Control of current supplied by a transistor to a pixel in an electroluminescent display device

• US 7,839,365 B2
• Filed: 08/22/2003
• Issued: 11/23/2010
• Est. Priority Date: 09/04/2002
• Status: Active Grant

• Current Assignee: Beijing Xiaomi Mobile Software Co., Ltd. (Xiaomi Corp.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Hoppenbrouwers, Jurgen J. L., Van Woudenberg, Roel, Knapp, Alan G., Van Der Vaart, Nijs C., Fish, David A.
• Primary Examiner(s): Nguyen; Chanh
• Assistant Examiner(s): WALTHALL, ALLISON N
• Application Number:
  US10/526,864
• Publication Number:
  US 20050264492A1
• Time in Patent Office: 2,650 Days
• Field of Search: 345 76- 83, 345/589
• US Class Current: 345/77
• CPC Class Codes: G09G 2300/0809 Several active elements per pixel in active matrix panels
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2300/0866 by means of changes in the pixel supply voltage
• CPC Class Codes: G09G 2310/027 Details of drivers for data electrodes, the drivers handling digital grey scale data, e.g. use of D/A converters
• CPC Class Codes: G09G 2320/0209 Crosstalk reduction, i.e. to reduce direct or indirect influences of signals directed to a certain pixel of the displayed image on other pixels of said image, inclusive of influences affecting pixels in different frames or fields or sub-images which constitute a same image, e.g. left and right images of a stereoscopic display
• CPC Class Codes: G09G 2320/0223 Compensation for problems related to R-C delay and attenuation in electrodes of matrix panels, e.g. in gate electrodes or on-substrate video signal electrodes
• CPC Class Codes: G09G 2320/0271 Adjustment of the gradation levels within the range of the gradation scale, e.g. by redistribution or clipping
• CPC Class Codes: G09G 2320/0276 for the purpose of adaptation to the characteristics of a display device, i.e. gamma correction
• CPC Class Codes: G09G 2320/0285 using tables for spatial correction of display data
• CPC Class Codes: G09G 2360/16 Calculation or use of calculated indices related to luminance levels in display data
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3241 the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror

Abstract

In an active matrix electroluminescent display device, an overall brightness level of an image to be displayed in a frame period is determined. A drive transistor of each pixel is controlled in dependence on an input drive signal for the pixel and on the overall brightness level, for example using a signal processor (30) to vary the pixel drive signals. This arrangement can control the pixels to limit the maximum currents drawn by the pixels, thereby limiting the cross talk effects resulting from voltage drops along row or column conductors. If an image is bright, the pixel drive levels across the image (or at least a part of the image) can be reduced, so that the maximum brightness is reduced.

Claims

1. An active matrix electroluminescent display device including an array of display pixels, comprising:

an electroluminescent (EL) display element;

active matrix circuitry including first and second drive transistors for driving a current through the display element, wherein the first and second drive transistors are in parallel, each connected between a respective power supply line and the EL display element, the input to the pixel being provided to the gates of the first and second drive transistors, and wherein the first drive transistor is supplied with a first supply voltage and the second drive transistor is supplied with a second supply voltage, at least one of the supply voltages being variable in dependence on the combined brightness level;

means for determining a combined brightness level of a multitude of pixels in an image to be displayed in a frame period; and

means for controlling the first and second drive transistors of each pixel individually in dependence on a respective input signal providing a drive level for the pixel and in dependence on the combined brightness level of the multitude of pixels in the image,

wherein the means for controlling the first and second drive transistors comprises a signal processing device for determining an combined brightness level and for processing the input signals for the pixels in dependence on the combined brightness level,

wherein the signal processing device is adapted to employ gamma characteristics for processing the input signals in dependence on the combined brightness level, wherein said gamma characteristics comprising a gamma correction LUT altered by a LUT generator to set a certain maximum brightness level depending on the combined brightness level.

2. The device as claimed in claim 1, wherein the signal processing device comprises a field store for storing the input signals for an image and a summation unit for summing the input signals for the multitude of pixels of the image in the field store to determine the combined brightness.

3. The device as claimed in claim 2, wherein the signal processing device further comprises a look up table for modifying the input signals for the stored image in dependence on the combined brightness level.

4. The device as claimed in claim 3, wherein the signal processing device is adapted to calculate or select the look-up table in dependence on the combined brightness level.

5. The device as claimed in claim 1, wherein the signal processing device operates to reduce the maximum brightness level to which any pixel is driven in response to an increase in the combined brightness of an image.

6. The device as claimed in claim 1, wherein the signal processing device comprises digital to analogue converter circuitry for converting digital inputs into the input signal, and wherein the digital to analogue converter circuitry is controllable in dependence on the combined brightness level.

7. The device as claimed in claim 1, wherein the input to the pixel is provided to the gates of the first and second drive transistors through an address transistor.

8. The device as claimed in claim 1, wherein the first supply voltage is fixed and the second supply voltage is variable.

9. The device as claimed in claim 8, wherein the first and second supply voltages can be equal.

10. A method of addressing an active matrix electroluminescent display device comprising an array of display pixels, an electroluminescent (EL) display element and active matrix circuitry including first and second drive transistors for driving a current through the display element, the method comprising:

determining a combined brightness level of a multitude of pixels in an image to be displayed in a frame period;

wherein the means for controlling the first and second drive transistors at least one comprises a signal processing device for determining an combined brightness level and for processing the input signals for the pixels in dependence on the combined brightness level,

wherein the signal processing device is adapted to employ gamma characteristics for processing the input signals in dependence on the combined brightness level, wherein said gamma characteristics comprising a gamma correction LUT altered by a LUT generator to set a certain maximum brightness level depending on the combined brightness level, and

controlling the first and second drive transistors of each pixel individually in dependence on a respective input signal providing a drive level for the pixel and in dependence on the combined brightness level of the multitude of pixels in the image, and

wherein the first and second drive transistors are in parallel, each connected between a respective power supply line and the EL display element, the input to the pixel being provided to the gates of the first and second drive transistors, and wherein the first drive transistor is supplied with a first supply voltage and the second drive transistor is supplied with a second supply voltage, at least one of the supply voltages being variable in dependence on the combined brightness level.

11. The method as claimed in claim 10, wherein controlling the first and second drive transistors comprises processing the input signals for the pixels in dependence on the combined brightness level and then applying the processed input signals to the pixels.

12. The method as claimed in claim 11, wherein determining the combined brightness level comprises storing the input signals for an image and summing them.

13. The method as claimed in claim 11, wherein processing the input signals comprising modifying the input signals using a look up table, the address of which is selected in dependence on the input signal and the combined brightness level.

14. The method as claimed in claim 11, wherein processing of the input signals is performed by employing gamma characteristics of the array of display elements.

15. The method as claimed in claim 10, wherein the control of the first and second drive transistors reduces the maximum brightness level to which any pixel is drive in response to an increase in the combined brightness of an image.

16. The method as claimed in claim 10, wherein the input signals are in digital form, and controlling the first and second drive transistors comprises controlling the digital to analogue conversion of the digital input signal in dependence on the combined brightness level and then applying the analogue input signals to the pixels.