Method of driving a liquid crystal display device, and a liquid crystal display device

• US 20010005192A1
• Filed: 12/05/2000
• Published: 06/28/2001
• Est. Priority Date: 12/07/1999
• 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: Walton, Harry Garth, Towler, Michael John
• Granted Patent: US 6,621,476 B2
• Time in Patent Office: Days
• US Class Current: 345/87
• CPC Class Codes: G02F 1/1395 Optically compensated birefringence [OCB]- cells or PI- cells
• CPC Class Codes: G09G 2300/0491 Use of a bi-refringent liquid crystal, optically controlled bi-refringence [OCB] with bend and splay states, or electrically controlled bi-refringence [ECB] for controlling the color
• CPC Class Codes: G09G 2310/061 for resetting or blanking
• CPC Class Codes: G09G 2310/08 Details of timing specific for flat panels, other than clock recovery
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: G09G 3/3648 using an active matrix G09G3/367 - G09G3/3696 take precedence

Abstract

A method of driving a display device makes use of the dynamic variation in the brightness of a surface mode liquid crystal device cell having liquid crystal layer with a non-zero twist angle. It has been found that when the applied voltage across such a cell is decreased, the brightness does not increase smoothly to its new value, but instead rises rapidly, reaches a non-equilibrium maximum value, and then slowly decays to the equilibrium value. According to the present invention, the liquid crystal layer is switched before the equilibrium value of the brightness has been reached, so as to make use of the non-equilibrium increased brightness and so increase the brightness of the display. The present invention also provides a liquid crystal display device having means for driving the liquid crystal layer so as to make use of the dynamic variation in the transmissivity of the liquid crystal device.

Claims

1. A method of driving a surface mode liquid crystal display device including a liquid crystal layer having a non-zero twist angle, the method comprising the step of:

applying a first voltage having a first magnitude across the liquid crystal layer to put the liquid crystal layer into a first liquid crystal state;

characterized in that it further comprises changing the magnitude of the voltage applied across the liquid crystal layer while the brightness of the liquid crystal device is greater than the equilibrium value of the brightness associated with the first liquid crystal state.

2. A method as claimed in

claim 1

wherein the step of changing the magnitude of the voltage applied across the liquid crystal layer is performed before the director of liquid crystal molecules in the centre in a thickness direction of the liquid crystal layer has substantially reached its equilibrium orientation associated with the first liquid crystal state.

3. A method as claimed in

claim 1

wherein the step of changing the magnitude of the voltage applied across the liquid crystal layer is carried out after no more than 50% of the time required for the brightness of the liquid crystal device substantially to reach the equilibrium value associated with the first liquid crystal state.

4. A method as claimed in

claim 1

wherein the step of changing the magnitude of the voltage applied across the liquid crystal layer is carried out after approximately 32% of the time required for the brightness of the liquid crystal device substantially to reach the equilibrium value associated with the first liquid crystal state.

5. A method as claimed in

claim 1

wherein the step of changing the magnitude of the voltage applied across the liquid crystal layer is carried out no later than approximately 16 msec after the step of applying the first voltage.

6. A method as claimed in

claim 1

and further comprising the step of putting the liquid crystal layer into a known liquid crystal state before performing the step of applying the first voltage.

7. A method as claimed in

claim 6

wherein the step of putting the liquid crystal layer into a known liquid crystal state comprises applying a blanking voltage to the liquid crystal layer.

8. A method as claimed in

claim 7

wherein the magnitude V_blank of the blanking voltage is dependent on the temperature of the liquid crystal layer.

9. A method as claimed in

claim 1

wherein the liquid crystal layer contains a chiral dopant.

10. A method as claimed in

claim 9

wherein the chiral dopant has a temperature dependent pitch.

11. A method as claimed in

claim 10

wherein the viscosity of the liquid crystal layer increases with the temperature of the liquid crystal layer, and the pitch induced by the chiral dopant in the liquid crystal decreases with temperature.

12. A method as claimed in

claim 9

wherein the chiral dopant induces a twist in the director of the liquid crystal molecules having a pitch p such that d/p=0.25, where d is the thickness of the liquid crystal layer.

13. A method as claimed in

claim 9

wherein the chiral dopant induces a twist in the director of the liquid crystal molecules having a pitch p such that d/p≧

0.25, where d is the thickness of the liquid crystal layer.

14. A method as claimed in

claim 9

wherein the chiral dopant induces a twist in the director of the liquid crystal molecules such that 0.25≦

d/p≦

0.5.

15. A method as claimed in

claim 1

wherein the twist angle of the liquid crystal layer is substantially 180°

.

16. A method as claimed in

claim 1

wherein the liquid crystal display device is an active matrix display device comprising an array of pixels each pixel defined by a corresponding pixel electrode, M strobe electrodes and N signal electrodes, the method comprising, in a frame, the steps of:

(a) applying a blanking voltage to each of the pixels;

(b) applying a respective signal voltage corresponding to desired image data to each of the pixels;

(c) allowing the liquid crystal molecules to switch states; and

(d) displaying the image.

17. A method as claimed in

claim 16

wherein step (a) comprises applying the blanking voltage substantially simultaneously to all pixels.

18. A method as claimed in

claim 16

wherein step (a) comprises applying the blanking voltage to pixels associated with each strobe electrode in sequence.

19. A method as claimed in

claim 16

wherein step (b) comprises applying the signal voltages to pixels associated with each strobe electrode in the sequence 1, 3, 5, . . . M, M−

1, M−

3, . . . 6, 4, 2 (M odd) or 1, 3, 5 . . . M−

1, M, M−

2, . . . 6, 4, 2 (M even).

20. A method as claimed in

claim 16

wherein the combined duration of steps (a) to (d) is substantially 16 msec and the duration of step (c) is substantially 7 msec.

21. A method as claimed in

claim 16

wherein the display device comprises a backlight, and the backlight is on during all of steps (a) to (d).

22. A method as claimed in

claim 16

wherein the display device comprises a backlight wherein the backlight is on for only part of a frame.

23. A method as claimed in

claim 22

wherein the backlight is off during step (a).

24. A method as claimed in

claim 16

wherein the device is a pi-cell.

25. A surface mode liquid crystal display device comprising:

a liquid crystal layer having a non-zero twist angle;

means for applying a first voltage having a first magnitude across a selected portion of the liquid crystal layer to put the selected portion of the liquid crystal layer into a first liquid crystal state; and

means for changing the magnitude of the voltage applied across the selected portion of the liquid crystal layer while the brightness of the part of the liquid crystal device corresponding to the selected portion of the liquid crystal layer is greater than the equilibrium value of the brightness associated with the first liquid crystal state.