while maintaining the ability to provide gray scale display. It is further desirable to reduce the maximum necessary supply voltage for the display plus driver combination.

In US6,323,849 and EP 0811866 a, prior art techniques for reducing power consumption of Liquid Crystal Displays (LCDs) are described. US6,323,849 describes an LCD display having a partial display mode in which a control circuit controls a display driver to switch off the part of the display where no useful information is displayed. When the LCD module is in a partial display mode, the line frequency can also be reduced while maintaining the same refresh rate, allowing a lower voltage to be used to generate the same amount of charge. However, the user must predetermine which part of the display is to be used, and thus additional control functions and software are often required in devices provided with such displays. EP 0811866 a describes a similar technique, albeit with a more flexible drive arrangement. In our co-pending uk patent application no 0209502.4, an improved reduced power consumption display driver is described which provides a more transparent user implementation.

US 4,823,121 describes an Electroluminescent (EL) panel drive system that detects the absence of a HIGH (HIGH) level signal representing the dot illumination of an EL panel in one line of image data, and in response thereto, prevents 4 circuits (a precharge circuit, a pull-up circuit, a write circuit, and a source circuit) from being activated. However, the power savings provided by this technique are specific to the drive arrangement for the electroluminescent panel type and are not easily generalized. Furthermore, the power savings are relatively modest.

Fig. 4a shows a typical light intensity-voltage curve 400 of an OLED, which curve 400 is non-linear as seen, and shows a dead zone corresponding to the turn-on voltage of the OLED (typically 1.5V-2V). It is desirable to operate an OLED display at a lower voltage rather than a higher voltage, as this increases the efficiency of the device (light output in terms of energy input) and reduces the degradation rate. The resistive losses are also reduced and, in the case of changing image data, the capacitive losses (which depend on the square of the voltage) are also reduced.

Fig. 4b shows a light intensity-current curve 402 for an OLED, the curve 402 being approximately linear compared to the curve 400.

Figure 4c diagrammatically shows a current driver 402 for one column line of a passive matrix OLED display such as the display 302 of figure 3. Typically, a plurality of such current drivers are provided in a column driver integrated circuit, such as y-driver 314 of figure 3, for driving a plurality of passive matrix display column electrodes.

A particularly advantageous form of current driver 402 is described in our co-pending uk patent application No.0126120.5 entitled Display driver circuits. The current driver 402 of fig. 4c summarizes the main features of the circuit and comprises a current driver block 406 incorporating a bipolar transistor 416, the emitter terminal of the transistor 416 being substantially directly connected to the current driver block at the supply voltage V_sThe lower power line 404. (this does not necessarily require that the emitter terminals are connected by the most direct route to the power supply lines or terminals for the drivers, but rather that there are preferably no intervening components between the emitter and the power rail, other than the inherent resistance of the lines or connections within the driver circuit). The column drive output 408 provides current drive to an OLED 412, which OLED 412 also has a ground connection 414, normally through a row driver MOS switch (not shown in fig. 4 c). A current control input 410 is provided to the current driver block 406 and for illustrative purposes the current control input 410 is shown connected to the base of a transistor 416, although in practice a current mirror arrangement is preferred. The signal on current control line 410 may comprise a voltage or current signal and is preferably provided by a digital to analogue converter (not shown in figure 4 c) for ease of connection.

The current source attempts to deliver a substantially constant current to the load to which it is connected, but it will be appreciated that when the output voltage of the current source reaches the supply voltage, a point will be reached at which this is no longer possible. The range of voltages over which the current source provides an approximately constant current to the load is referred to as the compliance of the current source. Because when V_s-V_oSmall, high compliance and vice versa, thus (V)_s-V_o) Can be characterized as compliance, wherein V_sIs the supply voltage, V_oEssentially the most current sourceA large output voltage. (for convenience, reference will be made to a current source in this document, but the current source may be replaced by a current sink).

The arrangement of fig. 4c is useful because the (optionally variable) current generator has a high compliance, i.e. a low V_s-V_oThe value is obtained. The lower the compliance of the current driver (i.e., V)_s-V_oThe larger) the greater the power loss due to limited drive compliance. The lower the driver circuit compliance, the greater the supply voltage supplied to the current driver in order to obtain the highest desired pixel brightness, and hence the greater the power dissipation. This is particularly the case where the pixel brightness is varied by varying the drive current, rather than by pulse width modulation for example.

As explained before, current control is preferred over voltage control for OLEDs because this helps to overcome the non-linearity of the light-voltage curve shown in fig. 4a, which is substantially linear. Fig. 4d shows a graph 420 of the current drawn from the power supply versus the power supply voltage for an organic LED display element driven by a controllable constant current source. The curve has an initial "dead" region in which substantially no current flows until the forward voltage is sufficient to turn on the OLED. The non-linear region 422 is then followed by a substantially flat curved portion 424 above the voltage indicated by dashed line 426, giving a generally 'S' -shaped curve. At the voltage indicated by dashed line 426, the supply voltage is sufficient to meet the compliance limit of the current source. In other words, the voltage indicated by dashed line 426 is the minimum supply voltage required to ensure that the constant current source operates well at the current it is controlled to provide.

