WO2008001288A1

WO2008001288A1 - Electrophoretic display devices

Info

Publication number: WO2008001288A1
Application number: PCT/IB2007/052424
Authority: WO
Inventors: Nigel D. Young
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-06-26
Filing date: 2007-06-22
Publication date: 2008-01-03
Also published as: TW200809718A

Abstract

An active matrix electrophoretic display device has pixel control circuits having a sensor (46) for detecting movement of the electrophoretic display particles (34) and timing circuitry for converting a drive signal into a time period during which a control signal is applied to the pixel following a detection event from the sensor. The sensing of particle movement compensates for the way pixel drive history influences the response of the pixel to a drive signal, but without requiring a complicated drive scheme. Instead, a drive signal is applied to the pixel, and the pixel circuit generates the appropriate duration of pixel driving to obtain the desired grey level.

Description

Electrophoretic display devices

This invention relates to electrophoretic display devices.

Electrophoretic display devices are one example of bistable display technology, which use the movement of particles within an electric field to provide a selective light scattering or absorption function.

In one example, white particles are suspended in an absorptive liquid, and the electric field can be used to bring the particles to the surface of the device. In this position, they may perform a light scattering function, so that the display appears white. Movement away from the top surface enables the color of the liquid to be seen, for example black. In another example, there may be two types of particle, for example black negatively charged particles and white positively charged particles, suspended in a transparent fluid. There are a number of different possible configurations.

It has been recognized that electrophoretic display devices enable low power consumption as a result of their bistability (an image is retained with no voltage applied), and they can enable thin display devices to be formed as there is no need for a backlight or polarizer. They may also be made from plastics materials, and there is also the possibility of low cost reel-to-reel processing in the manufacture of such displays.

If costs are to be kept as low as possible, passive addressing schemes are employed. The most simple configuration of display device is a segmented reflective display, and there are a number of applications where this type of display is sufficient. A segmented reflective electrophoretic display has low power consumption, good brightness and is also bistable in operation, and therefore able to display information even when the display is turned off. However, improved performance and versatility is provided using a matrix addressing scheme. An electrophoretic display using passive matrix addressing typically comprises a lower electrode layer, a display medium layer, and an upper electrode layer. Biasing voltages are applied selectively to electrodes in the upper and/or lower electrode layers to control the state of the portion(s) of the display medium associated with the electrodes being biased.

Another type of electrophoretic display device uses so-called "in plane switching". This type of device uses movement of the particles selectively laterally in the display material layer. When the particles are moved towards lateral electrodes, an opening appears between the particles, through which an underlying surface can be seen. When the particles are randomly dispersed, they block the passage of light to the underlying surface and the particle color is seen. The particles may be colored and the underlying surface black or white, or else the particles can be black or white, and the underlying surface colored. An advantage of in-plane switching is that the device can be adapted for transmissive operation, or trans flective operation. In particular, the movement of the particles creates a passageway for light, so that both reflective and transmissive operation can be implemented through the material. This enables illumination using a backlight rather than reflective operation. The in-plane electrodes may all be provided on one substrate, or else both substrates may be provided with electrodes.

This invention relates in particular to in-plane switching devices, in which there is lateral movement of particles.

Active matrix addressing schemes are also used for electrophoretic displays, and these are generally required when bright full color displays with high resolution greyscale are required. Such devices are being developed for signage and billboard display applications, and as (pixellated) light sources in electronic window and ambient lighting applications. Colors can be implemented using color filters, and the display pixels then function simply as greyscale devices. The description below refers to greyscales and grey levels, but it will be understood that this does not in any way suggest only monochrome display operation.

Electrophoretic displays are typically driven by complex driving signals. For a pixel to be switched from one grey level to another, often it is first switched to white or black as a reset phase and to then to the final grey level. Grey level to grey level transitions and black/white to grey level transitions are slower and more complicated than black to white, white to black, grey to white or grey to black transitions.

