Active Matrix Liquid Crystal Device

• US 20090102780A1
• Filed: 03/23/2007
• Published: 04/23/2009
• Est. Priority Date: 03/23/2006
• Status: Active Grant

• Current Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Original Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Inventors: Brown, Christopher J.
• Granted Patent: US 8,378,954 B2
• Time in Patent Office: Days
• US Class Current: 345/101
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 3/3614 Control of polarity reversal in general
• CPC Class Codes: G09G 3/3648 using an active matrix G09G3/367 - G09G3/3696 take precedence

Abstract

In one embodiment of the present invention, an active matrix liquid crystal device (AMLCD) includes an active matrix substrate including an active matrix area and a counter electrode substrate carrying a common electrode whose voltage may vary so as to reduce liquid crystal degradation. A layer of liquid crystal material is disposed between the substrates and a temperature measuring arrangement is provided on the active matrix substrate. This arrangement includes a temperature sensing liquid crystal capacitor which in turn includes a first electrode formed on the active matrix substrate outside the image generating region of the active matrix area and separated from the common electrode, which forms the second electrode of the capacitor, by the liquid crystal layer, which forms the capacitor dielectric. During operation of the active matrix, a measuring circuit repeatedly performs a precharging step, in which the capacitor is precharged to a fixed stable known voltage magnitude, and a step in which a signal representing the capacitance is formed. For example, the charge stored in the capacitor may be charge-shared with a transfer capacitor to form thereacross a voltage representing the capacitance of the liquid crystal capacitor and hence representing the liquid crystal temperature. The sampling is performed repeatedly, preferably in synchronism with addressing performed by the active matrix of the device. The resulting temperature measurement may be used, for example, to compensate the AMLCD for the effects of temperature variation in the liquid crystal properties.

Claims

1. An active matrix liquid crystal device comprising:

an active matrix first substrate having an active matrix area;

a second substrate carrying a common electrode for the active matrix;

a layer of liquid crystal material between the first and second substrates;

a temperature sensing first capacitor comprising a first electrode outside the image generating region of the active matrix area on the first substrate separated from the common electrode, which forms a second electrode of the first capacitor, by the liquid crystal layer, which forms the first capacitor dielectric; and

a capacitance measuring circuit arranged, during operation of the active matrix, repeatedly to perform the steps of precharging the first capacitor to a substantially fixed stable known first precharge voltage magnitude and forming a signal representing the capacitance of the first capacitor.

2. A device as claimed in claim 1, in which the measuring circuit is formed on the first substrate.

3. A device as claimed in claim 1, in which the measuring circuit is arranged to charge the first capacitor to the same magnitude of voltage during each precharging step.

4. A device as claimed in claim 3, in which measuring circuit is arranged to charge the first capacitor to the same polarity of voltage during each precharging step.

5. A device as claimed in claim 1, in which the measuring circuit is arranged to perform each precharging step at a same part of an active matrix addressing cycle.

6. A device as claimed in claim 5, in which the same part comprises the same part of a line addressing period.

7. A device as claimed in claim 1, in which the active matrix and the common electrode are arranged periodically to invert the polarity of drive voltages applied to pixel cells of the device.

8. A device as claimed in claim 4, in which the active matrix and the common electrode are arranged to invert the polarity during alternate line addressing periods.

9. A device as claimed in claim 8, in which the measuring circuit is arranged to perform the precharging step during alternate line addressing periods.

10. A device as claimed in claim 1, in which the forming step measures the charge stored in the first capacitor during the precharging step.

11. A device as claimed in claim 10, in which the measuring circuit is arranged to measure the stored charge by charge-sharing.

12. A device as claimed in claim 11, in which the measuring circuit is arranged, during each forming step of a first part of a conversion cycle, to share the stored charge with a transfer second capacitor during a first forming phase, and to make the first resulting voltage across the second capacitor available during a second forming phase.

13. A device as claimed in claim 12, in which the measuring circuit is arranged to charge a reference third capacitor to a second precharge voltage during the precharging step, to share the stored charge with a transfer fourth capacitor during the first forming phase, and to make the second resulting voltage across the fourth capacitor available during the second forming phase.

14. A device as claimed in claim 13, in which the third capacitor has a value less than or equal to a lowest-expected value of the first capacitor.

15. A device as claimed in claim 13, comprising means for forming the difference between the first and second resulting voltages.

16. A device as claimed in claim 15, in which the means comprises a summation fifth capacitor arranged to be connected temporarily between the second and fourth capacitors.

17. A device as claimed in claim 15, in which the means comprises a differential input of a following stage.

18. A device as claimed in claim 1, in which the measuring circuit comprises an analog/digital converter.

19. A device as claimed in claim 18, in which the converter is an integrating converter.

20. A device as claimed in claim 19, in which the converter is a dual-slope converter.

21. A device as claimed in claim 12, in which the measuring circuit comprises a dual-slope analog/digital converter and is arranged, during each of repeated discharge cycles of a second part of the conversion cycle, to charge a discharge sixth capacitor to a third precharge voltage during a third phase, to share the stored change with a transfer seventh capacitor during a fourth phase, and to make the third resulting voltage across the seventh capacitor available during a fifth phase.

22. A device as claimed in claim 21, in which the measuring circuit is arranged to charge the third capacitor to a fourth precharge voltage during the third phase, to share the stored charge with the fourth capacitor during the fourth phase, and to make the fourth resulting voltage across the fourth capacitor available during the fifth phase.

23. A device as claimed in claim 22, in which the sixth capacitor has a value less than that of the third capacitor.

24. A device as claimed in claim 1, in which at least one said precharge voltage is derived from a complement of a voltage on the common electrode.

25. A device as claimed in claim 1, in which at least one said precharge voltage is derived from a matrix element drive voltage.

26. A device as claimed in claim 12, in which the measuring circuit is arranged, during each of repeated calibration cycles of an initial part of the conversion cycle, to charge a calibration eighth capacitor to a fifth precharge voltage during a sixth phase, to share the stored charge with a transfer ninth capacitor during a seventh phase, and to make the fifth resulting voltage across the ninth capacitor available during an eighth phase.

27. A device as claimed in claim 26, in which the measuring circuit is arranged to charge the third capacitor to a sixth precharge voltage during the sixth phase, to share the stored charge with the fourth capacitor during the seventh phase, and to make the sixth resulting voltage across the fourth capacitor available during the eighth phase.

28. A device as claimed in claim 26, in which the measuring circuit comprises a reference voltage generator for generating a reference voltage from the fifth resulting voltage.

29. A device as claimed in claim 28, in which the generator comprises a tenth capacitor arranged to integrate the fifth resulting voltages from the calibration cycles.

30. A device as claimed in claim 21, in which the measuring circuit is arranged, during each of repeated calibration cycles of an initial part of the conversion cycle, to charge a calibration eighth capacitor to a fifth precharge voltage during a sixth phase, to share the stored charge with a transfer ninth capacitor during a seventh phase, and to make the fifth resulting voltage across the ninth capacitor available during an eighth phase.

31. A device as claimed in claim 30, in which the measuring circuit comprises a reference voltage generator for generating a reference voltage from the fifth resulting voltage.

32. A device as claimed in claim 31, in which the generator is arranged to supply the reference voltage to a comparator of the converter during the second part of the conversion cycle.

33. A device as claimed in claim 1, in which the signal representing the capacitance provides a measure of the liquid crystal material temperature.

34. A device as claimed in claim 33, comprising an arrangement, responsive to the measure of the liquid crystal material temperature, for supplying temperature-compensated drive signals to the cells of the matrix.