It can be seen that in region 424 of the curve of graph 420, increasing the power supply output voltage simply increases the excess, wasted power dissipation, and therefore it is preferable to operate at or near the compliance limit indicated by dashed line 426 in order to minimize this wasted power. However, the supply voltage relative to this compliance limit depends on many factors, including display age, display temperature, and, in the case of variable current drive, the current being supplied by the constant current source. For example, in case the OLED is at a constant brightness (i.e. at a substantially constant drive current), the voltage across the OLED decreases when the temperature of the OLED increases, and vice versa. To this end, large overhead is typically built into the supply voltage to ensure that the combination of the display and its drivers can operate in accordance with the desired specification and across a range of temperatures. The consequence of this is that the driven display is likely to operate at an efficiency well below its maximum efficiency over many prescribed temperature ranges and/or when at a brightness less than maximum brightness.

Disclosure of Invention

The applicant has realised that with emissive display technology, and in particular with organic light emitting diode based displays, significant power savings can be achieved by reading the drive voltage of the display and controlling the power supply to a constant current driver for the display. Applicants have recognized that particularly large power savings can be obtained by controlling the power supply such that the constant current driver operates at or near its compliance limit.

According to a first aspect of the present invention there is provided a display driver control circuit for controlling a display driver for an electroluminescent display, the display comprising at least one electroluminescent display element, the driver comprising at least one substantially constant current generator for driving the display element, the control circuit comprising: a drive voltage sensor for sensing a voltage on a first line, a current in the first line being regulated by the constant current generator; and a voltage controller connected to the driving voltage sensor, the voltage controller for controlling a voltage of a power supply for the constant current generator in response to the readout voltage, and the voltage controller configured to control the power supply voltage to improve efficiency of the display driver.

The supply voltage of at least one constant current generator, which may be a current source or a current sink, is controlled in response to the voltage on the line in which the current is regulated by the constant current generator, thereby allowing the supply voltage to vary automatically with external factors such as temperature, age of the display and current drive variations, in order to achieve more efficient operation of the display driver and in particular to achieve reduced power consumption of the display plus driver combination at the same perceived brightness level. Thus, when the supply voltage is greater than the voltage required by the constant current generator, the supply voltage is reduced to provide its regulated current, and if sufficient, the supply voltage is preferably increased. The display driver control circuitry may be retro-fitted to existing display driver circuitry to improve its efficiency, in which case the drive voltage sensor may be arranged to detect an external drive line of the driver, but in other embodiments the control circuitry may be integrated with other parts of the driver circuitry and the first line may be an "internal" line of the driver. Similarly, the power supply may comprise part of the driver of the control circuit, or may be powered by a separate controllable module. The constant current generator may comprise a constant current generator that is adjustable or controllable, for example to provide a variable pixel brightness for a color, or the constant current generator may provide a substantially fixed current source or sink, for example in a display in which the pixel brightness is varied by Pulse Width Modulation (PWM), or in a display in which the pixel brightness is fixed.

Preferably, the voltage controller is configured to reduce the supply voltage to the constant current generator when a reduction in the constant current generator supply voltage will not substantially reduce the regulated current sourced or sunk by the current generator and/or when a reduction in the constant current generator supply voltage does not substantially change the perceived brightness of the display element driven by the constant current generator. Broadly speaking this corresponds to allowing the voltage controller to control the power supply to reduce the supply voltage to the constant current generator when the current generator is operating at or below its compliance limit. Preferably, the voltage controller is configured to control the supply voltage such that the constant current generator operates near the compliance limit. Generally, operating either slightly above or slightly below the compliance limit, which may not necessarily be a hard limit, will provide satisfactory results, and in some embodiments the supply voltage may be controlled by means of a feedback mechanism that allows or requires the supply voltage to be on either side of the compliance limit at times. However, the supply voltage is preferably controlled such that it is substantially maintained at a voltage which represents a close enough approximation to the compliance limit for the control circuit that any change in pixel brightness due to supply voltage control is difficult to discern by a viewer under normal operating conditions. Preferably, the control circuit includes means for determining such a compliance limit, which, as will be appreciated, does not have to exactly correspond to a compliance limit which may be referred to as an actual compliance limit, for example, determined by examining a graph such as that shown in figure 4d (which is idealized to some extent).

