Display driver circuits

• US 20050140610A1
• Filed: 03/14/2003
• Published: 06/30/2005
• Est. Priority Date: 03/14/2002
• Status: Active Grant

• Current Assignee: Cambridge Display Technology Limited (Sumitomo Chemical Company Limited)
• Original Assignee: Cambridge Display Technology Limited (Sumitomo Chemical Company Limited)
• Inventors: Smith, Euan C, Routley, Paul R.
• Granted Patent: US 7,535,441 B2
• Time in Patent Office: Days
• US Class Current: 345/77
• 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 2330/021 Power management, e.g. power saving
• CPC Class Codes: G09G 3/3216 using a passive matrix
• CPC Class Codes: G09G 3/3266 Details of drivers for scan electrodes
• CPC Class Codes: Y02E 60/36 Hydrogen production from non-carbon containing sources, e.g. by water electrolysis

Abstract

Display driver circuits are described for driving organic light emitting diode displays, particularly passive matrix displays, with greater efficiency. The display drivers comprise a controllable current generator to provide a variable current drive output to an OLED display, the current generator comprising at least one bipolar transistor in series with the current drive output. The display brightness is adjustable by controlling the current generator to vary the current drive to the display. Preferably the bipolar transistor has an emitter terminal substantially directly connected to a power supply line of the driver to reduce losses in the driver. A corresponding method is also described. By employing a bipolar transistor current drive and varying display brightness by controlling the current an efficient driver-display combination is obtained.

Claims

1. A display driver for a passive organic electroluminescent display, the display driver comprising:

a controllable current generator to provide a variable current drive output to the display, the current generator comprising at least one bipolar transistor in series with the current drive output;

whereby the display brightness is adjustable by controlling the current generator to vary the current drive to the display.

2. A display driver as claimed in claim 1 wherein the passive organic electroluminescent display is a passive matrix display having a plurality of pixels addressed by row and column electrodes, the display driver having a plurality of said current generators for driving a plurality of ones of said row and column electrodes, whereby the brightnesses of said pixels are adjustable to provide a greyscale display.

3. A display driver as claimed in claim 1 wherein said bipolar transistor has an emitter terminal substantially directly connected to a power supply line of the driver.

4. A display driver as claimed in claim 3 wherein the controllable current generator includes a current mirror circuit.

5. A display driver as claimed in claim 1 further comprising control circuitry to control the current generator, the control circuitry comprising at least one method oxide semiconductor (MOS) transistor.

6. A display driver as claimed in claim 1 further comprising control circuitry to control the current generator;

wherein the current generator is controllable by a current input, and wherein the control circuitry is configured to provide a variable current output to the current generator current input.

7. A display driver as claimed in claim 6 wherein the control circuitry includes a digital-to-analog converter to convert a digital control signal input to an analog signal for providing said variable current output.

8. Display driver circuitry for an organic electroluminescent display, the display comprising a plurality of organic electroluminescent pixels, each pixel being addressed by one of each of a first plurality and a second plurality of driving electrodes, the display driver circuitry comprising:

a power supply connection;

a plurality of electrode driver outputs for driving a plurality of said display driving electrodes;

a plurality of variable current driver circuits, each having a control input and providing a variable current drive for a said electrode driver output, and each comprising a bipolar current drive transistor having an emitter terminal substantially directly connected to said power supply connection and a collector terminal coupled to a said electrode driver output; and

control circuitry coupled to the control inputs of said driver circuits and configured to provide an analog signal to each said control input for controlling the variable current drive provided by each said driver circuit;

whereby the brightness of each pixel is adjustable.

9. Display driver circuitry as claimed in claim 8 wherein each said current driver circuit comprises a current mirror having an output coupled to a said electrode driver output and a current control line coupled to said control input.

10. Display driver circuitry as claimed in claim 8 Of 9 wherein said bipolar current drive transistor comprises a Darlington transistor pair.

11. Display driver circuitry as claimed in claim 8, wherein said control circuitry comprises method oxide semiconductor (MOS) transistors.

12. Display driver circuitry as claimed in claim 11 wherein a said current driver control input comprises a current set input wherein the control circuitry for each of said current driver circuits comprises a controllable current setting means coupled to said current set input.

13. Display driver circuitry as claimed in claim 12 wherein the controllable current setting means comprises a plurality of field effect transistor (FET) switches each coupled to a respective current setting component.

14. Display driver circuitry as claimed in any one of claim 8 wherein said electrode driver outputs are configured for driving electrodes of one of said first and second pluralities of display driving electrodes, and further comprising a plurality of second bipolar transistors for switching current from electrodes of the other of said first and second pluralities of display driving electrodes.

15. Display driver circuitry as claimed in claim 14, further comprising a second power supply connection, and wherein each said second bipolar transistor has an emitter terminal substantially directly connected to said second power supply connection and a collecter terminal coupled to a connection for an electrode of the other of said first and second pluralities of display driving electrodes.

16. A column electrode display driver for a passive organic light-emitting diode (OLED) display, the display comprising a matrix of OLEDs and a plurality of row and column electrodes, each OLED having an anode coupled to a column electrode and a cathode coupled to a row electrode, the display driver comprising a plurality of independently adjustable, high compliance bipolar column current drivers each having an analog input for providing a variable column current drive to the display.

17. A column electrode display driver as claimed in claim 16 wherein each said column current driver comprises a bipolar transistor having an emitter directly connected to a power supply connection.

18. A column electrode display driver as claimed in claim 17 further comprising a metal oxide semiconductor (MOS) column current drive controller.

19. A column electrode display driver as claimed in claim 18 wherein each said column current driver comprises a current mirror with a current control connection, and wherein said MOS controller is configured to control a current in the current control connection of each column current driver.

20. Display driver circuitry for an organic electrolmninescent display, the display comprising a plurality of organic electroluminescent pixels, each pixel being addressed by one of each of a first plurality and a second plurality of driving electrodes, the display driver circuitry comprising:

a power supply connection;

a plurality of electrode driver outputs for driving a plurality of said display driving electrodes;

a plurality of current driver circuits for each driving electrode for providing a plurality of current drives for a said electrode driver output, each having a control input and each comprising a bipolar current drive transistor having an emitter terminal substantially directly connected to said power supply connection and a collector terminal coupled to a said electrode driver output; and

control circuitry coupled to the control inputs of said driver circuits and configured to provide a signal to each said control input for controlling the current drive provided to a said electrode driver output;

whereby the brightness of each pixel is adjustable.

21. A method of increasing the efficiency of a current driver for an organic electroluminescent display, the method comprising:

providing a current drive for the display using a bipolar transistor with an emitter terminal substantially directly connected to a power line; and

using a voltage for the power line such that when the organic electroluminescent display is at substantially a maximum desired brightness the bipolar transistor current drive is operating substantially at-its limit of compliance.

22. A method as claimed in claim 21 wherein the organic electroluminescent display is a variable brightness display and said current drive is a variable current drive.