Abstract:
This invention relates to methods, apparatus, and computer program code for driving an active matrix display, in particular an organic light emitting diode (OLED) display, with reduced power consumption.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods, apparatus, and computer program code for driving an active matrix display, in particular an organic light emitting diode (OLED) display, with reduced power consumption. 
     2. Related Technology 
     Displays fabricated using OLEDs provide a number of advantages over LCD and other flat panel technologies. They are bright, colorful, fast-switching (compared to LCDs), provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colors which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507. 
     A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative. 
     Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix (AM) displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Examples of polymer and small-molecule active matrix display drivers can be found in WO 99/42983 and EP 0,717,446A respectively. 
       FIG. 1   a  shows such an example OLED active matrix pixel circuit  150 . A circuit  150  is provided for each pixel of the display and ground  152 , V ss    154 , row select  124  and column data  126  busbars are provided interconnecting the pixels. Thus each pixel has a power and ground connection and each row of pixels has a common row select line  124  and each column of pixels has a common data line  126 . 
     Each pixel has an organic LED  152  connected in series with a driver transistor  158  between ground and power lines  152  and  154 . A gate connection  159  of driver transistor  158  is coupled to a storage capacitor  120  and a control transistor  122  couples gate  159  to column data line  126  under control of row select line  124 . Transistor  122  is a thin film field effect transistor (FET) switch which connects column data line  126  to gate  159  and capacitor  120  when row select line  124  is activated. Thus when switch  122  is on a voltage on column data line  126  can be stored on a capacitor  120 . This voltage is retained on the capacitor for at least the frame refresh period because of the relatively high impedances of the gate connection to driver transistor  158  and of switch transistor  122  in its “off” state. 
     Driver transistor  158  is typically an FET transistor and passes a (drain-source) current which is dependent upon the transistor&#39;s gate voltage less a threshold voltage. Thus the voltage at gate node  159  controls the current through OLED  152  and hence the brightness of the OLED. 
     The voltage-controlled circuit of  FIG. 1  suffers from a number of drawbacks, and some ways to address these are described in the applicant&#39;s WO03/038790. 
       FIG. 1   b , taken from WO03/038790, shows an example of a current-controlled pixel driver circuit  160  which addresses these problems. In this circuit the current through an OLED  152  is set by setting a drain source current for OLED driver transistor  158  using a reference current sink  162  and memorising the driver transistor gate voltage required for this drain-source current. Thus the brightness of OLED  152  is determined by the current, I col , flowing into reference current sink  162 , which is preferably adjustable and set as desired for the pixel being addressed. In addition, a further switching transistor  164  is connected between drive transistor  158  and OLED  152 . In general one current sink  162  is provided for each column data line. 
     It can be seen from these examples that an active matrix pixel circuit generally incorporates a thin film (driver) transistor (TFT) in series with an electroluminescent display element. 
     Referring now to  FIG. 2   a , this shows drain characteristics  200  for a FET TFT driver transistor of an active matrix pixel circuit. A set of curves  202 ,  204 ,  206 ,  208 , is shown each illustrating the variation of drain current of the FET with drain-source voltage for a particular gate-source voltage. After an initial non-linear portion of the curves become substantially flat, and the FET operates in the so-called saturation region. With increasing gate-source voltage the saturation drain current increases; below a threshold gate-source voltage V T  the drain current is substantially 0. The dashed line  230  indicates the separation between the initial non-linear portion of the curves and the saturation region. For each set of curves  202 ,  204 ,  206 ,  208 , a threshold point V T ( 202 ), V T ( 204 ), V T ( 206 ), V T ( 208 ) exists that indicates the point between the initial non-linear portion of the curve and the saturation region. Typical values of V T  are between 1V and 6V. Broadly speaking the FET acts as a voltage controlled current limiter. 
       FIG. 2   b  shows a drive portion  240  of a typical active matrix pixel circuit. A PMOS driver FET  242  is connected in series with an organic light emitting diode  244  between a ground line  248  and a negative power line V ss    246 . 
     It will be appreciated from the circuit of  FIG. 2   b  that, for a given OLED drive current, the greater V SS  the greater the excess (waste) power dissipation in driver transistor  242 . It is therefore preferable to reduce V SS  as much as possible to reduce this excess dissipated power. However it can be appreciated from  FIG. 2   a  that there is a limit, as indicated by dashed line  230 , below which V ss  may not be reduced, this limit being determined by the maximum available V GS  and the required OLED drive voltage. 
