Bistable reflective cholesteric liquid crystal displays utilizing super twisted nematic driver chips

• US 6,278,429 B1
• Filed: 09/11/1998
• Issued: 08/21/2001
• Est. Priority Date: 09/11/1998
• Status: Expired due to Term

• Current Assignee: Kent State University
• Original Assignee: Kent State University
• Inventors: Hewitt, Richard, Ruth, Jonathan C., Bos, Philip J.
• Primary Examiner(s): Hjerpe, Richard
• Assistant Examiner(s): NGUYEN, KIMNHUNG T
• Application Number:
  US09/151,420
• Time in Patent Office: 1,075 Days
• Field of Search: 345/94, 345/95, 345/208, 345/210, 349/96, 349/98
• 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
• CPC Class Codes: G09G 3/3681 suitable for passive matrices only
• CPC Class Codes: G09G 3/3692 suitable for passive matrices only

Abstract

A driving circuit for a reflective bistable cholesteric liquid crystal display which includes one substrate having a plurality of column or segment electrodes opposed by another substrate having a plurality of row or common electrodes. The intersecting column and row electrodes with the cholesteric material therebetween form a plurality of pixels. The driving circuit selectively applies a voltage to the row and column electrodes to control the appearance of the cholesteric material. In particular, the driving circuit includes at least one common driver coupled to respective common electrodes with each common driver having a first and a second common frame switch with corresponding high or low inputs. The first and second common frame switches are linked to one another by a common frame line. The high and low common inputs are connected to a plurality of common data switches, the first common frame switch a first and a second common voltage input and the second column frame switch having a third and a fourth common voltage input connected to each other. The driving circuit also includes at least one segment driver coupled to respective segment electrodes. The at least one segment driver is configured much the same as the common driver, except that it receives different input voltages. By selectively toggling the frame and data switches of each driver, a dynamic drive scheme can be applied to the display.

Claims

1. A bistable cholesteric liquid crystal display, comprising:

a pair of opposed substrates, one said substrate having a first plurality of electrodes, the other said substrate having a second plurality of electrodes oriented in a direction different than said first plurality of electrodes;

a cholesteric liquid crystal material disposed between said pair of opposed substrates and forming a pixel at each intersection of said first and second plurality of electrodes;

a first super twisted nematic driver having a plurality of outputs connected to said first plurality of electrodes; and

a second super twisted nematic driver having a plurality of outputs connected to said second plurality of electrodes, wherein both said drivers receive a plurality of voltage waveforms for selective transmission as respective voltage output waveforms to said first and second plurality of electrodes.

2. The display according to claim 1, wherein said voltage output waveforms are combined to form a pixel waveform at each said pixel.

3. The display according to claim 2, wherein said pixel waveform comprises:

a preparation phase for applying a preparation voltage to each said pixel for driving said cholesteric liquid crystal material into a homeotropic texture;

a hold phase for applying a hold voltage, different than said preparation voltage, for maintaining said cholesteric liquid crystal material in the homeotropic texture;

a selection phase for applying a selection voltage to each said pixel for predisposing said cholesteric liquid crystal material; and

an evolution phase for applying an evolution voltage to each said pixel to allow said predisposed liquid crystal material to relax into either a planar or a focal conic texture.

4. The display according to claim 1, wherein said plurality of voltage input waveforms applied to one of said drivers comprises an upper level and a lower level preparation waveform, an upper level and a lower level select waveform, and an upper and a lower level hold/evolve waveform.

5. The display according to claim 4, wherein said plurality of voltage input waveforms applied to one of said drivers comprises a non-select waveform.

6. The display according to claim 1, wherein said plurality of voltage input waveforms applied to one of said drivers comprises a lower level preparation voltage and an upper level and a lower level data waveform.

7. The display according to claim 1, wherein said voltage output waveforms applied to one of said plurality of electrodes comprises:

an alternating upper and lower level preparation waveform, an alternating upper and lower level select waveform, and an alternating upper and lower level hold/evolve waveform.

