Unipolar waveform drive method and apparatus for a bistable liquid crystal display

• US 6,268,840 B1
• Filed: 04/21/1998
• Issued: 07/31/2001
• Est. Priority Date: 05/12/1997
• Status: Expired due to Term

• Current Assignee: Kent Displays Incorporated
• Original Assignee: Kent Displays Incorporated
• Inventors: Huang, Xiao-Yang
• Primary Examiner(s): CHOW, DOON Y
• Application Number:
  US09/063,907
• Time in Patent Office: 1,197 Days
• Field of Search: 345/87, 345/94, 345/95, 345/96, 345/97, 345/98, 345/99, 345/100, 345/210, 349/169, 349/179, 349/33, 349/34, 349/37
• US Class Current: 345/94
• CPC Class Codes: G09G 2300/0486 Cholesteric liquid crystals, including chiral-nematic liquid crystals, with transitions between focal conic, planar, and homeotropic states
• CPC Class Codes: G09G 2310/06 Details of flat display driving waveforms
• CPC Class Codes: G09G 3/3629 using liquid crystals having memory effects, e.g. ferroelectric liquid crystals

Abstract

A flat-panel liquid crystal display having unipolar drive circuitry. The display includes a flat sheet of bistable chiral nematic liquid crystal material activated by a drive circuit that individually controls the display state of multiple picture elements at a refresh rate 1000 scan lines per second. The driver circuitry activates the liquid crystal domains into homeotropic states over a relatively long activation period and then, during a short (˜1 msec.) selection period, either keeps the domains in a homeotropic state or initiate a transition to the transient twisted planar state. The drivers then activate the domains in an evolution phase to provide either focal conic or twisted planar end states across the two-dimensional array of picture elements. The drive circuitry includes a plurality of unipolar display drivers which generate substantially square wave, unipolar waveforms which are applied to row and column electro segments. The frequency and timing of the unipolar waveforms is controlled by the display driver circuitry to generate desired bi-polar voltages across picture elements or pixels of the display. A pipelining scheme wherein a number of pixel rows are addressed simultaneously during preparation, and evolution stages is used to reduce total updating time for the display.

Claims

1. A method of activating a bistable cholesteric liquid crystal material disposed between a first set of electrodes and a second set of electrodes arranged on opposed sides of said liquid crystal material that are adapted to selectively apply an electric field through said liquid crystal material, said method comprising the steps of:

a) energizing said electrodes to establish a preparation voltage across said liquid crystal during a preparation interval, thereafter energizing said electrodes to establish a bipolar selection voltage across said liquid crystal during a selection interval for selecting a final display state for said liquid crystal;

thereafter energizing said electrodes to establish an evolution voltage across said liquid crystal during an evolution interval, and thereafter permitting said liquid crystal to exhibit its final display state during a holding interval; and

b) said step of energizing during the selection interval accomplished by combining a first unipolar voltage waveform coupled to at least one electrode of the first set of electrodes and a second unipolar voltage waveform coupled to at least one electrode of the second set of electrodes to apply the bipolar selection voltage across the liquid crystal material.

2. The method of claim 1 wherein there are eight or fewer discrete voltage levels that make up the first and second unipolar voltage waveforms applied to electrodes on opposite sides of the liquid crystal material.

3. The method of claim 2 wherein the eight discrete voltage levels are applied by driver circuits coupled to the electrodes that are configured to convert three bits of digital signal data into a chosen one of the eight discrete voltage levels.

4. The method of claim 3 wherein the bistable cholesteric liquid crystal material, the first and second set of electrodes and the driver circuits comprise a liquid crystal display and a driver circuit has outputs coupled to multiple electrodes and includes a memory for storing digital data corresponding to a energization of multiple adjacent picture elements of the display only some of which are being energized with a selection voltage waveform.

5. The method of claim 4 wherein the driver circuit includes a clocking mechanism for shifting the data within the memory to provide an updating of an image displayed by the display.

6. The method of claim 1 wherein at least one of the first and second unipolar voltage waveforms comprises at least two discrete voltage levels.

7. The method of claim 6 wherein the first unipolar voltage waveform comprises at least two discrete voltage levels and second unipolar voltage waveforms comprises at least two discrete voltage levels.

8. The method of claim 7 wherein the step of applying the first and second unipolar voltage waveforms to electrodes on opposite sides of the liquid crystal material produces the bipolar selection voltage by a step of coordinating an application of the at least two discrete voltage levels of the first unipolar waveform with an application of the at least two discrete voltage levels of the second unipolar waveform to produce the bipolar selection voltage.

9. The method of claim 7 wherein the preparation, selection, and evolution voltages all switch polarity due to control of an application of the at least two discrete voltage levels of the first unipolar waveform and an application of the at least two discrete voltage levels of the second unipolar waveform so as to produce bipolar preparation, selection, and evolution voltages.