     In an active matrix driver multiple factors contribute to increasing the supply voltage of an AM OLED display above that which is necessary at a given time. In principal a supply voltage might only need to be ˜0.5V above that required to drive the highest voltage OLED (˜4V for polymer, ˜7V for small molecule and phosphorescent systems). However in practice the supply needs to be sufficient to hold the drive TFT in saturation, and possess enough overhead to cope with increases in OLED threshold voltage with time which can result in supply voltages as high as 14V for small molecule. This extra voltage is dropped entirely over the drive TFT increasing (doubling in the example given) the power consumption and stressing the TFT both with the enhanced field drop and heating. We have previously described, in WO03/107313, some techniques for addressing these difficulties. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is therefore provided a method of reducing the power consumption of an active matrix electroluminescent display, the method comprising: controlling a power supply voltage to the display; and monitoring a power supply current to the display; and wherein said controlling further comprises progressively reducing said power supply voltage until said power supply current reduces by greater than a threshold. 
     In embodiments this method provides enhanced efficiency of the display and reduced stress on the drive thin film transistor. This also helps to reduce threshold voltage drift with time. Thus, broadly speaking embodiments of the method provide reduced power consumption and/or increased display lifetime. 
     The current threshold may be an absolute current value threshold or a relative threshold such as a percentage (such as 90 percent) of a saturation current determined as, for example, a current value which is substantially constant for small changes in supply voltage. Alternatively the threshold may be defined in terms of a rate of reduction of supply current—that is, for example, a percentage change in supply current with a step reduction in supply voltage. In a further alternative a response curve of an active matrix pixel (drive transistor and electroluminescent display element) may be stored, for example in non-volatile memory, and the threshold determined by a position on such a characteristic curve, which may in turn be determined by the monitored power supply current. 
     Preferably the monitoring and controlling maintains the active matrix display in an operating region in which a highest driven driver transistor (that is, a driver transistor with a maximum drive) is just within saturation. Preferably the monitoring and controlling is performed substantially continuously, for example in a computer program controlled feedback loop. 
     Where the active matrix display is a multi-colored display with at least two, and preferably three sub-pixels of different colors each of the sub-pixels may be provided with a different respective power supply line so that the power supplies for the different sub-pixels may be controlled substantially independently. This is advantageous because, in general, different color sub-pixels have different threshold voltages and by driving them from separate power supply lines a separate optimisation may be supplied for each. Additionally or alternatively different spatially separate regions of the display may be provided with their own respective power supply lines for separate respective power supply control along the lines outlined above. This may be advantageous where, for example, different regions of the display are substantially dedicated to different tasks. 
     In embodiments the method also controls a drive level to one or more pixels of the display. This allows a further reduction in power supply voltage providing the drive level of one or more pixels, which might otherwise be brought out of saturation is increased to compensate. 
     In a related aspect the invention provides a controller for an active matrix electroluminescent display driver, the display having a plurality of pixels each with an electroluminescent display element and an associated drive transistor, the display having a power supply line for providing power to the driver transistors of said pixels; the driver comprising a pixel data driver to drive said display pixels with data for display, a controllable voltage power supply to provide a power supply to said power supply line, and a current sensor to sense a current in said power supply line; the controller comprising: a current sense input for said current sensor; a voltage control output for said controllable power supply; and a voltage controller to provide a voltage control signal for said voltage control output responsive to a current sense signal from said current sense input. 
     Preferably the voltage controller is configured to adjust the power supply control signal to progressively reduce the sensed current to a threshold point, and to then adjust the control signal to maintain the sensed current in the region of this threshold point. Generally the power supply voltage is determined with respect to a ground line of the active matrix display, although it may in principle be determined with respect to some other power supply line. Optionally the driver may include a voltage sensor to sense the power supply voltage and to provide an input to the controller which may be used, for example, to facilitate determination of an operating point of the display. In this case the control output may also be responsive to the sensed power supply voltage. 
     As mentioned above, the display may have a plurality of power supply lines driving different portions of the display such as different sub-pixels or different spatially separate regions of the display, in which case the controller (or separate controllers) may control the power supply voltage to each separate power supply line. Optionally, as mentioned above, the pixel drive data may be adjusted in coordination with the voltage control signal, in particular to compensate (the hardest or highest driven drive transistors) for a reduction in power supply voltage. 
     The invention further provides an active matrix electroluminescent display driver incorporating the above described controller in combination with the above described pixel data driver, controllable voltage power supply, and current sensor. 