8. The display according to claim 7, wherein said voltage output waveforms applied to one of said plurality of electrodes further comprises a non-select waveform.

9. The display according to claim 1, wherein said voltage output waveforms applied to one of said plurality of electrodes comprises:

an alternating upper and lower level preparation waveform, and an alternating upper and lower level data waveform.

10. A method for addressing a cholesteric liquid crystal display which has a pair of opposed substrates, one of the substrates having a first plurality of electrodes, the other substrate having a second plurality of electrodes oriented in a direction different than the first plurality of electrodes, the substrates having cholesteric liquid crystal material disposed therebetween to form a pixel at each intersection of the first and the second plurality of electrodes, the first plurality of electrodes having at least a first driver coupled thereto and the second plurality of electrodes having at least a second driver coupled thereto, the method comprising the steps of:

applying a plurality of voltage input waveforms to the first driver and the second driver, wherein said voltage input waveforms are about 60 volts RMS or less;

selectively transmitting said plurality of voltage input waveforms through the first and the second drivers to generate respective first and second output waveforms;

combining said first and second output waveforms at the intersecting electrodes to generate a pixel waveform that drives the cholesteric liquid crystal material to the desired appearance, wherein said step of combining further comprises the steps of;

applying preparation voltages to each pixel for driving the cholesteric material into a homeotropic texture;

applying hold voltages to each pixel wherein at least one of said hold voltages is different than said preparation voltages, to maintain the cholesteric material in the homeotropic texture;

applying selection voltages to each pixel for predisposing the cholesteric material; and

applying evolution voltages to each pixel to allow the predisposed cholesteric material to relax into either a planar or a focal conic texture.

11. The method according to claim 10, wherein said step of applying further comprises the steps of:

applying an upper level and a lower level preparation waveform;

applying an upper level and a lower level select waveform; and

applying an upper level and a lower level hold/evolve waveform.

12. The method according to claim 11, wherein said step of applying further comprises the step of:

applying a non-select waveform.

13. The method according to claim 10, wherein said step of selectively transmitting further comprises the steps of:

transmitting upper and lower level preparation waveforms;

transmitting upper and lower level select waveforms; and

transmitting upper and lower level hold/evolve waveforms.

14. The method according to claim 13, wherein said step of selectively transmitting further comprises the step of:

transmitting a non-select waveform.

15. The method according to claim 10, wherein said step of selectively transmitting further comprises the steps of:

transmitting upper and lower level preparation waveforms; and

transmitting upper and lower level data waveforms.

16. A driving circuit for a reflective bistable cholesteric liquid crystal display which includes one substrate having a first plurality of electrodes opposed by another substrate having a second plurality of electrodes, wherein the intersection of the first and the second plurality of electrodes with bistable cholesteric liquid crystal material disposed therebetween form a plurality of pixels, the driving circuit selectively applying voltages to the first and the second plurality of electrodes to control the appearance of each pixel, the driving circuit comprising:

at least one common driver coupled to the first plurality of electrodes, each common driver having a first and a second common frame switch, each said common frame switch having a high input and a low input, said first and second frame switches linked to one another by a common frame line, said high and low inputs connected to a plurality of common data switches, each having a common output, said first common frame switch receiving a first and a second common voltage input and said second common frame switch receiving a third and a fourth common voltage input connected to each other, wherein said common frame line is toggled to selectively pass through said common voltage inputs for use as said common voltage output from each said common data switch; and

at least one segment driver coupled to the second plurality of electrodes, said at least one segment driver having a plurality of segment voltage outputs which are selectively applied to the second plurality of electrodes to drive the cholesteric liquid crystal material to a desired texture.

