Generated by PDFKit.NET Evaluation
`
`(121UK Patent Application (191GB (1112 389 952 (13)A
`
`(43) Date of A Publication
`
`24.12.2003
`
`(21) Application No:
`
`(22) Date of Filing:
`
`0213989.7
`
`18.06.2002
`
`(51) INTCL7 :
`G09G 3/ 32 / / H05B 33/ 14
`
`(71) Applicant(s):
`Cambridge Display Technology Limited
`(Incorporated In the United Kingdom)
`Greenwich House, Madlngley Rise,
`Madlngley Road, CAMBRIDGE, CB3 OTX,
`United Kingdom
`
`(72) lnventor(s):
`Paul Richard Routley
`Euan Christopher Smith
`
`(74) Agent and/or Address for Service:
`Marks & Clerk
`66/68 HIiis Road, CAMBRIDGE,
`Cambrideshlre, CB21LA, United Kingdom
`
`(52) UK CL (Edition V ):
`G5CCHBN
`
`(56) Documents Cited:
`EP 1227467 A2
`JP2001236040 A
`
`WO 2001/027910 A1
`US 5594463 A
`
`(58) Field of Search:
`UK CL (Edition T ) G5C CHBM CHBN
`INT CL7 G09F9/30 9/33, G09G 3/12 3/20 3/30 3/32
`3/36, H05B 33/12 33/14
`Other: Online: EPODOC, JAPIO, WPI
`
`(54) Abstract litle: Driver circuits for electroluminescent displays with reduced power consumption
`
`(57) Display driver circuits for an organic light emitting
`diode display, particularly a passive matrix display
`with greater efficiency. The display 302 comprises at
`least one electroluminescent display element, and
`the driver including at least one constant current
`generator 520 for driving the display element. The
`display driver control circuitry comprises a drive
`voltage sensor 526 for sensing a voltage on a first line
`in which the current is regulated by said constant
`current generator. A voltage controller 528 is coupled
`to the drive voltage sensor for controlling the voltage
`of a supply 514, 515 for the constant current
`generator in response to the sensed voltage, and
`configured to control said supply voltage to increase
`the efficiency of the display driver. The voltage
`controller may be adapted to control the display on a
`row by row basis, or as a complete frame.
`
`00
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`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 001
`
`

`

`Generated by PDFKit.NET Evaluation
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`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 002
`
`

`

`Generated by PDFKit.NET Evaluation
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`LG Display Co., Ltd.
`Exhibit 1014
`Page 003
`
`

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`(PRIOR ART)
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 004
`
`

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`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 005
`
`

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`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 006
`
`

`

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`
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`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 007
`
`

`

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`
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`
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`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 008
`
`

`

`8/10
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`
`....................................... --................................................................ -.......................... . ....... ---------.. ..... -------·-·
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`
`Figure 8
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 009
`
`

`

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`
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`
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`
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`
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`
`Figure 9
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 010
`
`

`

`(
`
`10/10
`
`S1000
`
`''
`
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`AND RESET PEAK DETECTOR
`
`S1002
`
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`FOR ROW
`
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`
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`
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`VARY POWER SUPPLY Vs
`
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`
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`
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`VARIATION IN MAX Xi
`
`'
`S1008
`DETERMINE WHETHER CURRENT
`GENERATOR AT OR BEYOND
`COMPLIANCE LIMIT
`
`S1010
`YES
`
`COMPLIANCE LIMIT
`EXCEEDED?
`
`NO
`
`S1012
`
`S1014
`
`OUTPUT CONTROL SIGNAL TO
`INCREASE POWER SUPPLY Va
`
`OUTPUT CONTROL SIGNAL TO
`REDUCE POWER SUPPLY Va
`
`Figure 10
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 011
`
`