     In all of the above aspects of the invention the electroluminescent display device preferably comprises an organic light emitting diode-based display such as a small molecule, polymer and/or dendrimer-based display. 
     In a further aspect the invention provides an active matrix OLED display wherein each said pixel comprises at least first and second sub-pixels of different colors, and wherein said two portions comprises said first and second sub-pixels respectively. 
     The invention further provides a carrier medium carrying processor control code to implement the above described methods and display drivers. This code may comprise conventional program code, for example source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (Trade Mark) or VHDL (Very high speed integrated circuit Hardware Description Language). Such code may be distributed between a plurality of coupled components. The carrier medium may comprise any conventional storage medium such as a disk or programmed memory (for example firmware such as Flash RAM or ROM), or a data carrier such as an optical or electrical signal carrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the of the invention will now be further described, by way of example only, with the reference to the accompanying figures in which: 
         FIG. 1  shows an example of an active matrix OLED pixel circuit; 
         FIGS. 2   a  and  2   b  show, respectively drain characteristics for a TFT driver transistor of an active matrix pixel circuit, and a drive portion of a generalised active matrix pixel circuit; 
         FIG. 3  shows an active matrix display driver according to an embodiment of the present invention; and 
         FIG. 4  shows a flow diagram of a power supply voltage control procedure for the driver of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Broadly speaking we will describe a technique for reducing power consumption of an active matrix OLED display by means of active monitoring and adjustment of the supply voltage. In outline, test reductions of the supply voltage are made and the current drawn monitored. The voltage at which the current starts to dip significantly is the point at which the highest driven TFT is just within saturation. If the supply voltage is then held at this point then no additional allowance in supply voltage need be made for OLED ageing (and/or temperature effects) and/or possible TFT process/characteristic variations. In embodiments the active supply monitoring automatically compensates for this over time resulting in lower stresses on the TFTs and a reduced power consumption. 
     In some preferred embodiments these advantages are enhanced by providing separate monitors and adjustments on red, green and blue sub-pixel power supply lines. This is because the operational voltages of each color can differ considerably—for example a red sub-pixel may require a drive voltage of 3.6V while a green sub-pixel may require 4.2V and a blue sub-pixel 5.15V in which case a power supply voltage of at least 6.15V (allowing 1V overhead for driver transistor compliance and other losses) might be needed were only a single power supply line used. Alternatively, where two of the sub-pixel colors have a similar IV characteristic (for example the red and green sub-pixels) and only one differs (for example the blue sub-pixel then two rather than three sub-pixel power supplies may be provided). This can simplify electrode line routing on the display glass (substrate), sometimes significantly. 
     Additionally or alternatively sub-sections of the display may be supplied and monitored separately in applications where peak luminescences and thus drive levels, can vary significantly (and systematically) between different areas of the display, thus enabling further savings to be made. 
     In addition to the above techniques it can also be possible to drop the supply voltage further and compensate the lower OLED drive currents on some of the drive transistors by increasing the corresponding gate voltages in response. Preferably this is done with knowledge of the (average) electrical characteristics of the drive transistors, so that this information (in effect a graph) can be used to determine an increase in gate voltage needed to compensate a particular supply voltage reduction. Such characteristics may, for example, be stored in non-volatile memory in the driver. 
       FIG. 3  shows a block diagram  300  of a display driver for an active matrix display  302 , configured to control V ss  in accordance with the available active matrix pixel drive voltage to increase the power efficiency of the display plus driver combination. 
     In  FIG. 3  the active matrix display  302  has a plurality of row electrodes  304   a - e  and a plurality of column electrodes  308   a - e  each connecting to internal respective row and column lines  306 ,  310  of which, for clarity, only two are shown. Power (V ss )  312  and ground  318  connections are also provided, again connected to respective internal conducting traces  314  and  316  to provide power to the pixels of the display. For clarity a single pixel  320  is illustrated, connected as shown to V ss , ground, row, and column lines  314 ,  316 ,  306 , and  310 . It will be recognised that in practice a plurality of such pixels is provided generally, but not necessarily, arranged in a rectangular grid and addressed by row and column electrodes  304 ,  308 . The active matrix pixel  320  may comprise any conventional active matrix pixel driver circuit. 
     In operation each row of active matrix display  302  is selected in turn by appropriately driving row electrodes  304  and, for each row, the brightness of each pixel in a row is set by driving, preferably simultaneously, column electrodes  308  with brightness data. This brightness data as described above, may comprise either a current or a voltage. Once the brightnesses of the pixels in one row have been set the next row may be selected and the process repeated, the active matrix pixels including a memory element, generally a capacitor, to keep the row illuminated even when not selected. Once data has been written to the entire display, the display only needs to be updated with changes to the brightness of pixels. 
