Active Matrix Displays

• US 20110181630A1
• Filed: 08/13/2009
• Published: 07/28/2011
• Est. Priority Date: 08/15/2008
• Status: Active Grant

• Current Assignee: Cambridge Display Technology Limited (Sumitomo Chemical Company Limited)
• Original Assignee: Cambridge Display Technology Limited (Sumitomo Chemical Company Limited)
• Inventors: Rankov, Aleksandra, Smith, Euan, Thompson, Barry
• Granted Patent: US 8,791,887 B2
• Time in Patent Office: Days
• US Class Current: 345/690
• CPC Class Codes: G09G 2300/0408 Integration of the drivers onto the display substrate
• CPC Class Codes: G09G 2300/0465 Improved aperture ratio, e.g. by size reduction of the pixel circuit, e.g. for improving the pixel density or the maximum displayable luminance or brightness
• CPC Class Codes: G09G 2310/0248 Precharge or discharge of column electrodes before or after applying exact column voltages
• 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 3/3216 using a passive matrix
• 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
• CPC Class Codes: G09G 3/3283 in which the data driver supplies a variable data current for setting the current through, or the voltage across, the light-emitting elements

Abstract

This invention relates to active matrix OLED (Organic Light Emitting Diode) displays, in particular to display panels with integrated negative capacitance circuits and to active capacitance compensation. We describe an active matrix OLED display comprising a glass panel bearing a plurality of lines of OLED pixels, each with an associated active matrix driver circuit having a programming connection for programming a brightness of the associated OLED, programming connections of a line of pixels being connected to a programming line of said display, and wherein said active matrix OLED display further comprises a plurality of capacitors on said glass panel, each having a first plate connected to an end of a respective said programming line and having a second plate for connecting to a negative capacitor circuit to compensate for a capacitance of said programming line.

Claims

1. An active matrix organic light emitting diode (OLED) display, the display comprising a glass panel bearing a plurality of lines of OLED pixels, each pixel comprising an OLED and an associated active matrix driver circuit, each said active matrix driver circuit including a programming connection for programming a brightness of the associated OLED, programming connections of a line of said OLED pixels being connected to a programming line of said display, and wherein said active matrix OLED display further comprises a plurality of capacitors on said glass panel, each having a first plate connected to an end of a respective said programming line and having a second plate for connecting to a negative capacitor circuit to compensate for a capacitance of said programming line.

2. An active matrix OLED display as claimed in claim 1 wherein a said programming line has an external connection to provide a programming connection to said display, and wherein a said capacitor is connected to an opposite end of a said programming line to a said external connection.

3. An active matrix OLED display as claimed in claim 1 wherein a said capacitor is connected to each end of a said programming line for connecting to a respective said negative capacitance circuit at each end of a said programming line.

4. An active matrix OLED display as claimed in claim 1 further comprising a plurality of said negative capacitor circuits, each coupled to a said second plate of a said capacitor connected to a said programming line, and wherein said negative capacitor circuits are fabricated on said glass panel.

5. An active matrix OLED display as claimed in claim 1 further comprising a plurality of said negative capacitor circuits, each coupled to a said second plate of a said capacitor connected to a said programming line, and wherein said negative capacitor circuits are fabricated on chiplets attached to said glass panel.

6. An active matrix OLED display as claimed in claim 1 wherein said negative capacitor circuit comprises an amplifier circuit, and wherein said first plate of a said capacitor is coupled to an input of said amplifier and said second plate of said capacitor is coupled to an output of said amplifier.

7. An active matrix OLED display as claimed in claim 1 wherein a said negative capacitor circuit comprises a plurality of transistors, and wherein said transistors comprise only NMOS devices such that said negative capacitor circuit lacks any PMOS transistors.

8. An active matrix OLED display as claimed in claim 1 wherein a said negative capacitor circuit comprises a plurality of transistors and includes circuitry to compensate for a threshold voltage change in one or more of said transistors due to ageing.