`

`1
`
`DISPLAY DRNER CIRCUITS
`
`This invention generally relates to display driver circuits for electro-optic displays, and
`
`more particularly relates to circuits and methods for driving organic light emitting diode
`
`displays, especially passive matrix displays, with greater efficiency.
`
`Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of
`
`electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing
`
`angle and are easy and cheap to fabricate on a variety of substrates. Organic LEDs may
`
`be fabricated using either polymers or small molecules in a range of colours ( or in
`
`multi-coloured displays), depending upon the materials used. Examples ofpolymer(cid:173)
`
`based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160;
`
`examples of so called small molecule based devices are described in US 4,539,507.
`
`A basic structure 100 of a typical organic LED is shown in Figure 1 a. A glass or plastic
`
`substrate 102 supports a transparent anode layer 104 comprising, for example, indium
`
`tin oxide (ITO) on which is deposited a hole transport layer 106, an electroluminescent
`
`layer I 08, and a cathode 110. The electroluminescent layer 108 may comprise, for
`
`example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer 106, which
`
`helps match the hole energy levels of the anode layer 104 and electroluminescent layer
`
`108, may comprise, for example, PEDOT:PSS (polystyrene-sulphonate-doped
`
`polyethylene-dioxythiophene). Cathode layer 110 typically comprises a low work
`
`function metal such as calcium and may include an additional layer immediately
`
`adjacent electroluminescent layer 108, such as a layer of aluminium, for improved
`
`electron energy level matching. Contact wires 114 and 116 to the anode the cathode
`
`respectively provide a connection to a power source 118. The same basic structure may
`
`also be employed for small molecule devices.
`
`In the example shown in Figure 1 a light 120 is emitted through transparent anode 104
`
`and substrate 102 and such devices are referred to as "bottom emitters". Devices which
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 012
`
`

`

`2
`
`emit through the cathode may also be constructed, for example by keeping the thickness
`
`of cathode layer 110 less than around 50-100 run so that the cathode is substantially
`
`transparent.
`
`Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or
`
`multi-colour pixellated display. A multicoloured display may be constructed using
`
`groups of red, green, and blue emitting pixels. In such displays the individual elements
`
`are generally addressed by activating row ( or column) lines to select the pixels, and
`
`rows (or columns) of pixels are written to, to create a display. So-called active matrix
`
`displays have a memory element, typically a storage capacitor and a transistor,
`
`associated with each pixel whilst passive matrix displays have no such memory element
`
`and instead are repetitively scanned, somewhat similarly to a TV picture, to give the
`
`impression of a steady image.
`
`Figure 1 b shows a cross section through a passive matrix OLED display 150 in which
`
`like elements to those of Figure la are indicated by like reference numerals. In the
`
`passive matrix display 150 the electroluminescent layer 108 comprises a plurality of
`
`pixels 152 and the cathode layer 110 comprises a plurality of mutually electrically
`
`insulated conductive lines 154, running into the page in Figure 1 b, each with an
`
`associated contact 156. Likewise the ITO anode layer 104 also comprises a plurality of
`
`anode lines 158, of which only one is shown in Figure lb, running at right angles to the
`
`cathode lines. Contacts (not shown in Figure lb) are also provided for each anode line.
`
`An electroluminescent pixel 152 at the intersection of a cathode line and anode line may
`
`be addressed by applying a voltage between the relevant anode and cathode lines.
`
`Referring now to Figure 2a, this shows, conceptually, a driving arrangement for a
`
`passive matrix OLED display 150 of the type shown in Figure 1 b. A plurality of
`
`constant current generators 200 are provided, each connected to a supply line 202 and to
`
`one of a plurality of column lines 204, of which for clarity only one is shown. A
`
`plurality of row lines 206 (of which only one is shown) is also provided and each of
`
`these may be selectively connected to a ground line 208 by a switched connection 210.
`
`As shown, with a positive supply voltage on line 202, column lines 204 comprise anode
`
`connections 158 and row lines 206 comprise cathode connections 154, although the
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 013
`
`