This reset operation is used in order to achieve satisfactory grey level accuracy. Typically, the display pixels are reset to either the positive or the negative rail depending on the final image (i.e. to either black or white). The pixel is reset to black if the target grey level is closer to black than to white, and vice versa. This results in a visually more attractive image transition compared to the more simply example of always resetting to black or to white, because the result of such a reset sequence is to produce momentarily a black and white image of the final greyscale image.

It has also been proposed to provide additional control signals before the transition to the final grey level, to implement a so-called shaking phase, which functions as a preparatory drive phase before the particles are moved to implement new grey levels. This is used in order to speed up the subsequent grey level transition phase, and to reduce the dependency of the response of the display to the previous history.

Further discussion of known drive schemes can be found in WO 2005/071651 and WO 2004/066253.

This invention aims to provide a simplified drive scheme which can overcome the problem of the display output being dependent on history, which is the result of so-called "sticking" of the ink particles to the electrodes at which they are located at the beginning of the drive phase.

According to the invention, there is provided an active matrix electrophoretic display device, comprising: an array of rows and columns of display pixels; and control means for supplying drive signals to the pixels to drive the pixels to predetermined optical states corresponding to an image to be displayed, wherein each pixel comprises a pixel control circuit, comprising: a sensor for detecting movement of the electrophoretic display particles; and timing circuitry for converting a drive signal into a time period during which a control signal is applied to the pixel following a detection event from the sensor.

The device of the invention includes pixel circuitry which enables a desired drive level to be converted into a time period for application of a control signal (for example a constant voltage rail) to the pixel, following initial movement of the pixel particles. The sensing of particle movement compensates for the way pixel drive history influences the response of the pixel to a drive signal, but without requiring a complicated drive scheme. Instead, a drive signal is applied to the pixel, and the pixel circuit generates the appropriate duration of pixel driving to obtain the desired grey level.

Each pixel preferably comprises a storage electrode and a drive electrode, and the electrophoretic display particles are adapted to be collected at the storage electrode before the pixel is driven to a selected optical state. The sensor then detects when particles have started to move away from the storage electrode.

The sensor can comprise an optical sensor for detecting the presence of particles at a location adjacent the storage electrode, and the detection event comprises the detection of the presence of particles by the sensor after the control signal is applied to the pixel to drive the pixel from a reset state to a display state. The sensor may comprise a photodiode, which can easily be integrated into the pixel circuitry.

The timing circuitry may comprise a data storage capacitor and a discharge path, wherein the discharge path is enabled by the sensor signal. The discharge path discharges the storage capacitor at a given rate (which may or may not be constant) and the time taken is thus a function of the original data stored on the data storage capacitor.

The timing circuitry can further comprise a buffer circuit which is adapted to apply a substantially constant drive voltage to the pixel until the data storage capacitor has been discharged to a predetermined voltage by the discharge path. The drive of the pixel is thus constant, and the data signal is converted entirely into a time signal, to implement a pulse width modulation type drive scheme.

The discharge path can comprise a constant current source having a switching transistor, with the switching transistor controlled in dependence on the sensor output.

However, in an alternative embodiment, the discharge path can comprise a current source transistor which is controlled in dependence on the particle movement velocity. This enables the drive scheme additionally to compensate for temperature and other conditions (such as pressure and age of the device) which affect the particle movement characteristics. The sensor then comprises first and second detectors each having a detection event, and the time between the two detection events is used to control the current source transistor.

To do this, the sensor can comprise a storage capacitor for providing a drive voltage to the current source transistor, and the storage capacitor is adapted to be partially discharged during the time between the two detection events, thereby to control the current output of the current source transistor. The invention also provides a method of driving an electrophoretic display device, comprising an array of rows and columns of display pixels, the method comprising, for each pixel: supplying a drive signal to the pixel to drive the pixel to a predetermined optical state corresponding to an image to be displayed; applying a control signal to the pixel; detecting movement of the electrophoretic display particles in response to the control signal; and converting the drive signal into a time period during which the control signal is applied to the pixel following the detection.