Preferably, the control circuit further comprises a supply voltage sensor for reading the supply voltage of the constant current generator; in an embodiment, the voltage on the output (44 sink) of the current generator and the voltage on the input of the power supply of the current generator may be read using the same sensor. The voltage controller may comprise means for determining the difference between the supply voltage and the drive voltage on the first line to facilitate determining whether the constant current generator is operating near its compliance limit. Although the control circuit may be used with a display driver having only a single constant current generator, it is advantageous that the display driver has a plurality of constant current generators for driving a corresponding plurality of display elements simultaneously, such as display elements in a row of a passive matrix display. The control circuit preferably determines a maximum voltage at the output of one of the constant current generators and controls the supply voltage in response to the maximum sensed voltage. In summary, the display element or pixel driven at this maximum voltage will be the least efficient pixel display element among those having the greatest brightness at any one time. In the case where the simultaneously driven display elements comprise display elements in a row of a pixellated display, the supply voltage may be controlled according to the maximum voltage of the current generator driving that row, effectively controlling the supply voltage row by row. Alternatively, as is usual for a pixellated passive matrix display, where the rows are driven sequentially, the maximum voltage may be that of all the rows of the display, which is the maximum voltage of the display frame, and the supply voltage may be controlled on a frame-by-frame basis. This option is available because pixellated passive matrix displays typically drive only one row at a time, although because of the rapidity of row refresh, it appears to provide a uniform display to the viewer. Therefore, the supply voltage can be lowered when this will not reduce the adjustment current or pixel brightness of the pixel having the highest drive voltage in the special row being driven. Thus, the supply voltage can be varied when each row of the display is driven according to the needs (i.e., brightness, efficiency, etc.) of the pixels in that particular row. It will be appreciated that this potentially provides improved power savings. Again, the supply voltage may be sensed and controlled in response to the difference between the supply voltage and the maximum determined drive line voltage, or in response to the minimum difference between the supply voltage and the drive line sense voltage, which are mathematically equivalent.

Preferably, the display is a passive electroluminescent display, such as a small molecule or polymer based Organic Light Emitting Diode (OLED) display. The display driver control circuitry may comprise part of the circuitry of an integrated circuit on which the row drivers and/or column drivers of the passive matrix display may also be included. Those skilled in the art will recognize that the representation of pixel lines or display element lines as rows and columns is essentially arbitrary, and in passive matrix displays the matrix need not be rectangular. Those skilled in the art will further recognize that the control circuit may be used with fixed or variable constant current generators. The power supply for the constant current generator is preferably of the voltage converter type, such as a switched mode power supply, so that the supply voltage can be reduced without substantially affecting the efficiency of the power supply. In case a switched mode power supply is used, this will preferably have a high switching frequency, e.g. more than 1MHz, in order to facilitate a fast change of the supply voltage.

The lower the compliance of the current driver (i.e., V)_s-V_oLarger), the greater the power loss due to limited drive compliance. Therefore, it is preferred to employ a constant current generator or driver with a high compliance, as this will allow the use of a lower power supply output voltage. Thus, preferably the current generator for the display comprises, in series with the current drive to the display, at least one bipolar transistor, and preferably the emitter terminal of the transistor is connected substantially directly to the power supply input or connection and the collector terminal of the transistor is connected to the electrode driver output. Preferably, the voltage drop between the emitter terminal and the power supply connection is less than V of the transistor_beTypically less than 100mV, possibly less than 50 mV.

Preferably, the controllable current generator comprises a current mirror, as this allows V_oTypically to within less than 0.5V of the power supply, and sometimes to within 0.1V of the power supply. There is no need to provide a pair of bipolar transistors for each driver circuit (although in some embodiments this is preferred) since the current mirror circuit may in fact be shared by a plurality of driver circuits, for example across a plurality of display column electrodes. The current mirror has a finite output impedance and thus the output current can be varied over a range of 25% beyond the output compliance range (in general terms, because for a given drive current, V_beA small amount of variation with collector voltage).

This effect can be reduced by using a Wilson current mirror, although the compliance is then reduced.

The functions of the above-described display driver control circuit may be implemented using discrete components and/or integrated circuits in silicon, or in an Application Specific Integrated Circuit (ASIC) or Field Programmable Gate Array (FPGA), or by means of a dedicated processor with appropriate processor control code.

According to another aspect of the present invention there is provided a method of reducing power consumption of a display driver driving an electroluminescent display, the display comprising at least one electroluminescent display element, the driver comprising at least one substantially constant current generator for driving the display element and having a power supply for supplying power to the current generator at a supply voltage, the method comprising: sensing a voltage on a first line connected to a current generator, the current in the first line being regulated by the current generator; and controlling the supply voltage in response to the sense voltage to reduce the supply voltage when a reduction in the supply voltage is obtained without substantially changing the regulated current.

Broadly speaking, this approach provides similar advantages and benefits as the display driver control circuit described above. The first line will typically be the output of a current generator that provides a substantially constant current from a current source for the "output", the current flowing into the current source being controlled by a current sink. Preferably, the control controls the supply voltage such that the current generator operates at or near its compliance limit. However, the voltage sensing does not require a direct sensing of the voltage at the output of the current generator, since the compliance limit may be determined, for example, by obtaining an inflection point in the current-voltage curve of the current generator, rather than by detecting an absolute voltage value. The compliance limit may be determined by determining the variation of the sense voltage with the supply voltage (since below the compliance limit, as the supply voltage decreases, the sense voltage drop remains approximately constant), or a sense voltage limit based on a known or assumed compliance limit may be employed. In some embodiments, the method includes determining a current generator compliance limit for use in controlling the supply voltage.