`

`cotU1ections would be reversed if the power supply line 202 was negative and with
`
`respect to ground line 208.
`
`3
`
`As illustrated pixel 212 of the display has power applied to it and is therefore
`
`illuminated. To create an image cotU1ection 210 for a row is maintained as each of the
`
`column lines is activated in tum until the complete row has been addressed, and then the
`
`next row is selected and the process repeated. Alternatively a row may be selected and
`
`all the columns written in parallel, that is a row selected and a current driven onto each
`
`of the column lines simultaneously, to simultaneously illuminate each pixel in a row at
`
`its desired brightness. Although this latter arrangement requires more column drive
`
`circuitry it is preferred because it allows a more rapid refresh of each pixel. In a further
`
`alternative arrangement each pixel in a column may be addressed in tum before the next
`
`column is addressed, although this is not preferred because of the effect, inter alia, of
`
`column capacitance as discussed below. It will be appreciated that in the arrangement
`
`of Figure 2a the functions of the column driver circuitry and row driver circuitry may be
`
`exchanged.
`
`It is usual to provide a current-controlled rather than a voltage-controlled drive to an
`
`OLEO because the brightness of an OLEO is determined by the current flowing through
`
`it, this determining the number of photons it outputs. In a voltage-controlled
`
`configuration the brightness can vary across the area of a display and with time,
`
`temperature, and age, making it difficult to predict how bright a pixel will appear when
`
`driven by a given voltage. In a colour display the accuracy of colour representations
`
`may also be affected.
`
`Figures 2b to 2d illustrate, respectively, the current drive 220 applied to a pixel, the
`
`voltage 222 across the pixel, and the light output 224 from the pixel over time 226 as
`
`the pixel is addressed. The row containing the pixel is addressed and at the time
`
`indicated by dashed line 228 the current is driven onto the column line for the pixel.
`
`The column line (and pixel) has an associated capacitance and thus the voltage
`
`gradually rises to a maximum 230. The pixel does not begin to emit light until a point
`
`232 is reached where the voltage across the pixel is greater than the OLEO diode
`
`voltage drop. Similarly when the drive current is turned off at time 234 the voltage and
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 014
`
`

`

`4
`
`light output gradually decay as the column capacitance discharges. Where the pixels in
`
`a row are all written simultaneously, that is where the columns are driven in parallel, the
`
`time interval between times 228 and 234 corresponds to a line scan period.
`
`It is desirable for many applications, but by no means essential, to be able to provide a
`
`greyscale-type display, that is one in which the apparent brightness of individual pixels
`
`may be varied rather than simply set either on or off. Here "greyscale" refers to such a
`
`variable brightness display, whether a pixel is white or coloured.
`
`The conventional method of varying pixel brightness is to vary pixel on-time using
`
`Pulse Width Modulation (PWM). In the context of Figure 2b above the apparent pixel
`
`brightness may be varied by varying the percentage of the interval between times 228
`and 234 for which drive current is applied. In a PWM scheme a pixel is either full on or
`
`completely off but the apparent brightness of a pixel varies because of time integration
`
`within the observer's eye.
`
`Pulse Width Modulation schemes provide a good linear brightness response but to
`
`overcome effects related to the delayed pixel tum-on they generally employ a pre(cid:173)
`
`charge current pulse (not shown in Figure 2b) on the leading edge 236 of the driving
`
`current waveform, and sometimes a discharge pulse on the trailing edge 238 of the
`
`waveform. As a result, charging (and discharging) the column capacitance can account
`
`for roughly half the total power consumption in displays incorporating this type of
`
`brightness control. Other significant factors which the applicant has identified as
`
`contributing to the power consumption of a 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 a practical circuit, the effects of a limited
`
`current driver compliance, as explained in more detail later.
`
`Figure 3 shows a schematic diagram 300 of a generic driver circuit for a passive matrix
`
`OLED display. The OLED display is indicated by dashed line 302 and comprises a
`
`plurality n ofrow lines 304 each with a corresponding row electrode contact 306 and a
`
`plurality m of column lines 308 with a corresponding plurality of column electrode
`
`contacts 310. An OLED is connected between each pair of row and column lines with,
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 015
`
`