This provides a method of operating the display device of the invention, with a simple drive scheme which externally simply requires the provision of a drive signal (in addition to a preceding reset signal). A reset signal can be applied to the pixel before supplying the drive signal, thereby to collect the particles at a storage electrode. As outlined above, detecting movement can comprise detecting (for example optically sensing) the presence of particles at a location adjacent the storage electrode. Converting the drive signal into a time period can take into account the particle movement velocity.

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows schematically one known type of device to explain the basic technology; Fig. 2 shows in schematic form the electric circuit control for the device of

Fig. 1;

Fig. 3 shows a first example of display device of the invention; Fig. 4 is used to explain when particle movement is detected within the device of Fig. 3; Fig. 5 shows a first example of pixel control circuit for the device of Fig. 3;

Fig. 6 is a timing diagram to explain the operation of the circuit of Fig. 5; Fig. 7 shows a second example of pixel control circuit for the device of Fig. 3; Fig. 8 shows a second example of display device of the invention; and Fig. 9 shows an example of pixel control circuit for the device of Fig. 8.

The same references are used in different Figures to denote the same layers or components, and description is not repeated. The invention provides an active matrix electrophoretic display device and drive method in which each pixel comprises a pixel control circuit which detects movement of the electrophoretic display particles and converts a drive signal into a time period for a pulse width modulation type drive scheme which starts from when particle movement is detected. This enables compensation for previous history.

Before describing the invention in more detail, one example of the type of display device to which the invention relates will be described briefly.

Figure 1 diagrammatically shows a cross section of a portion of an electrophoretic display device 1, for example showing only a few display elements, comprising a base substrate 2, an electrophoretic film with an electronic ink which is present between two transparent substrates 3,4 for example PET (polyethylenenapthalate). One of the substrates 3 is provided with transparent picture electrodes 5 and the other substrate 4 with a transparent counter electrode 6.

The electronic ink comprises multiple micro capsules 7, of about 10 to 50 microns. Each micro capsule 7 comprises positively charged white particles 8 and negatively charged black particles 9 suspended in a fluid F. When a positive field is applied to the picture electrode 5, the white particles 8 move to the side of the micro capsule 7 directed to the counter electrode 6 and the display element become visible to a viewer.

Simultaneously, the black particles 9 move to the opposite side of the microcapsule 7 where they are hidden to the viewer. By applying a negative field to the picture electrodes 5, the black particles 9 move to the side of the micro capsule 7 directed to the counter electrode 6 and the display element becomes dark to a viewer (not shown). When the electric field is removed, the particles 8,9 remain in the acquired state and the display exhibits a bi-stable character and consumes substantially no power. Figure 2 shows diagrammatically an equivalent circuit of a display device 1 incorporating the display pixels of Figure 1, and comprising an electrophoretic film laminated on the base substrate 2 provided with active switching elements, a row driver 16 and a column driver 10. Preferably, the counter electrode 6 is provided on the film comprising the encapsulated electrophoretic ink, but it could be alternatively provided on a base substrate in the case of operation using in-plane electric fields.

The display device 1 is driven by active switching elements, in this example thin film transistors 19. The display thus comprises a matrix of display elements 18 at the area of crossing of row (selection) electrodes 17 and column (data) electrodes 11. The row driver 16 consecutively selects the row electrodes 17, while a column driver 10 provides a data signal to the column electrode 11. Preferably, a processor 15 firstly processes incoming data 13 into the data signals. Mutual synchronization between the column driver 10 and the row driver 16 takes place via drive lines 12. Select signals from the row driver 16 select the pixel electrodes 22 via the thin film transistors 19 whose gate electrodes 20 are electrically connected to the row electrodes 17 and the source electrodes 21 are electrically connected to the column electrodes 11.