The method can be applied to existing display drivers by reading out the voltage on the control lines or electrodes of the display without modifying the display driver. Preferably, the display comprises a plurality of simultaneously drivable display elements, such as a row of a passive matrix display, and the method further comprises: sensing the voltage on the drive line for each of these elements; and controlling the supply voltage of the constant current generators driving the drive lines in response to the maximum read voltage from the drive lines. It is also possible to measure the supply voltage (or a voltage dependent on the supply voltage) and control the supply voltage in response to the voltage difference between the voltage on the current drive line and the read supply voltage, or in the case of multiple drive lines the voltage difference between the maximum drive voltage and the read supply voltage. In the case of a plurality of simultaneously driven display elements, this voltage difference may be determined by determining the maximum read voltage, or by determining the minimum difference between the supply voltage and the read drive voltage, so that for a set current drive level, the display element or pixel requiring maximum drive may be driven by a power supply providing only the necessary additional voltage required by the constant current generator of the display element.

In a preferred embodiment of the method, the one or more electroluminescent display elements comprise OLEDs, such as small molecule or polymer OLEDs.

The invention further provides a display driver circuit configured to implement the above method.

The invention further provides processor control code, and a carrier medium carrying the code, for implementing the above method and display driver control circuit functions. The code may comprise conventional program code, or microcode, or code for setting up or controlling an ASIC or FPGA. The carrier may comprise a storage medium such as a hard or floppy disk, Compact Disc (CD) or DVD-ROM, or a programmed memory such as read only memory (firmware), or a data carrier such as an optical or electrical signal carrier. It will be appreciated by those skilled in the art that the code may be distributed between a plurality of connected components in communication with each other.

Drawings

These and other aspects of the invention will be further described, by way of example only, with reference to the accompanying drawings, in which:

FIGS. 1a and 1b show, respectively, a cross-section through an organic light emitting diode and a passive matrix OLED display;

figures 2a to 2d show, respectively, a conceptual driver arrangement for a passive matrix OLED display, a plot of current drive versus time for a pixel of the display, a plot of pixel voltage versus time, and a plot of pixel light output versus time;

FIG. 3 shows a schematic diagram of a generic driver circuit for a passive matrix OLED display, according to the prior art;

FIGS. 4a to 4d show, respectively, the light-voltage curves for OLED display elements, the light-current curves for OLED display elements, current drivers for columns of a passive matrix OLED display, and the current-voltage curves for an OLED display and its associated current sources;

FIG. 5 shows a schematic diagram of a passive matrix OLED driver circuit according to a first embodiment of the present invention;

FIG. 6 shows a part of a schematic diagram of a passive matrix OLED driver circuit according to a second embodiment of the present invention;

figure 7 shows a part of a schematic diagram of a passive matrix OLED driver circuit according to a third embodiment of the present invention;

FIG. 8 shows a circuit diagram of a maximum voltage detector for use with an embodiment of the invention;

FIG. 9 shows a general schematic diagram of a passive matrix OLED driver circuit according to an embodiment of the present invention; and

fig. 10 shows a flow chart of a supply voltage control process according to an embodiment of the invention.

Detailed Description

Turning now to fig. 5, fig. 5 shows a schematic diagram of a passive matrix OLED driver 500 according to an embodiment of the invention, the driver 500 implementing display drive voltage sensing to control the power supplied to the display to provide improved efficiency.

In fig. 5, a passive matrix OLED display 302 similar to the display described with reference to fig. 3 has: row electrodes 306 driven by row driver circuitry 512; and column electrodes 310 driven by a column driver 510. The driver for each row typically comprises MOS transistors for selectively grounding the row electrodes; the driver for each column in the preferred embodiment comprises a substantially constant current generator 520 (a current source as shown), such as described with reference to fig. 4 c. In fig. 5, only one of a plurality of constant current sources is shown for clarity, one provided for each column. The current generator 520 is powered by the supply voltage on line 515 and is controlled by the analog output from the digital-to-analog converter 522. A digital input to digital to analog converter 522 is provided by control input 509. A digital to analog converter 522 may be provided for each column electrode line, such as line 524, or a single digital to analog converter may be shared between column lines, for example by time multiplexing.

As shown in fig. 5, the current source is a controllable current source to provide a variable brightness or gray scale display, but in other embodiments a fixed current source may be used. In these other embodiments, pulse width modulation may be used to give the human eye a representation of variable brightness, or alternatively, the pixels of the display may all have substantially the same relative brightness, i.e., the display need not be a grayscale display. In other embodiments, the display may employ different color pixels to provide a variable color display.

The row driver circuit 512 has a control input 511 for selecting one (or more) of the rows of electrodes to be grounded. The column driver 510 has a control input 509 for setting the current drive to one or more of the column electrodes. Preferably, for ease of connection, control inputs 509 and 511 are digital inputs, and preferably control input 509 sets the current drive for all m columns of display 302. The two-dimensional image may be rendered on the display 302 by: each row is selected in turn and all pixels in the selected row are driven by the column driver 510, then the next row is selected and the process is repeated to build up an image using the conventional raster scan pattern. In the case of gray scale and color display to be provided, a variable current drive is provided for each column, depending on the desired pixel brightness. In some embodiments of the row driver circuit 512, the raster scan function may be provided automatically by the row driver under control of the control input 511.

The power supply unit 514 supplies power to the various elements of the display driver 500 and has, inter alia, an output 515 for powering the column driver 510. The power supply unit 514 also has a control input 516 for controlling the output voltage supplied on line 515 to the column driver.