     Power to the display is provided by a battery  324  and a power supply unit  322  to provide a regulated V ss  output  328 . Power supply  322  has a voltage control input  326  to control the voltage on output  328 . Preferably power supply  322  is a switch mode power supply with rapid control of the output voltage  328 , typically on a microsecond time scale where the power supply operates at a switching frequency 1 MHz or greater. Use of a switch mode power supply also facilitates use of a low battery voltage which can be stepped up to the required V ss  level, thus assisting compatibility with, for example low voltage consumer electronic devices. 
     The row select electrodes  304  are driven by row select drivers  330  in accordance with a control input  332 . Likewise the column electrodes  308  are driven by column data drivers  334  in response to a data input  336 . In the illustrated embodiment each column electrode is driven by an adjustable constant current generator  340 , in turn controlled by a digital-to-analogue converter  338  coupled to input  336 . For clarity only one such constant current generator is shown. 
     The constant current generator  340  has a current output  344  to source or sink a substantially constant current. The constant current generator  340  is connected to a power supply drive V drive    342 , which may be equal and connected to V ss  or which may be greater than (here, more negative than) V ss  to allow active matrix pixel  320  to be driven harder than V ss . The voltage for V drive  may be provided, for example, by a separate output from power supply unit  322 . 
     The embodiment of the display driver illustrated in  FIG. 3  shows a current-controlled active matrix display in which a column electrode current to set a pixel brightness. It will be appreciated that a voltage-controlled active matrix display, in which the brightness of a pixel is set by the voltage on a column line, could also be employed by using voltage rather than current drivers for column data drivers  334 . 
     The control input  332  of row select drivers  330  and the data input  336  of column data drivers  334  are both driven by display drive logic circuitry  346  which may, in some embodiments, comprise a microprocessor. The display drive logic  346  is clocked by a clock  348  and, in the illustrated embodiment, has access to a frame store  350 . Pixel brightness and/or color data for display on display  302  is written to display drive logic  346  and/or frame store  350  by means of data bus  352 . 
     The display drive logic has a sense input  356  driven from the output of a current sensing device  354 . This may comprise, for example, an analogue-to-digital converter configured to sense the voltage drop across a resistor. This is used to monitor the current drawn by display  302  from output  328  of power supply  322 . In embodiments in which a plurality of power supply lines are monitored a plurality of converters or a multiplexed converter may be employed. Optionally (but not shown in  FIG. 3 ) the supply voltage V ss  may also be monitored. 
     The display drive logic  346  (which may be implemented by a processor under stored program control or in hardware or in a combination of the two) includes a current sense unit  358  and a power controller  360  (in this example both implemented by processor control code stored in non-volatile memory). The current sense unit  358  inputs a current signal on sense input  356  and the power controller  360  outputs a voltage control signal to input  326  of power supply unit  322  to control power supply voltage V ss  in response to the sensed input voltage. Operation of the power controller is described in more detail below with reference to  FIG. 4 . 
       FIG. 4  shows a flow diagram of a procedure which may be implemented by power controller  360  in embodiments of a display driver for driving an active matrix display. The general procedure is suitable for both current- and voltage-programmed active matrix displays. 
     Referring to  FIG. 4 , the display controller  346 , at step S 400 , inputs a current sense signal which it then compares (step S 402 ) with a control condition. This control condition comprises a test to determine whether the current has begun to dip significantly and, in one embodiment, may therefore be implemented by determining a change in sensed current since a previous measurement, either in absolute terms or as a percentage, and then comparing this with a threshold such as two percent, five percent, ten percent. 
     If comparison with the control condition indicates that the power supply voltage can be reduced without significant loss of TFT driver transistor saturation, for example because the change in current is less than a pre-determined threshold, then at step S 404  V ss  is reduced and the procedure loops back to step S 400 . If, however, comparison with the control condition indicates that the one or more TFT driver transistors with the highest drive (which should be closest to saturation) are just leaving saturation then, at step S 406 , V ss  is increased and the procedure again loops back to step S 400 . 
     The skilled person will appreciate that a variety of conditions will be employed as the control condition, depending upon the particular application. In embodiments where the active matrix display has two or more separate power supply lines, for example for two or more separate sub-pixels of the display then separate control loops are shown in  FIG. 4 , optionally with different control conditions, may be employed for each separate power supply lines. 
     No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.