A data signal present at the column electrode 11 is transferred to the pixel electrode 22 of the display element coupled to the drain electrode via the TFT. In the embodiment shown, an additional capacitor 23 is provided at the location at each display element 18, and is connected to one or more storage capacitor lines 24.

Instead of TFTs, other switching elements can be applied such as diodes, MIMs devices, etc.

This is an example of transverse field active matrix device, but the invention will be described with reference to its preferred implementation in an in-plane switching transmissive display device.

A known addressing scheme comprises a plurality of frames of "shaking" pulses, a plurality of frames during which a reset signal is applied to each pixel (reset phase) and a plurality of frames of drive signals to produce the desired grey level (drive phase). The invention aims to provide a simplified drive scheme and provides a display device designed for this purpose.

Figure 3 shows a first example of display device 30 of the invention, and shows one electrophoretic display cell.

The cell is bounded by side walls 32 to define a cell volume in which the electrophoretic ink particles 34 are housed. The example of Figure 3 is an in-plane switching transmissive pixel layout, with illumination 36 from a light source (not shown), and through a color filter 38.

The particle position within the cell is controlled by an electrode arrangement comprising a data (drive) electrode 40 and a storage electrode 42 which can be a common electrode to all pixels. The relative voltages on the electrodes 40 and 42 determine whether the particles move under electrostatic forces to the storage electrode 42 or the drive electrode 40.

The storage electrode 42 defines a region in which the particles are hidden from view, by a light shield 44. With the particles over the storage electrode 42, the pixel is in an optically transmissive state allowing the illumination 36 to pass to the viewer on the opposite side of the display, and the pixel aperture is defined by the size of the light transmission opening relative to the overall pixel dimension.

The invention provides a sensor 46 for detecting movement of the electrophoretic display particles 34, and this movement is used to start a timing operation, so that the drive scheme involves applying a drive signal to the drive electrode 40 for a length of time to provide the desired grey level. The light sensor is in the form of a photodiode and is shielded from ambient light 48 from the user side of the display by the light shield 44. The light sensor is close to the storage electrode 42 so that initial particle movement is detected and so that the light shield does not need to be enlarged significantly (which would reduce pixel aperture).

In a reset phase, the particles are collected at the storage electrode, and Figure 4 shows the display pixel of Figure 3 shortly after the application of a drive voltage following the reset phase. As schematically shown, particles have started to be driven towards the drive electrode 40 and start blocking the passage of light to the sensor 46, thereby reducing the photodiode output, and this change in photodiode current can be used as a particle detection mechanism.

Figure 5 is a first example of pixel circuit for controlling the pixel of Figures 3 and 4. Each pixel has a pixel control circuit, comprising the sensor 46 and timing circuitry for converting a drive signal into a time period during which a control signal is applied to the pixel following a detection event from the sensor.

The timing circuitry comprises a data storage capacitor 50 (C2) and a discharge path 52. The discharge path 52 discharges the storage capacitor 50 and is controlled in dependence on the sensor signal. The time taken for the capacitor 50 is discharge is a function of the original data stored on the data storage capacitor 50.

The timing circuitry has a level shift buffer circuit 54 formed from transistors Tl to T6, and which drives the electrophoretic cell (represented as capacitor 56 (Cl) between the two electrodes 40,42) by means of the drive electrode. The buffer circuit 54 operates to couple a high voltage rail, shown as 15V by way of example, to the cell 56, or couple a low voltage rail to the cell. The cell is thus driven full on or off, and a pulse width modulation type scheme is implemented. The circuit is configured as an open loop differential amplifier, which is triggered when the input crosses the threshold voltage of the an inverter 58 at the input of the buffer circuit 54.

The inverter input is initially high, at the data voltage on the capacitor 50. As the capacitor is discharged through the path 52, the voltage on the capacitor 50 drops, and the inverter switches when the threshold voltage is reached. This toggles the buffer circuit output from high to low.

