Low power circuits for active matrix emissive displays and methods of operating the same

• US 20050225519A1
• Filed: 04/06/2005
• Published: 10/13/2005
• Est. Priority Date: 04/12/2004
• Status: Active Grant

• Current Assignee: Siliconfile Technologies Incorporated (SK Hynix Inc.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Naugler, W. Edward Jr.
• Granted Patent: US 7,129,938 B2
• Time in Patent Office: Days
• US Class Current: 345/77
• CPC Class Codes: G09G 2300/0814 used for selection purposes, e.g. logical AND for partial update
• CPC Class Codes: G09G 2300/0819 used for counteracting undesired variations, e.g. feedback or autozeroing
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2310/0259 with use of an analog or digital ramp generator in the column driver or in the pixel circuit
• 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 2320/0285 using tables for spatial correction of display data
• CPC Class Codes: G09G 2330/021 Power management, e.g. power saving
• CPC Class Codes: G09G 2330/028 Generation of voltages supplied to electrode drivers in a matrix display other than LCD
• CPC Class Codes: G09G 2360/148 the light being detected by light detection means within each pixel
• CPC Class Codes: G09G 3/2011 by amplitude modulation
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3266 Details of drivers for scan electrodes
• CPC Class Codes: G09G 3/3291 in which the data driver supplies a variable data voltage for setting the current through, or the voltage across, the light-emitting elements

Abstract

The embodiments of the present invention provide a flat panel display having a plurality of pixels, each comprising a light-emitting device configured to emit light in accordance with a current flowing through the light-emitting device, a transistor coupled to the light-emitting device and configured to provide the current through the light-emitting device, the current increasing with a ramp voltage applied to a control terminal of the transistor, and a switching device configured to switch off in response to the luminance of the light-emitting device having reached a specified level, thereby disconnecting the ramp voltage from the transistor and locking the brightness at the specified level. The switching device is further configured to stay off thereby allowing the luminance of the light-emitting device to be kept at the specified level until the pixel is rewritten in a different frame.

Claims

1. A display having a plurality of pixels, each pixel comprising:

a light-emitting device configured to emit light in response to a current flowing through the light-emitting device, a luminance of the light-emitting device being dependent upon the current;

a transistor coupled to the light-emitting device and configured to provide the current through the light-emitting device, the current increasing with a ramp voltage applied to a control terminal of the transistor; and

a first switching device configured to switch off in response to the luminance of the light-emitting device having reached a specified level, thereby disconnecting the ramp voltage from the transistor; and

wherein the first switching device is further configured to stay off thereby allowing the luminance of the light-emitting device to be kept at the specified level until the pixel is rewritten.

2. The display of claim 1, wherein the light-emitting device is an organic light-emitting diode.

3. The display of claim 1, wherein each pixel further comprises a capacitor coupled to the transistor and configured to keep the luminance of the light-emitting device at the specified level after the ramp voltage is disconnected from the transistor.

4. The display of claim 1, further comprising an optical sensor associated with each pixel, the optical sensor positioned to receive a portion of the light from the light-emitting device and having an electrical parameter dependent on the luminance of the light-emitting device.

5. The display of claim 4, wherein the pixels are arranged in rows and columns and the display further comprises a resistor associated with each column and serially coupled with the optical sensor in each of the pixels in the column.

6. The display of claim 5, wherein each pixel further comprises a second switching device serially coupled with the optical sensor and having a control terminal connected to a conductive line associated with a row of pixels.

7. The display of claim 6, wherein the first and second switching devices are thin-film transistors.

8. The display of claim 4, wherein the pixels are arranged in rows and columns and the first switching device in each pixel has a first control terminal coupled to a conductive line associated with a row of pixels and a second control terminal connected to a voltage that is dependent upon the luminance of the light-emitting device.

9. The display of claim 8, further comprising a voltage comparator associated with each column of pixels and having an output connected to the second control terminal of the first switching device in each pixel in the column, a first input receiving a reference voltage corresponding to a specified luminance of a pixel in the column, and a second input connected to the optical sensor associated with each pixel in the column.

10. A method for controlling the brightness of a pixel in a display, the method comprising:

switching on a switching device by applying a first control voltage to a first control terminal and a second control voltage to a second control terminal of the switching device;

applying a ramp voltage through the switching device to a gate of a transistor serially coupled with the light-emitting device thereby causing a luminance of the light-emitting device to increase with the ramp voltage; and

illuminating an optical sensor with the light from the light-emitting device thereby causing an electrical parameter associated with the optical sensor to change according to the luminance of the light-emitting device; and

wherein the second control voltage is dependent on the electrical parameter and changes to a different value in response to the luminance of the light-emitting device having reached a specified level for the pixel, thereby switching off the switching device.

11. The method of claim 10, further comprising:

charging a capacitor coupled to the transistor with the ramp voltage, the capacitor keeping the brightness of the light at the specified level after the switching device is switched off.

12. The method of claim 10, further comprising:

changing the first control voltage to keep the switching device off and the brightness of the light at the specified level.

13. The method of claim 10, wherein the transistor and the light-emitting device are serially coupled with each other between a variable voltage source and ground, and the method further comprising:

varying a voltage output from the variable voltage source as the display ages.

14. The method of claim 13, wherein varying the voltage output comprising:

recording a value of ramp voltage required to cause the light-emitting device in each pixel in the display to reach the specified level of luminance for the pixel; and

varying the voltage output based on a statistical measure calculated from the changes in the recorded values for some or all of the pixels in the display.

15. A display having a plurality of pixels, each pixel comprising:

a light-emitting device configured to emit light in response to a current flowing through the light-emitting device, a luminance of the light-emitting device being dependent upon the current;

a transistor configured to provide the current through the light-emitting device, the current increasing with a ramp voltage applied to a control terminal of the current source; and

a first switching device configured to disconnect the ramp voltage from the transistor in response to the luminance of the light-emitting device having reached a specified level; and

wherein the transistor and the light-emitting device are serially coupled with each other between a variable voltage source and ground.

16. The display of claim 15, wherein the variable voltage source is configured to output a voltage that changes as the display ages.

17. The display of claim 16, wherein the voltage output from the variable voltage source changes based on a statistical evaluation of the changes in ramp voltages required to cause the luminance of the light-emitting devices to reach specified levels in some or all of the pixels in the display.

18. The display of claim 15, further comprising:

a storage capacitor configured to be charged by the ramp voltage;

a second switching device configured to disconnect the second ramp voltage from the capacitor in response to the luminance of the light-emitting device having reached the specified value; and

a buffer configured to record the voltage across the storage capacitor after the storage capacitor is disconnected from the second ramp voltage.

19. The display of claim 15, further comprising:

a capacitor coupled to the transistor and configured to be charged by the ramp voltage until the luminance of the light-emitting device has reached the specified level and to keep the luminance of the light-emitting device at the specified level.

20. A display having a plurality of pixels, each pixel comprising:

a light-emitting device;

means for allowing a ramp voltage to control a current through the light-emitting device so that the luminance of the light-emitting device increases with the ramp voltage;

means for disconnecting the ramp voltage from the light-emitting device in response to the luminance having reached a specified level; and

means for keeping the luminance at the specified level after the ramp voltage is disconnected; and

wherein the means for keeping comprises means for isolating the pixel from other pixels in the display.