9. An active matrix OLED display as claimed in claim 1 wherein a said capacitor has a value of between 0.05 times and 2 times said capacitance of said programming line.

10. A method compensating for capacitance of a programming line of an active matrix organic light emitting diode (OLED) display, the display comprising a glass panel bearing a plurality of lines of OLED pixels, each pixel comprising an OLED and an associated active matrix driver circuit, each said active matrix driver circuit including a programming connection for programming a brightness of the associated OLED, programming connections of a line of said OLED pixels being connected to a programming line of said display, the method comprising driving each said programming line using a negative capacitor circuit including a respective capacitor to supply charge to compensate for said capacitance of said programming line, locating said capacitors in a border portion of said glass panel of said display.

11. A circuit for use in an active matrix organic light emitting diode (AMOLED) display, the display comprising a glass panel and a plurality of programming lines on the panel, the circuit comprising a capacitor and an amplifier coupled to the capacitor, such that, in operation, said capacitor and said amplifier provide a negative capacitance to a said programming line, wherein at least said capacitor is disposed within a boundary of the glass panel.

19. A method as claimed in claim 11 wherein said OLED display comprises a passive matrix OLED display.

12. A method of compensating for capacitance of a programming line of an organic light emitting diode (OLED) display when driving the OLED display, the method comprising:

using a reference capacitance to mimic said capacitance of said programming line; and

driving both said programming line and said reference capacitance in tandem responsive to a comparison between a voltage on said programming line and a voltage on said reference capacitor.

13. A method as claimed in claim 12 further comprising discharging said reference capacitance towards a voltage on said programming line to reduce a drive on said programming line.

14. A method as claimed in claim 12 further comprising controlling said driving to reduce instability due to positive feedback between said capacitance of said programming line and said reference capacitance.

15. A method as claimed in claim 12 wherein said reference capacitor has a capacitance of less than 1/10 of said capacitance of said programming line.

16. A method as claimed in claim 12 wherein said driving comprises controlling respective current drives to said reference capacitance and to said programming line.

17. A method as claimed in claim 16 wherein said controlling of said current drive to said reference capacitance comprises applying a current drive to said reference capacitance with a duty cycle of less than 100%.

18. A method as claimed in claim 16 wherein said controlling of said respective current drives includes matching a current drive to said reference capacitance to a current drive to said capacitance of said programming line in a ratio of a capacitance of said reference capacitance to said capacitance of said programming line.

20. A method as claimed in claim 12 wherein said driving comprises using a comparator to perform said comparison and using said comparator to control respective switching devices switching current generators supplying said reference capacitance and said programming line.

23. A method as claimed in claim 12 wherein said reference capacitor has a capacitance of less than 1/20 of said capacitance of said programming line.

24. A method as claimed in claim 12 wherein said reference capacitor has a capacitance of less than 1/50 of said capacitance of said programming line.

25. A method as claimed in claim 12 wherein said reference capacitor has a capacitance of less than 1/100 of said capacitance of said programming line.

26. A method as claimed in claim 12 wherein said OLED display comprises a passive matrix OLED display.

21. A system for compensating for capacitance of a programming line of an organic light emitting diode (OLED) display when driving the OLED display, the system comprising:

means for using a reference capacitance to mimic said capacitance of said programming line; and

means for driving both said programming line and said reference capacitance in tandem responsive to a comparison between a voltage on said programming line and a voltage on said reference capacitor.

22. An organic light emitting diode (OLED) display driver for compensating a capacitance of a programming line of an OLED display, the OLED display driver including a capacitance compensation circuit, wherein said capacitance compensation circuit comprises a reference capacitance, a comparator having a first input coupled to said reference capacitance and a second input coupled to a connection for said programming line and an output, a first current driver for providing a first current drive to said programming line and a second current driver for providing a second current drive to said reference capacitance, and first and second devices both driven by said output of said comparator, such that, in use, charge injected from said first current drive into said programming line compensates for said capacitance of said programming line.