The discharge path 52 comprises a discharge load TlO and a control transistor T9. The control transistor has a gate voltage (Node N) which depends on the photodiode output. The photodiode is in series with a source transistor T8 between power rails, and during the addressing of the pixel, the source transistor is turned on. The source transistor T8 is connected to a common electrode line 59 for this purpose. The source transistor tends to pull the gate voltage of the control transistor high, but when the photodiode is conducting, this voltage is pulled down, and the voltage difference between the voltage rails is dropped mainly across the source transistor. It can be seen therefore that as the light is blocked from the sensor photodiode, the conduction drops and the voltage drop across the photodiode increases. Thus, initially the photodiode is illuminated and the gate voltage of the control transistor T9 is low, with the discharge path disabled. At a certain point in time, there is sufficient blocking of light from the photodiode for the gate voltage on the transistor T9 to rise to a level that turns on the transistor and enables the discharge path.

This commences the discharge of the capacitor 50, and this continues until it has discharged sufficiently for the buffer circuit to toggle. The time for this to take place depends on the voltage on the capacitor 50.

The circuit includes an addressing transistor 60 (T7) for loading data onto the capacitor 50 from a data (column) line 62. This is carried out row by row in conventional manner, and the gate of the addressing transistor 60 is connected to a row line 64 for this purpose.

The example of Figure 5 has a mixture of NMOS and PMOS transistors, although a circuit could equally be designed with only one transistor type. The operation of the circuit thus comprises: a reset phase ("Reset"), which can be implemented by positive biasing of the common storage electrode 42 (in the case of negatively charged particles). This can thus perform a parallel reset of all pixels. A reset switch is shown in Figure 5 which connects the electrode 42 either to OV for normal operation or 15V for the reset operation. a data loading phase ("Load") in which the full pixel array is loaded with data. During this time, the photodiode is illuminated because the pixels are reset, so that the discharge path is not turned on. During data loading, the biasing of the common storage electrode 42 at the reset voltage can be maintained so that no particle movement is initiated. - a timing control phase ("Run"), in which the loaded data is converted to a time duration, and the common storage electrode 42 is at normal voltage.

Figure 6 shows these phases "Reset", "Load" and "Run" and shows the voltage on the common electrode 42, on the drive electrode 40 and the data voltage "Data".

During the reset phase, the common electrode 42 is switched to the high 15V rail so that all pixels are driven to the same state, which is the state that collects all particles at the common electrode terminal.

The reset voltage is maintained on the common electrode 42 during the data Load phase, when a voltage is loaded on the data storage capacitor 50 (C2). As shown in the Data plot, this voltage takes different values, and two values are shown in Figure 6. At this time, the drive voltage is applied to the drive electrode 40 by the operation of the circuit 54, and this is independent of the desired data voltage. This independent drive voltage 68 may thus be considered to be a control voltage for controlling movement of the particles away from the storage electrode. Thus, the drive voltage level is the same for the two examples shown in Figure 6. The data loading phase is implemented row by row in conventional active matrix addressing manner.

During the Run phase, the common electrode 42 is brought low to OV to start particle movement, and start the discharge of the capacitor 50. This takes a length of time which is dependent on the data voltage, so that as shown in the plot 40, the drive voltage is maintained for a different duration for the two examples.

In the example of Figure 5, the current in the discharge path 52 will depend on the drain source voltage of the load TlO which will vary as the capacitor discharges. The load transistor TlO is configured as a diode-connected p-type transistor.

Figure 7 shows a modification to the circuit of Figure 5, in which the discharge path 52 is modified to form a constant current source. The node voltage N is used to control a second inverter 70, and the control transistor T9 is made p-type (so that the circuit function is the same). The result is that the control transistor T9 is turned fully on and off with a fast transition time. A bias voltage is also shown applied to the gate of the discharge load transistor TlO, and this discharge load transistor TlO functions as a current mirror device with a large channel length providing a current that is largely independent of the drain source voltage. This means the data storage capacitor is discharged linearly.