The power supply unit 514 is preferably a switched mode power supply having an input from the battery 602, preferably having a relatively low voltage, such as 3 volts, for compatibility with typical portable consumer electronic devices. The voltage provided on power supply output line 515 will generally be higher than the battery voltage, typically between 5 and 10 volts, for driving a passive matrix polymer OLED display to provide the desired brightness, although so-called small molecule based OLED displays generally require higher voltages, for example 30 volts or more.

Data for display on the display 302 is provided on a data and control bus 502, the bus 502 including, for example, at least one data line and a write line. Bus 502 may be a parallel bus, or a serial bus. The bus 502 provides input to a frame memory 504, and the frame memory 504 stores display data for each pixel of the display 302, in effect forming an image of the display data in the memory. Thus, for example, one or more bits of memory may be associated with each pixel, defining a gray scale pixel brightness level or pixel color. The data in the frame memory 504 is stored in such a way that the luminance values of the pixels in a row can be read out, and in the embodiment shown the frame memory 504 is dual ported, outputting the data read out from the frame memory onto a second, read data bus 505. In other embodiments, the functions of the data bus 502 and the data bus 505 may be combined into a single data bus.

The passive matrix OLED driver 500 also incorporates display drive logic 506 for providing display data to a control input 509 of a column driver 510 and for providing a row select or scan control output to a control input 511 of a row driver 512 for controlling the raster scanning of the display. The timing or processing performed by the display driver logic 506 is controlled by a clock signal from a clock generator 508. Display drive logic 506 is also coupled to read data and control bus 505 to read data from frame memory 504.

The display drive logic 506 operates in a conventional manner to read data from the frame memory 504 and to provide control data signals to the control inputs 509 and 511 for displaying the data on the passive matrix display 302. However, the display driver logic 506 also includes a drive voltage sense circuit or control code 526, and a power control circuit or control code 528 responsive to the drive voltage sense unit 526, as described in more detail below.

The analog to digital converter 530 is provided with a plurality of inputs 532, one for each of the column electrode lines 310a-310e and one for the switched mode power supply 514 supply voltage output line 515. Analog-to-digital converter 530 senses the voltages on lines 310a-310e and 515 and provides a digital output corresponding to each of these voltages to output 534, which output 534 may comprise a serial or parallel bus. The analog-to-digital converter 530 may comprise a separate analog-to-digital converter for each of the lines being read out, or may comprise a single analog-to-digital converter that is shared, e.g., based on time multiplexing. Thus, display drive logic 506 has inputs that include digital values corresponding to the sensed voltages on each of drive line 310 and power line 515. The display driver logic 506 may be implemented on, for example, an ASIC (application specific integrated circuit) with the aid of a conventional clock or combinational logic and/or processed with a microprocessor.

In operation, whenever a row is selected and the pixels 312 in that selected row are driven by the constant current generator 520 of the column data driver 510, a drive voltage readout module, which may be implemented with dedicated logic or by means of control code for a microprocessor, controls the analog-to-digital converter 530, for example reading the voltages on lines 310a-310e and line 515 using a control bus (not shown). For simplicity, only a single constant current driver 520 is shown in fig. 5, but it will be appreciated that the display drive logic 506 is capable of reading the supply voltage 515 of the current generator, and the voltage on the outputs 524, 310e of the current generator, which provides a substantially constant regulated current. The same applies to other constant current generators of the column driver 510, not shown in fig. 5. In this way, display drive logic 506 is able to determine whether current generator 520 is at or near its compliance limit.

The column data driver of figure 5 allows a variable current driver to be applied to the column electrodes 310 so that in any given row some pixels may be brighter than others. Although the column electrodes are current driven, it will nevertheless be appreciated that, in general, the brighter the pixel, the greater the voltage applied to the pixel, in accordance with figure 4 a. However, because in practice the characteristics of the OLEDs in a display are not uniform, pixels driven with the same current may require different voltages, depending on their efficiency, age (depending on use), and other factors. Current generator 520 attempts to provide a programmed current level to the pixel and changes its output voltage accordingly. Assuming that the supply voltage of constant current generator 520 is sufficient, the output voltage of constant current generator 520 will be sufficient to maintain the programmed current. As the supply voltage decreases, the output voltage of constant current generator 520 will remain approximately constant until the compliance limit of the current generator is fixed, at which point a further decrease in the supply voltage will result in a substantial decrease in the output voltage of constant current generator 520, with the following effects: constant current generator 520 is no longer able to provide the current (source or sink) that it has been programmed to generate.

From the above discussion, it will be appreciated that the supply voltage of the power supply unit 514 should be sufficient to allow the current generator used to drive the pixels in the selected row requiring the maximum current generator output voltage to substantially provide that voltage. Again, the power control module 528, which may comprise dedicated logic or processor control code (or a combination of both), provides an output signal on line 516 to control the switched mode power supply unit 514 to provide a supply voltage output on line 515 to accomplish this. In one embodiment, power control module 528 determines the maximum voltage sensed on column lines 310a-310e and compares this maximum voltage to the power supply voltage sensed on line 515 to determine if any constant current driver 520 is at or near the compliance limit. In another embodiment, the power control module 528 determines the voltage across each constant current generator 520 by determining the difference between the input voltage (on line 515) and the output (e.g., on line 524), and identifying the minimum voltage across any one of the constant current generators, and then examining the minimum voltage to determine whether the minimum voltage is sufficient to meet the compliance limits of the constant current generators. The compliance limit of the constant current generator may be known, at least approximately known, or may be determined by the power supply control module 528 or the drive voltage sensing module 526 or some other part of the display drive logic 506, or indeed by the power supply unit 514. As will be described in more detail later.