10. The method of claim 9 wherein at least one of the preparation and the evolution intervals includes application of a bipolar voltage that changes polarity at a frequency less than a frequency of polarity change of the bipolar selection voltage applied during the selection interval.

11. Display apparatus comprising:

a) a layer of bistable cholesteric liquid crystal material;

b) a first set of electrodes and a second set of electrodes spaced on opposite sides of the liquid crystal layer for applying selected energization voltages across multiple picture elements of the liquid crystal layer; and

c) control electronics for setting a display state of multiple picture elements of the liquid crystal layer comprising circuitry for;

i) applying a preparation voltage across a selected picture element of the liquid crystal layer during a preparation interval, the selected picture element defined by a region of the layer of liquid crystal material adjacent an overlapping region of one electrode of the first set of electrodes and one electrode of the second set of electrodes during a preparation interval;

ii) applying a bipolar selection voltage across said selected picture element during a selection interval to select a predetermined final display state, the bipolar selection voltage resulting from application of a first unipolar waveform applied to the one electrode of the first set of electrodes and a second unipolar waveform applied to the one electrode of the second set of electrodes; and

iii) applying an evolution voltage across said selected picture element during an evolution interval.

12. The display apparatus of claim 11 wherein the control electronics comprises one or more application specific integrated circuits for application of voltages to electrodes of the first set and one or more application specific integrated circuits for application of voltages to electrodes of the second set.

13. The display apparatus of claim 12 wherein the application specific integrated circuit includes a control input for configuring said circuit for applying voltages to a chosen one of the two sets of electrodes.

14. The apparatus of claim 11 wherein the control electronics comprises a driver circuit for applying discrete voltage levels to the electrodes that are configured based upon a digital control signal coupled to the driver circuit.

15. The apparatus of claim 14 wherein the driver circuit applies 8 or fewer discrete voltages levels based upon a three bit control signal applied to said driver circuit.

16. The apparatus of claim 14 wherein the driver circuit has outputs coupled to multiple electrodes and includes a memory for storing digital data corresponding to a energization of multiple adjacent picture elements only some of which are being energized with the selection voltage.

17. The apparatus of claim 16 wherein the driver circuit includes a clocking mechanism for shifting the data within the memory to provide an updating of an image of the display.

18. The apparatus of claim 14 wherein the driver circuit comprises means for applying discrete energization voltages during both a preselection interval and the selection interval.

19. The apparatus of claim 14 wherein during at least one of the preparation and the evolution intervals the driver circuit applies a bipolar voltage that changes polarity at a frequency less than a frequency of polarity change of the bipolar selection waveform applied during the selection interval.

20. The method of claim 11 wherein at least one of the first and second unipolar voltage waveforms comprises at least two discrete voltage levels.

21. The method of claim 20 wherein the first unipolar voltage waveform comprises at least two discrete voltage levels and second unipolar voltage waveforms comprises at least two discrete voltage levels.

22. A method of presenting an image on a display having a bistable cholesteric liquid crystal material disposed between electrodes arranged on opposed sides of said liquid crystal material by applying an electric field through said liquid crystal material, said method comprising the steps of:

a) providing a display having a bistable cholesteric liquid crystal material disposed between electrodes arranged on opposed sides of said liquid crystal material;

b) energizing said electrodes with display driver voltages to i) apply a preparation voltage across said liquid crystal material during a preparation phase, ii) apply a bipolar selection voltage across said liquid crystal material during a selection phase for selecting a final display state for said liquid crystal, iii) applying an evolution voltage across said liquid crystal material during an evolution phase, and iv) permitting said liquid crystal material to exhibit its final display state;

wherein the bipolar selection voltage results from application of a first unipolar waveform applied to one or more electrodes on one side of the liquid crystal material and a second unipolar waveform applied to one or more electrodes on an opposite side the liquid crystal material.

23. The method of claim 22 wherein at least one of the first and second unipolar voltage waveforms comprises at least two discrete voltage levels.

24. The method of claim 23 wherein the first unipolar voltage waveform comprises at least two discrete voltage levels and second unipolar voltage waveforms comprises at least two discrete voltage levels.

25. The method of claim 22 wherein the preparation voltage results from application of a first unipolar waveform applied to one or more electrodes on one side of the liquid crystal material and a second unipolar waveform applied to one or more electrodes on an opposite side the liquid crystal material one electrode of the second set of electrodes.

26. The method of claim 22 wherein the display driver voltages include eight or fewer discrete voltage levels all of the same voltage polarity with respect to a reference potential.

27. The method of claim 26 wherein the electrodes are arranged on opposite sides of the liquid crystal material and wherein an overlapping region of electrodes on opposites sides of the liquid crystal material defines a picture element and further wherein one of said eight distinct voltage levels is applied to each electrode in the overlapping region during the selection phase to discriminate between the final display state of said picture element.

28. The method of claim 26 wherein an electric field direction is periodically reversed through application of different ones of said eight or fewer voltages.