DRIVING CIRCUIT OF ELECTRO-OPTICAL DEVICE, DRIVING METHOD FOR ELECTRO-OPTICAL DEVICE, AND ELECTRO-OPTICAL DEVICE AND ELECTRONIC EQUIPMENT EMPLOYING THE ELECTRO-OPTICAL DEVICE

• US 20020003521A1
• Filed: 12/17/1998
• Published: 01/10/2002
• Est. Priority Date: 04/18/1997
• Status: Active Grant

• Current Assignee: 138 East LCD Advancements Limited (Vector Capital Corporation)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: MATSUEDA, YOJIRO, OZAWA, TOKUROH
• Granted Patent: US 6,380,917 B2
• Time in Patent Office: Days
• US Class Current: 345/89
• CPC Class Codes: G09G 2310/027 Details of drivers for data electrodes, the drivers handling digital grey scale data, e.g. use of D/A converters
• CPC Class Codes: G09G 2320/0276 for the purpose of adaptation to the characteristics of a display device, i.e. gamma correction
• CPC Class Codes: G09G 3/2011 by amplitude modulation
• CPC Class Codes: G09G 3/3614 Control of polarity reversal in general
• CPC Class Codes: G09G 3/3688 suitable for active matrices only

Abstract

A driving circuit of an electro-optical device such as a liquid crystal device is compatible with digital image signals and implements a DA converting function and a γ correcting function by a relatively simple and small-scale circuit configuration. The driving circuit of the liquid crystal device is provided with a DAC 3 for issuing a voltage signal V_C corresponding to N bits of digital image data D_A that indicate a gray scale value to a signal line of the liquid crystal device. Depending on whether the value of a most significant bit is “0” or “1,” the DAC 3 brings the output driving voltage characteristic close to the optical characteristics of the liquid crystal device according to the a pair of first or second reference voltages so as to make a γ correction.

Claims

1. A driving circuit of an electro-optical device that supplies an analog image signal, which has a driving voltage corresponding to an arbitrary step of gray scale among 2^N (where N is a natural number) steps of gray scale, to a signal line of an electro-optical device in which the changes in the optical characteristic thereof with respect to the changes in said driving voltage are nonlinear;

said driving circuit of an electro-optical comprising;

an input interface to which an N-bit digital image signal indicative of said arbitrary step of gray scale is applied; and

a digital-to-analog converter that generates a voltage within a range of a pair of first reference voltages according to the bit value of said digital image signal to produce said driving voltage within a first driving voltage range corresponding to the step of gray scale of said digital image signal so that the changes in said driving voltage with respect to the changes in the step of gray scale of said digital image signal are nonlinear if said applied digital image signal indicates a step of gray scale from a first to m-1th (where “

m”

is a natural number and 1<

m≦

2N), and generates a voltage within a range of a pair of second reference voltages according to the bit value of said digital image signal to produce said driving voltage corresponding to the step of gray scale of said digital image signal within a second driving voltage range adjacent to said first driving voltage range so that the changes in said driving voltage with respect to the changes in the gray scale of said digital image signal are nonlinear if said digital image signal indicates a step of gray scale from an m-th to 2^N-th gray scale, and supplies said analog image signal having said generated driving voltage to said signal line.

2. The driving circuit of an electro-optical device according to claim 1, wherein the voltage polarity of said pair of first reference voltages and the voltage polarity of said pair of second reference voltages supplied to said digital-to-analog converter are set to be opposite from each other so that a change in said driving voltage corresponding to a change in the gray scale has an inflection point between said first and second driving voltage ranges.

3. The driving circuit of an electro-optical device according to claim 1, wherein:

the value of said “

m”

is equal to 2<sup>N−

1</sup>;

lower N-1 bits of said digital image signal are selectively applied to said digital-to-analog converter as they are or after being inverted according to the value of a most significant bit of said digital image signal;

said digital-to-analog converter generates a voltage in the range of said first reference voltage if said lower N-1 bits are applied thereto as they are, and generates a voltage in the range of said second reference voltage if said lower N-1 bits are inverted before being applied thereto.

4. The driving circuit of an electro-optical device according to claim 3, further comprising a selective inverting circuit for selectively inverting said lower N-1 bits depending upon the value of said most significant bit, said selective inverting circuit being provided between said interface and said digital-to-analog converter.

5. The driving circuit of an electro-optical device according to claim 1, further comprising a selective voltage supply circuit for selectively supplying either said first or second reference voltage to said digital-to-analog converter according to the value of the most significant bit of said digital image signal.

6. The driving circuit of an electro-optical device according to claim 1, wherein said digital-to-analog converter comprises a switched capacitor type digital-to-analog converter adapted to generate the voltages in the ranges of said first and second reference voltages, respectively, by means of charging a plurality of capacitors.

7. The driving circuit of an electro-optical device according to claim 6, wherein said first reference voltage is composed of a pair of voltages that enable a voltage in said first driving voltage range to be selectively generated, and said second reference voltage is composed of a pair of voltages that enable a voltage in said second driving voltage range to be selectively generated.