In addition, this modification enables the data voltage to be in a standard range, for example OV to 5V, as there will no longer be a diode voltage drop across the load TlO.

Figure 8 shows an alternative embodiment in which two sensors are used to enable the particle velocity to be taken into account. This enables compensation for temperature effects, for example.

Figure 8 corresponds to Figure 3, but shows two sensors 46a, 46b. Each sensor has its own detection timing events, and the time between the two detection events is used to control the discharge of the storage capacitor.

Figure 9 shows an example circuit for controlling the pixel layout of Figure 8.

The level shift buffer circuit 54 of Figure 5 is represented as a block, again providing the drive signal to the display cell 56 (Cl), and a storage capacitor 50 (C2) holds the data voltage.

The row conductor not only turns on the address transistor, but also turns on a charging transistor T23 which charges a further capacitor C3 to the fixed voltage (10V in this example) at the same time that the pixel data is loaded to the capacitor 50 (C2).

The circuit has two discharge paths - the path 52 for discharging the data storage capacitor, but also a second discharge path 82 for discharging a further capacitor C3.

The first sensor 46a is connected in a similar manner to the sensor 46 in Figure 6, and when the sensor current drops, the transistor T24 is turned on to activate the second discharge path 82 to start the discharge of the further capacitor C3, as well as starting the discharge of the main path 52. The second sensor 46b is connected in the second discharge path and discharges the capacitor C3 until it is covered by particles and stops conducting.

The main discharge path comprises first and second transistors T20, T21. The first T20 is controlled by the first sensor 46a to commence the discharge path operation when the first sensor becomes covered. The second T21 has a gate voltage which is a function of the time delay between the two sensor outputs. This means that the operating point of the second transistor T21 depends on the amount by which the capacitor C3 has been discharged. The less the discharge of C3, the harder on the transistor T21. This means that the quicker the particle speed, the smaller the time difference, therefore the higher the end gate voltage on T21, which in turn means a higher current in the path 52, and in turn a shorter addressing time. This tends to cancel out the effects which give rise to higher particle speed.

This circuit thus implements the main discharge path 52 so that the current in the path is controlled in dependence on the particle movement velocity. Essentially, the transistor T20 functions to enable the discharge path 52, and the transistor T21 functions as a variable current source transistor.

The operation of this circuit thus comprises: the reset phase, which can again be implemented by positive biasing of the common storage electrode 42 (in the case of negatively charged particles). This can thus perform a parallel reset of all pixels. the data loading phase combined with a charging phase for the capacitor C3, in which the full pixel array is loaded with data/charged. During this time, the photodiodes are illuminated because the pixels are reset, and the illumination of sensor 46a turns off the discharge paths by means of T24 and T20 (preventing sensor 46b discharging the capacitor C3). During data loading, the biasing of the common storage electrode 42 can be maintained so that no particle movement is initiated. the timing control phase ("Run"), in which the loaded data is converted to a time duration dependent on the particle velocity, and the common storage electrode 42 is at normal voltage. The circuit operates until the inverter at the input of the level shift circuit 80 toggles, as before.

After the particles pass the first sensor 46a, discharge of the capacitor 50 commences, before the current in the discharge path has been regulated in dependence on velocity. This provides an offset which can easily be corrected, for example by gamma correction or other method.

The invention has been described in connection with an in-plane switching arrangement, but the concepts can be extended to other configurations.

One example of display has been given with row and columns in a particular orientation. The orientation is however somewhat arbitrary. The row is in the example given the conductor to which the pixel address signal is applied and the column is the conductor to which the data signal is applied. These may be switched around, and it should therefore be understood that a "row" may run from top to bottom, and a "column" may run from side to side. The scope of the claims should be understood accordingly.