17. The driving circuit according to claim 16, wherein said at least one segment driver has a first and a second segment firmed switch, each said segment frame switch having a high input and a low input, said first and second segment frame switches linked to one another by a segment frame line, said high and low segment inputs connected to a plurality of segment data switches, each segment data switch having a segment output, said first segment frame switch receiving a first and a second segment voltage input and said second segment frame switch receiving a third and a fourth segment voltage input connected to each other, wherein said segment frame line is toggled to selectively pass through said segment voltage inputs for use as said segment voltage output.

18. The driving circuit according to claim 17, wherein said third and fourth common voltage inputs provide a first arbitrary waveform for said corresponding common voltage output, and wherein said third and fourth segment voltage inputs provide a second arbitrary waveform for said corresponding segment voltage output.

19. The driving circuit according to claim 18, wherein said first common frame switch receives a preparation voltage and a hold/evolve voltage.

20. The driving circuit according to claim 18, wherein said first segment frame switch receives a preparation voltage and a data voltage.

21. The driving circuit according to claim 18, wherein said second common frame switch receives at least a selection voltage.

22. The driving circuit according to claim 18, wherein said second segment frame switch receives a data voltage.

23. The driving circuit according to claim 18, wherein said second row frame switch receives three different voltage values so that indicia appears on the display row by row.

24. The driving circuit according to claim 18, wherein said second common frame switch receives two different voltage values so that indicia appears on the display all at once.

25. A method for addressing a liquid crystal display having a plurality of common electrodes orthogonally positioned with respect to a plurality of segment electrodes with cholesteric liquid crystal material disposed therebetween, the method comprising the steps of:

connecting at least one common driver to the plurality of common electrodes;

connecting at least one segment driver to the plurality of segment electrodes; and

toggling a pair of frame switches in each said common and segment driver such that a first of said pair of frame switches applies one of two waveforms to the corresponding electrodes or such that a second of said pair of frame switches applies an arbitrary waveform to the corresponding electrodes.

26. The method according to claim 25, other comprising the step of:

switching a plurality of data switches in each said common and segment driver to apply either the one of two waveforms or said arbitrary waveform to the corresponding electrodes.

27. The method according to claim 26 further comprising the step of:

transmitting a preparation voltage and a hold/evolve voltage through said first frame switch of said common driver.

28. The method according to claim 26 further comprising the step of:

transmitting a preparation voltage or a data voltage through said first frame switch of said segment driver.

29. The method according to claim 26, further comprising the step of:

transmitting at least a selection voltage through said second frame switch of said common driver.

30. The method according to claim 26, further comprising the step of:

transmitting a data voltage through said second frame switch of said segment driver.

31. The method according to claim, 26, further comprising the step of:

transmitting three different voltage values through said second frame switch of said common driver so that indicia appears on the display row by row.

32. The method according to claim 26, further comprising the step of:

transmitting two different voltage values through said second frame switch of said common driver so that indicia appears on the display all at once.

33. A driving circuit for a reflective bistable cholesteric liquid crystal display which includes one substrate having a first plurality of electrodes opposed by another substrate having a second plurality of electrodes, wherein the intersection of the first and the second plurality of electrodes with bistable cholesteric liquid crystal material disposed therebetween form a plurality of pixels, the driving circuit selectively applying voltages to the first and the second plurality of electrodes to control the appearance of each pixel, the driving circuit comprising:

a plurality of common drivers for generating and selectively applying voltages to the first plurality of electrodes, each said common driver having two inputs connected to each other to form a superinput;

a plurality of segment drivers for generating and selectively applying voltages to the second plurality of electrodes; and

a mode select switch coupled to each said common driver to select one of two modes to transfer a select or non-select voltage to said superinput, wherein said plurality of common drivers and said plurality of segment drives apply the voltages to drive the cholesteric liquid crystal material to a desired texture.

34. The driving circuit according to claim 33, further comprising:

a common select waveform generator switch coupled to said mode select switch for generating said select voltage in a high value or a low value.

35. The driving circuit according to claim 33, further comprising:

a prep/write select switch coupled to said plurality of segment drivers to pass through a preparation signal when said switch is in a first position and a writing signal when said switch is in a second position.