`

`5
`
`in the illustrated arrangement, its anode 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, selectively connecting the row lines to ground. The y-driver 314 and x-driver
`
`316 are typically both under the control of a processor 318. A power supply 320
`
`provides power to the circuitry and, in particular, to y-driver 314.
`
`Specific examples of OLED display drivers are described in US 6,014,119, US
`
`6,201,520, US 6,332,661, EP l,079,361A and EP 1,091,339A; OLED display driver
`
`integrated circuits are also sold by Clare Micronix of Clare, Inc., Beverly, MA, USA.
`
`The Clare Micronix drivers provide a current controlled drive and achieve greyscaling
`
`using a conventional PWM approach; US 6,014,119 describes a driver circuit in which
`
`pulse width modulation is used to control brightness; US 6,201,520 describes driver
`
`circuitry in which each column driver has a constant current generator to provide digital
`
`(on/off) pixel control; US 6,332, 661 describes pixel driver circuitry in which a
`
`reference current generator sets the current output of a constant current driver for a
`
`plurality of columns, but again this arrangement is not suitable for variable brightness
`
`displays; and EP l,079,361A and EP l,091,339A both describe similar drivers for
`
`organic electroluminescent display elements in which a voltage drive rather than a
`
`current drive is employed.
`
`It is generally desirable to reduce the power consumption of the display plus driver
`
`combination, particularly whilst retaining the ability to provide a greyscale display. It is
`
`further desirable to reduce the maximum required power supply voltage for the display
`
`plus driver combination.
`
`Prior art techniques for reducing the power consumption of liquid crystal displays
`
`(LCDs) are described in US 6,323,849 and EP O 811 866A. US 6,323,849 describes an
`
`LCD display with a partial display mode in which a control circuit controls display
`
`drivers to tum off a portion of the display which does not show useful information.
`
`When the LCD module is in a partial display mode the line frequency may also be
`
`reduced whilst maintaining the same frame refresh rate, allowing a lower voltage to be
`
`used to produce the same amount of charge. However, a user must predetermine which
`
`portion of the display is to be used, which will typically require additional control
`
`LG Display Co., Ltd.
`Exhibit 1014
`Page 016
`
`