Once power control module 528 has determined whether any of constant current generators 520 are at or near their compliance limits, power control module 528 can control the supply voltage on line 515, either decreasing the supply voltage when the voltage is greater than that required to drive the necessary current into the brightest/least significant pixel, or increasing the supply voltage when the voltage is sufficient to meet the necessary current drive of at least one of the pixels in the row. For row-by-row supply voltage control, it should be appreciated that the power supply unit 514 should be able to respond to the control signal on line 516 fast enough to achieve some power savings during the intervals in which the rows are illuminated, which are often referred to as line periods. Taking the example of a 320 column by 240 row display operating at 60 frames per second (240 x 60 rows per second), the row period is approximately 70 microseconds, and 140 microseconds in the case of using a 120 row double scan to reduce the enable loss. A switched mode power supply operating at a switching frequency of 1MHz or higher and employing approximately 10 cycles smoothing is able to respond in 10 microseconds, 10 microseconds being sufficient for such a display. For higher resolution displays, a switched mode power supply operating at a higher frequency, for example 10MHz, may be employed.

In a variation of the above embodiment, the display drive logic 506 stores the voltage sensed on each column electrode line 310 as each row is accessed. In this way, the maximum required drive voltage for a complete display frame can be determined, and thus the switched mode power supply voltage can be reduced to the minimum required for the maximum required drive voltage for any pixel in the display frame. Thus, the power control module 528 in this embodiment operates on a frame-by-frame basis, rather than a row-by-row basis, and the supply voltage V on line 515_sIs controlled more slowly. This operation may be preferable when a slower control loop is desired, for example to allow the display driver logic (or microprocessor) to run more slowly, thereby providing further power savings. It should be appreciated, however, that row-by-row control potentially allows maximum power savings in constant current generator 520.

It will be appreciated that embodiments of the power saving method can be applied to column data drivers that employ fixed constant current generators rather than variable constant current generators, and to driver circuits that employ on/off pulse width modulated brightness control using fixed constant current generators. However, where variable brightness is achieved by driving the display with a variable substantially constant current generator, adaptively controlling the supply voltage in dependence on the displayed pixel brightness (i.e. the pixel drive voltage from the constant current generator) provides the greatest benefit.

Referring now to fig. 6, fig. 6 shows a portion 600 of a schematic circuit diagram of a variation of the passive matrix OLED display driver of fig. 5. The same elements as in fig. 5 are denoted by the same reference numerals.

In fig. 6, the analog-to-digital converter 530 has two inputs: a first input 602 from the switched mode power supply unit power line 515, and a second input 604 from the maximum voltage detection module 606, as previously described. As previously described, digitized versions of the signals on inputs 602 and 604 are provided to display drive logic 506 on sense line 534. Again, analog-to-digital converter 530 may actually include more than one analog-to-digital converter.

The maximum or peak voltage detection module 606 has a plurality of inputs 608, one for each of the column electrode lines 310a-310e, and the detection module 606 provides an output 604 corresponding to the maximum voltage on these separate input lines. The maximum detection module 606 has a reset input 610 driven by display drive logic 5Q6 to allow the maximum value detected from the column lines to be reset when each new row is selected. It will be appreciated that the maximum detection module performs some of the processing performed by display drive logic 506 (either by drive voltage sense unit 526 or by power controller 528) in FIG. 5. This simplifies the processing load on the display drive logic 506 and reduces the number (or speed) of analog converters 530. As described above, power supply controller 528 provides an output on line 512 to control power supply 514 in response to a minimum difference between the voltage on line 515 and the voltages on lines 310a-310 e. This minimum voltage difference may be found by determining the maximum voltage on any of the column electrode lines 310a-310e and then determining the difference between this maximum voltage and the voltage on the power supply output line 515.

Fig. 7 shows a portion 700 of a schematic circuit diagram of a variation of the passive matrix OLED display driver of fig. 6, like elements to those of fig. 6 being denoted by like reference numerals.

In the arrangement of fig. 7, the output 604 of the maximum detection module 606 is directly connected to the voltage control input 516 of the power supply unit 514, and the necessary supply voltage control functions are implemented in the switched mode power supply, rather than in the display drive logic 506. Broadly speaking, these functions may be implemented digitally, optionally by using an input (not shown in fig. 7) from the row driver output 511 of the display drive logic 506 to the switched mode power supply 514, in a manner similar to that described above with reference to fig. 5 and 6, to determine when a new row is selected. However, the desired control function may be implemented more directly in the power supply unit 514 by means of an analog control circuit. Thus, for example, the difference between the supply voltage output 515 and the maximum detection voltage on the column electrode lines, n-line 516, may be determined by means of a differential amplifier. The difference may then be compared to a threshold, such as an estimated compliance limit or constant current generator 520, or to a dynamically determined compliance limit. For example, a small variation may be superimposed on the supply voltage on line 515 and the magnitude of the variation on the detected output 604 (since changing the supply voltage will have little effect on the electrode line voltage when the supply voltage is greater than the desired voltage). The supply voltage on line 515 can then be adjusted, either increased or decreased as needed, based on the comparison.