8. The driving circuit of an electro-optical device according to claim 7, wherein:

the value of said “

m”

is equal to 2<sup>N−

1</sup>;

the lower N-1 bits of said digital image signal are selectively applied to said switched capacitor type digital-to-analog converter as they are or inverted before being applied thereto according to the value of the most significant bit of said digital image signal; and

said switched capacitor type digital-to-analog converter generates a voltage in the range of said first reference voltage if said lower N-1 bits are applied thereto as they are, and generates a voltage in the range of said second reference voltage if said lower N-1 bits are inverted before being applied thereto.

9. The driving circuit of an electro-optical device according to claim 6, wherein said switched capacitor type digital-to-analog converter comprises:

a first through an N-1th capacitive elements respectively having a pair of opposed electrodes, wherein one of said paired first reference voltages or one of said paired second reference voltages is selectively applied to one of said paired opposed electrodes according to the value of said most significant bit;

a capacitive element resetting circuit for short-circuiting said pair of opposed electrodes in each of said first through N-1th capacitive elements so as to discharge electric charges therein;

a signal line potential resetting circuit for selectively resetting the potential of said signal line to the other of said paired first reference voltages or the other of said paired second reference voltages according to the value of said most significant bit; and

a selective switching circuit including a first through N-1th switches that selectively connect said first through N-1th capacitive elements to said signal lines, respectively, according to the values of said lower N-1 bits after the discharge by said capacitive element resetting circuit and the resetting by said signal line potential resetting circuit.

10. The driving circuit of an electro-optical device according to claim 9, wherein:

the capacitance of said first through N-1th capacitive elements is set to

C×

2<sup>i−

1</sup>

(C;

Predetermined unit capacitance;

i=1, 2 . . . , N-1).

11. The driving circuit of an electro-optical device according to claim 1, wherein the values of said first and second reference voltages are set so that the difference between said driving voltage corresponding to the m-1th step of gray scale and said driving voltage corresponding to the m-th step of gray scale is smaller than a predetermined value.

12. The driving circuit of an electro-optical device according to claim 11, wherein the values of said first and second reference voltages are set so that the ratio of said optical characteristic in the case where said electro-optical device is driven by said driving voltage corresponding to the m-1th step of gray scale to said optical characteristic in the case where said electro-optical device is driven by said driving voltage corresponding to the m-th step of gray scale is equivalent to one step of gray scale obtained by dividing the variation range of said optical characteristic by (2^N-1).

13. The driving circuit of an electro-optical device according to claim 1, wherein said digital-to-analog converter comprises a resistance ladder that divides said first and second reference voltages, respectively, by a plurality of resistors connected in series.

14. The driving circuit of an electro-optical device according to claim 13, further comprising a selective voltage supply circuit for selectively supplying either said first or second reference voltages to said digital-to-analog converter according to the value of the most significant bit of said digital image signal, wherein:

said digital-to-analog converter is further provided with a decoder that decodes the lower N-1 bits of said digital image signal and outputs decoded signals through 2<sup>N−

1</sup> output terminals, and 2<sup>N−

1</sup> switches, one terminal of each of which is connected to each of a plurality of taps drawn out among said plurality of resistors and the other terminal thereof is connected to each of said signal lines and which are respectively operated according to the decoded signals output through said 2<sup>N−

1</sup> output terminals.

15. The driving circuit of an electro-optical device according to claim 1, wherein said signal lines are provided with predetermined capacitors in addition to the parasitic capacitance of said signal lines.

16. The driving circuit of an electro-optical device according to claim 1, wherein said electro-optical device is a liquid crystal device composed of liquid crystal held between a pair of substrates, and said driving circuit is formed on one of said paired substrates.

17. The driving circuit of an electro-optical device according to claim 16, wherein said first and second reference voltages are respectively supplied to said digital-to-analog converter with a voltage polarity with respect to a predetermined reference potential being inverted for each horizontal scanning period.

19. An electro-optical device comprising the driving circuit according to claim 1.

20. Electronic equipment comprising the electro-optical device according to claim 17.

18. A driving method of an electro-optical device having a digital-to-analog converter that supplies an analog image signal, which has a driving voltage corresponding to an arbitrary step of gray scale among 2^N (where N is a natural number) steps of gray scale, to a signal line of the electro-optical device in which the changes in the optical characteristic with respect to the changes in said driving voltage are nonlinear, said driving method comprising the steps of:

supplying an N-bit digital image signal indicative of said arbitrary step of gray scale to said digital-to-analog converter;

generating a voltage within a range of a pair of first reference voltages according to the bit value of said digital image signal to produce said driving voltage within a first driving voltage range corresponding to the step of gray scale of said digital image signal by said digital-to-analog converter so that the changes in said driving voltage with respect to the changes in the gray scale of said digital image signal are nonlinear if said applied digital image signal indicates a first to m-1th step of gray scale (where “

m”

is a natural number and 1<

m≦

2^N);

generating a voltage within a range of a pair of second reference voltages according to the bit value of said digital image signal to produce said driving voltage that corresponds to the step of gray scale of said digital image signal and also lies within a second driving voltage range adjacent to said first driving voltage range by said digital-to-analog converter so that a change in said driving voltage with respect to a change in the gray scale of said digital image signal is nonlinear if said digital image signal indicates an m-th to 2^N-th step of gray scale; and

supplying said analog image signal having said generated driving voltage to said signal line.