Various modifications will be apparent to those skilled in the art.

Claims

CLAIMS:

1. An active matrix electrophoretic display device, comprising: an array of rows and columns of display pixels; and control means (10,16) for supplying drive signals to the pixels to drive the pixels to predetermined optical states corresponding to an image to be displayed, wherein each pixel comprises a pixel control circuit, comprising: a sensor (46) for detecting movement of the electrophoretic display particles (34); and timing circuitry (50,52,54) for converting a drive signal into a time period during which a control signal (68) is applied to the pixel following a detection event from the sensor.

2. A device as claimed in claim 1, wherein each pixel comprises a storage electrode (42) and a drive electrode (40), and wherein the electrophoretic display particles (34) are adapted to be collected at the storage electrode (42) before the pixel is driven to a selected optical state.

3. A device as claimed in claim 2, wherein the sensor comprises (46) an optical sensor for detecting the presence of particles at a location adjacent the storage electrode (42).

4. A device as claimed in claim 3, wherein the detection event comprises the detection of the presence of particles by the sensor after the control signal (68) is applied to the pixel to drive the pixel from a reset state ("Reset") to a display state ("Run").

5. A device as claimed in any preceding claim, wherein the sensor (46) comprises a photodiode.

6. A device as claimed in any preceding claim, wherein the timing circuitry (50,52,54) comprises a data storage capacitor (50) and a discharge path (52), wherein the discharge path (52) is enabled by the sensor signal.

7. A device as claimed in claim 6, wherein the timing circuitry (50,52,54) further comprises a buffer circuit (54) which is adapted to apply a substantially constant drive voltage to the pixel until the data storage capacitor (50) has been discharged to a predetermined voltage by the discharge path (52).

8. A device as claimed in claim 6 or 7, wherein the discharge path (52) comprises a constant current source (TlO) having a switching transistor (T9), with the switching transistor controlled in dependence on the sensor output.

9. A device as claimed in claim 6 or 7, wherein the discharge path (52) comprises a current source transistor (T21) which is controlled in dependence on the particle movement velocity.

10. A device as claimed in claim 9, wherein the sensor comprises first and second detectors (46a,46b) each having a detection event, and wherein the time between the two detection events is used to control the current source transistor (T21).

11. A device as claimed in claim 10, wherein the sensor comprises a storage capacitor (C3) for providing a drive voltage to the current source transistor (T21), and wherein the storage capacitor (C3) is adapted to be partially discharged during the time between the two detection events, thereby to control the current output of the current source transistor (T21).

12. A method of driving an electrophoretic display device, comprising an array of rows and columns of display pixels (30), the method comprising, for each pixel: supplying a drive signal ("Data") to the pixel to drive the pixel to a predetermined optical state corresponding to an image to be displayed; applying a control signal (68) to the pixel; detecting movement of the electrophoretic display particles in response to the control signal (68); and converting the drive signal ("Data") into a time period during which the control signal (68) is applied to the pixel following the detection.

13. A method as claimed in claim 12, further comprising applying a reset signal

("Reset") to the pixel before supplying the drive signal, thereby to collect the particles (34) at a storage electrode.

14. A method as claimed in claim 13, wherein detecting movement comprises detecting the presence of particles (34) at a location (46) adjacent the storage electrode (42).

15. A method as claimed in claim 12, 13 or 14, wherein detecting movement comprises optically sensing the presence of particles (34).

16. A method as claimed in any one of claims 12 to 15, wherein converting the drive signal into a time period comprises discharging a data storage capacitor (50), with the discharge path (52) enabled by the sensor signal.

17. A method as claimed in any one of claims 12 to 15, wherein converting the drive signal into a time period takes into account the particle movement velocity.

18. A method as claimed in claim 17, wherein the particle movement velocity is derived from the time between first and second detection events of first and second detectors (46a,46b).