Fig. 8 shows a passive matrix OLED display 302 connected to a maximum voltage detector 800, the detector 800 having a sample/hold circuit 806 suitable for use as the maximum detection module 606 of fig. 6 and 7.

In FIG. 8, each column electrode 310a-310e is connected to a respective diode 802a-802e to sample a respective voltage X1, X2, X3, X4, and Xm on a respective column line. The diode or arrangement provides a maximum voltage MAX X (less than the diode drop) on any one of the column electrode lines on output line 804. The peak detection circuit 805 includes: a capacitor 806 for storing the voltage on line 804; and a controllable switch 808, the switch 808 being closed in response to a signal on a reset line 810 to reset the charge on the capacitor 806. A high input impedance amplifier may be used to buffer the maximum detected voltage output on line 804.

Fig. 9 shows a general circuit diagram of a passive matrix OLED driver incorporating power control embodying aspects of the present invention. In fig. 9, the same elements as those of fig. 5 are denoted by the same reference numerals.

Each column line 310 is driven by a respective adjustable constant current generator 520. The voltages on each of column lines 1, 2, 3, 4.. m are denoted X1, X2, X3, X4,. Xm, respectively, and these voltages are tapped off by lines 524a-524 e. Input or supply voltage V on line 515 to constant current column driver 520_sIs tapped by line 904. Control circuit 902 has an input from line 904 and inputs from lines 524a-524e and provides a control output on line 516 to control the switched mode power supply 514. In other arrangements, an internal column driver tap, such as line 906, may be used to read the supply voltage to the constant current generator. The control circuit controls the power supply as described above so that the minimum (V) is reached_s-X_i) Substantially in a position for X_iThe compliance limit of the drive. Thus, when the minimum value increases, the power supply is controlled to decrease the power supply voltage and vice versa.

Fig. 10 shows a flow diagram of a process that may be implemented by display drive logic, such as display drive logic 506 of fig. 5, to control the supply voltage of a current-controlled passive-matrix display driver in order to improve the efficiency of a driven display. Where the display driver logic 506 comprises a microprocessor, the program of FIG. 10 may be implemented using suitable processor control code.

The procedure of fig. 10 assumes line-by-line power control, but a similar procedure can be used for frame-by-frame power control. For progressive control, the steps of FIG. 10 are performed for each row in turn; for frame-by-frame control, the steps of FIG. 10 are performed for each frame.

In step S1000, the processor reads the maximum column electrode voltage X_iAnd a column driver supply voltage V for the rows_sThe peak detector 805 is then reset. Then, the processor is driven from V_s(for rows) minus the maximum value X_iTo determine the minimum supply current for the column driver constant current generatorThe overhead is compressed.

Steps S1004 to S1008 provide a method of determining whether a current generator is near its compliance limit. In step S1004, a control signal is provided to the power supply to make the power supply voltage V_sChanged by a small amount, and then at step S1006, the maximum voltage X is read_iIs detected (sample-and-hold reset, if necessary), and the maximum voltage X is determined_iA change in (c). If the change is small, the current generator is within its compliance limit, and if the change is above a certain threshold, the compliance limit of the constant current generator has been exceeded. In step S1008, this determination is made.

In step S1010, the routine determines whether the compliance limit has been exceeded. If the compliance limit has been exceeded, then in step S1012 a control signal is provided to the column driver to increase the supply voltage V_s(ii) a If the compliance limit has not been exceeded, then in step S1014 a control signal is provided to the column constant current driver to reduce the supply voltage V_s. In both cases, the program ends and returns to step S1000 to repeat the program for the same line, or if the next line of the display has already been selected, the program for the next line is executed. During each row or line period, with multiple cycles through the program, better supply voltage control is achieved, although this will depend on the speed of the processor and the duration of the line period.

Clearly, there will be many effective alternatives for the skilled person. For example, display drive logic 506, and in particular drive voltage sense and power control functions 526 and 528, may be implemented, at least in part, using a state machine implemented on a Programmable Logic Array (PLA). Where a microprocessor is employed in the drive logic 506, the buses 502 and 505 may be combined in a shared address/data/control bus, although again the frame memory 504 is preferably dual-ported to simplify connecting the display to other devices.

It is understood that the invention is not limited to the described embodiments, but encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims (27)

1. Display driver control circuitry for controlling a display driver for an electroluminescent display, the display comprising at least one electroluminescent display element, the driver comprising at least one substantially constant current generator for driving the display element, the control circuitry comprising:

a drive voltage sensor for sensing a voltage on a first line, a current in the first line being regulated by the constant current generator; and

a voltage controller connected to the driving voltage sensor, the voltage controller for controlling a voltage of the constant current generator power supply in response to the readout voltage, and the voltage controller configured to control the power supply voltage to improve efficiency of the display driver.