`

`6
`
`functions and software in the device for which the display is provided. EP O 811 866A
`
`describes a similar technique, albeit with a more flexible driving arrangement. An
`
`improved reduced power consumption.display driver which provides for more
`
`transparent user implementation is described in the applicant's co-pending UK patent
`
`application number 0209502.4.
`
`US 4,823,121 describes an electroluminescent (EL) panel driving system which detects
`
`the absence of a HIGH level signal representing a spot illumination of the EL panel in
`
`the image data of a line and, in response to this, prevents four circuits (a pre-charge
`
`circuit, a pullup circuit, a write-in circuit and a source circuit) from being activated.
`
`However the power savings provided by this technique are specific to the drive
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`arrangement for the type of electroluminescent panel described and are not readily
`
`generalisable. Furthermore the savings are relatively modest.
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`Figure 4a shows a typical light intensity-voltage curve 400 for an OLEO which, as can
`
`be seen, is non-linear and exhibits a dead region corresponding to the OLEO turn-on
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`voltage (typically 1.5V - 2V). It is desirable to operate an OLEO display at a lower
`
`rather than a higher voltage as this increases the device's efficiency (light output in
`
`terms of energy input) and reduces the degradation rate. Resistive losses are also
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`reduced and, where image data is changing, capacitive losses (which depend upon the
`
`square of the voltage) are also reduced.
`
`Figure 4b shows a light intensity-current curve 402 for an OLEO which, by contrast
`
`with curve 400, is approximately linear.
`
`Figure 4c shows, schematically, a current driver 402 for one column line of a passive
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`matrix OLEO display, such as the display 302 of Figure 3. Typically a plurality of such
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`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.
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`A particularly advantageous fonn of current driver 402 is described in the applicant's
`
`co-pending British patent application no. 0126120.5 entitled" Display Driver Circuits".
`
`The current driver 402 of Figure 4c outlines the main features of this circuit and
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`Exhibit 1014
`Page 017
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`7
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`comprises a current driver block 406 incorporating a bipolar transistor 416 which has an
`
`emitter terminal substantially directly connected to a power supply line 404 at supply
`
`voltage Vs· {This does not necessarily require that the emitter terminal should be
`
`connected to a power supply line or terminal for the driver by the most direct route but
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`rather that there should preferably be no intervening components, apart from the
`
`intrinsic resistance of tracks or connections within the driver circuitry between the
`
`emitter and a power supply rail). A column drive output 408 provides a current drive to
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`OLED 412, which also has a ground connection 414, normally via a row driver MOS
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`switch (not shown in Figure 4c ). A current control input 410 is provided to current
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`driver block 406 and, for the purposes of illustration, this is shown connected to the
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`base of transistor 416 although in practice a current mirror arrangement is preferred.
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`The signal on current control line 410 may comprise either a voltage or a current signal
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`and this is preferably provided from a digital-to-analogue converter (not shown in
`
`Figure 4c) for ease of interfacing.
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`A current source attempts to deliver a substantially constant current to the load to which
`
`it is connected but it will be appreciated that there will come a point as its output
`
`voltage approaches the supply voltage, at which this is no longer possible. The range of
`
`voltages over which a current source provides an approximately constant current to a
`
`load is termed the compliance of the current source. The compliance can be
`
`characterised by (Vs-V 0) where Vs is the supply voltage and VO is substantially the
`maximum output voltage of the current source in that when Vs-V O is small the
`
`compliance is high, and vice-versa. (For convenience in this document reference will
`
`be made to a current source and to current sources but these may be substituted by a
`
`current sinks or sinks).
`
`The arrangement of Figure 4c is useful because the (optionally variable) current
`
`generator has a high compliance, that is a low value of Vs-Vo. The lower the current
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`driver compliance (i.e. the greater V5-V0 ), the greater the power losses due to limited
`
`driver compliance. The lower the driver circuit compliance the greater the supply
`
`voltage to the current driver must be in order to obtain a maximum desired pixel
`
`brightness, and hence the greater the power loss. This is particularly the case where
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`Exhibit 1014
`Page 018
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`pixel brightness is varied by varying the drive current rather than by, for example, pulse
`
`width modulation.
`
`8
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`As previously explained current control is preferable to voltage control for an OLED
`
`because this helps to overcome the non-linearity of the light voltage curve shown in
`
`Figure 4a, the light-current curve for an OLED being substantially linear. Figure 4d
`
`shows a graph 420 of current drawn from a power supply against a power supply
`
`voltage for an organic LED display element driven from a controllable constant current
`
`source. This curve has an initial "dead" region in which substantially no current flows
`
`until the forward voltage is sufficient to turn the OLED on. A non-linear region 422 is
`
`then followed by a substantially flat portion 424 of the curve above a voltage indicated
`
`by dashed line 426, giving a generally 'S' shaped curve. At the voltage indicated by
`
`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
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`voltage required to ensure that the constant current source is well behaved 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 merely increased the excess, wasted power dissipation and it is therefore
`
`preferable to operate at or near the compliance limit indicated by dashed line 426 to
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`minimise this wasted power. However, the power supply voltage for this compliance
`
`limit depends upon a number of factors including display age, display temperature and,
`
`where a variable current drive is employed, upon the current being provided by the
`
`constant current source. For example with an OLED at a constant brightness (that is at
`
`a substantially constant drive current) the voltage across the OLED falls as its
`
`temperature increases, and vice-versa. For those reasons a large overhead is generally
`
`built into the supply voltage to ensure that the combination of the display and its driver
`
`is able to perform according to a desired specification and across a temperature range.
`
`A consequence of this is that over much of a specified temperature range and/or when at
`
`less than maximum brightness the driven display is likely to be operating at
`
`significantly less than its maximum efficiency.
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`Exhibit 1014
`Page 019
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`9
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`The applicants have recognised that significant power savings may be achieved with
`
`emissive display technology, and in particular with organic light emitting diode-based
`
`displays, by sensing a drive voltage to the display and controlling a power supply to a
`
`constant current driver for the display. The applicants have recognised that especially
`
`significant savings may be made by controlling the power supply so that the constant
`
`current driver operates at or near its compliance limit.
`
`According to a first aspect of the present invention there is therefore provided display
`
`driver control circuitry for controlling a display driver for an electroluminescent display,
`
`the display comprising at least one electroluminescent display element, the driver
`
`including 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 in which the current is regulated by said constant current generator; and a
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`voltage controller coupled to said drive voltage sensor for controlling the voltage of a
`
`supply for said constant current generator in response to said sensed voltage, and
`
`configured to control said supply voltage to increase the efficiency of said display
`
`driver.
`
`Controlling the supply voltage to the at least one constant current generator, which may
`
`be a current source or a current sink, in response to a voltage on a line in which the
`
`current is regulated by the constant current generator allows the supply voltage to be
`
`varied automatically as external factors such as temperature, display age and current
`
`drive change in order to achieve more efficient operation of the display driver and more
`
`particularly a reduced power consumption for the display and driver combination for the
`
`same perceived level of brightness. Thus the power supply voltage may be reduced
`
`when it is greater than that needed by the constant current generator in order to provide
`
`its regulated current, and preferably also increased where the supply voltage is
`
`insufficient. The display driver control circuit may be retro-fitted to existing display
`
`driver circuitry to increase its efficiency, in which case, the drive voltage sensor may be
`
`arranged to sense 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 or of the control circuitry or power may be supplied by a separate,
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`Exhibit 1014
`Page 020
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`10
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`controllable module. The constant current generator may comprise an adjustable or
`
`controllable constant current generator, for example to provide variable pixel brightness
`
`for colour, or it may provide a substantially fixed current source or sink, for example in
`
`displays in which pixel brightness is varied by pulse width modulation (PWM) or where
`
`pixel brightness is fixed.
`
`Preferably the voltage controller is configured to reduce the supply voltage to the
`
`constant current generator when such a reduction will not substantially reduce the
`
`regulated current sourced or sunk by the current generator and/or when such a reduction
`
`will not substantially change the perceived brightness of the display element driven by
`
`the constant current generator. Broadly speaking this amounts to permitting 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 limit of
`
`compliance. Preferably the voltage controller is configured to control the supply
`
`voltage so that the constant current generator operates in the vicinity of the compliance
`
`limit. Generally operating either slightly above or slightly below the compliance limit,
`
`which may not be a hard limit, will provide satisfactory results and, in some
`
`embodiments, the supply voltage may be controlled by means of a feedback mechanism
`
`which allows or requires the supply voltage at times to be either side of the compliance
`
`limit. However, preferably the supply voltage is controlled so that it is held
`
`substantially at a voltage which, for the purposes of the control circuitry, represents a
`
`sufficiently close approximation to the compliance limit that any variations in pixel
`
`brightness due to the supply voltage control are difficult to discern by a human observer
`
`under normal operating conditions. Preferably the control circuitry includes means to
`
`determine such a compliance limit which, as will be appreciated, need not exactly
`
`correspond with what might be termed an actual compliance limit determined, for
`
`example, by inspection of a graph such as that shown in Figure 4d (which to some
`
`extent is an idealisation).
`
`The control circuitry preferably further includes a supply voltage sensor for sensing the
`
`supply voltage to the constant current generator; in embodiments the same sensor may
`
`be employed for sensing both the voltage on an output (44 sink) of the current generator
`
`and the voltage on an input for power supply to the current generator. The voltage
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`Exhibit 1014
`Page 021
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`11
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`controller may then include means to determine a difference between the supply voltage
`
`and the drive voltage on the first line, to facilitate determination of whether or not the
`
`constant current generator is operating in the vicinity of its compliance limit. Although
`
`the control circuitry can be employed with a display driver having only a single constant
`
`current generator, advantageously the display driver has a plurality of constant current
`
`generators for simultaneously driving a corresponding plurality of display elements,
`
`such as the display elements in a row of a passive matrix display. Then the control
`
`circuitry preferably determines the maximum voltage on an output of one of the
`
`constant current generators and controls the power supply voltage in response to this
`
`maximum sensed voltage. The display element or pixel driven at this maximum voltage
`
`will, broadly speaking, be the most inefficient display element for pixel amongst those
`
`having the maximum brightness at any one time. Where the simultaneously driven
`
`display elements comprise display elements in a row of a pixellated display, the supply
`
`voltage may be controlled based upon the maximum voltage of current g