2. The display driver control circuit of claim 1, wherein the voltage controller is configured to reduce the supply voltage when the supply voltage will not substantially reduce the regulated current and/or the display brightness.

3. The display driver control circuit according to claim 2, wherein the voltage controller is configured to control the supply voltage such that the constant current generator operates near its compliance limit.

4. The display driver control circuit of claim 3, further comprising means for determining a compliance limit for use by the voltage controller.

5. The display driver control circuit according to any one of claims 1 to 4, characterized by further comprising: a power supply voltage sensor for reading out the power supply voltage; and means for determining a difference between the supply voltage and the first line voltage, and the voltage controller is configured to control the supply voltage in response to the difference.

6. A display driver control circuit according to any one of claims 1 to 4 wherein the display has a plurality of electroluminescent display elements and the display driver has a plurality of substantially constant current generators for driving the plurality of display elements simultaneously, each constant generator being configured to regulate the current on an associated display drive line, the display driver control circuit further comprising a drive voltage sensor for sensing the voltage on each of the display drive lines and the voltage controller being configured to control the supply voltage in response to the sensed voltage on the drive line having the maximum voltage of the drive line sense voltage.

7. The display driver control circuit according to claim 6, further comprising: a power supply voltage sensor for reading out the power supply voltage; and means for determining a difference between the supply voltage and the maximum voltage, and the voltage controller is configured to control the supply voltage in response to the difference.

8. Display driver control circuit according to claim 6 or 7, wherein the display comprises a passive matrix display and the voltage controller is configured to control the supply voltage frame by frame.

9. Display driver control circuit as claimed in claim 6 or 7, characterized in that the display comprises a passive matrix display having a plurality of rows of display elements, and the voltage controller is configured to control the supply voltage row by row.

10. A display driver control circuit according to any preceding claim, wherein the display has at least one control line for controlling the illumination of the at least one electroluminescent display element, wherein the drive voltage sensor is configured to sense the voltage on the display control line, and the voltage controller has an output for controlling an adjustable power supply configured to provide the supply voltage.

11. A display driver comprising the display driver control circuit of any one of claims 1 to 10.

12. A display driver control circuit as claimed in any preceding claim wherein the electroluminescent display elements comprise organic light emitting diodes.

13. A method of reducing power consumption of a display driver driving an electroluminescent display, the display comprising at least one electroluminescent display element, the driver comprising at least one substantially constant current generator for driving the display element and having a power supply for supplying power to the current generator at a supply voltage, the method comprising:

sensing a voltage on a first line connected to a current generator, the current in the first line being regulated by the current generator; and

controlling the supply voltage in response to the sense voltage to reduce the supply voltage when a reduction in the supply voltage is obtained without substantially changing the regulated current.

14. A method according to claim 13, wherein the control controls the supply voltage such that the current generator operates at or near its compliance limit.

15. The method of claim 14, further comprising determining the current generator compliance limit for use in the controlling.

16. A method according to claim 13, 14 or 15, characterized in that the method further comprises:

sensing a voltage on a second line, the voltage on the second line being dependent on the supply voltage; and

determining a voltage difference between voltages sensed on the first line and the second line; and

wherein the controlling is responsive to the voltage difference.

17. A method as claimed in claim 13, 14 or 15, wherein said display comprises a plurality of simultaneously drivable electroluminescent display elements, each of said display elements being driven by a said substantially constant current generator, each of said substantially constant current generators having an associated drive line in which the current is regulated by the current generator, the method further comprising:

sensing a voltage on each of the associated drive lines; and

controlling the supply voltage to reduce the supply voltage in response to the sense voltage when the reduction in the supply voltage is obtained without substantially changing the regulated current in the associated drive line having the maximum sense voltage.

18. The method of claim 17, further comprising:

sensing the electrical line on another line, the voltage on the other line being dependent on the supply voltage; and

determining a voltage difference between a voltage sensed on the other line and the maximum sensing voltage; and

wherein the controlling is responsive to the voltage difference.

19. A method according to any one of claims 13 to 18, wherein the display has at least one control line for controlling the illumination of the at least one electroluminescent display element, wherein the driver drives the control line, and wherein the reading comprises reading the voltage on the control line.

20. A method according to any one of claims 13 to 19, wherein the substantially constant current generator comprises a current source.

21. A method according to any one of claims 13 to 19, wherein the substantially constant current generator comprises a current sink.

22. A method according to any one of claims 13 to 21, wherein the display comprises a passive matrix display having a plurality of electroluminescent display elements and a plurality of row electrodes and a plurality of column electrodes for accessing the display elements, and the driver is connected to at least one of the row electrodes and the column electrodes for driving the display.

23. The method of claim 22, wherein said sensing and controlling are performed row by row.

24. The method according to claim 22, characterized in that said reading out and controlling are performed frame by frame.

25. A method according to any one of claims 13 to 24, wherein said electroluminescent display element comprises an organic light emitting diode.

26. A carrier carrying processor control code to implement the method of any one of claims 13 to 25.

27. A display driver circuit configured to implement the method of any one of claims 13 to 25.