Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
2000-12-20
2003-11-18
Awad, Amr (Department: 2675)
Computer graphics processing and selective visual display system
Display driving control circuitry
Intensity or color driving control
C345S088000, C345S213000
Reexamination Certificate
active
06650341
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal driving method and a liquid crystal driving circuit, and more specifically to a liquid crystal driving method and a liquid crystal driving circuit, which can control unevenness in color in a liquid crystal panel, attributable to a voltage shift occurring in the case of carrying out an AC driving on the basis of a potential on an opposing electrode in the liquid crystal panel.
In general, a liquid crystal panel is written with several tens frames (several tens screen images) per second, and an output signal of a liquid crystal drive circuit carries out an AC drive on the basis of a potential on an opposing electrode in the liquid crystal panel, in units of scan line or in units of frame. Namely, if an DC voltage continues to be applied to the liquid crystal, ions are accumulated in one electrode, with the result that the liquid crystal becomes immediately deteriorated. In order to avoid this deterioration, the AC drive is carried out by inverting, in units of one frame or a few frames, the positive
egative polarity of the liquid crystal drive circuit output signal, which is a video signal voltage to be applied to the liquid crystal.
FIG. 7
shows an example of the liquid crystal drive circuit for carrying out the AC drive on the basis of the potential on the opposing electrode in the liquid crystal panel in the above mentioned manner. This liquid crystal drive circuit is a technology disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-09-218671, and is a switched capacitor type D/A converter having a sample period and a hold period. This D/A converter is mainly constituted of a differential operational amplifier
304
connected to an output terminal
303
and having a non-inverted input terminal connected to a first reference voltage input terminal
300
. An inverted input terminal of the differential operational amplifier
304
is connected to a first capacitor group
305
including a plurality of capacitors which are constituted of unitary capacitors as a basic element. A second capacitor or a second capacitor group (which will be expediently generically called the second capacitor group in this description)
306
is connected between the non-inverted input terminal and the output terminal
303
of the differential operational amplifier
304
. In addition, the following switch group is constituted for on-off switching between the differential operational amplifier
304
, the first capacitor group
305
and the second capacitor group
306
.
Namely, a first switch group
307
is provided in which one end of each switch is connected to one end of a corresponding capacitor in the first capacitor group
305
and the other end of each switch is connected in common to a second reference voltage input terminal
301
. A second switch group
308
is provided in which one end of each switch is connected to one end of a corresponding capacitor in the first capacitor group
305
and the other end of each switch is connected in common to a connection node between third and fourth switches
309
and
310
explained hereinafter. There are provided the third switch
309
having one end connected to the other end of the second switch group
308
and the other end connected to the first reference voltage input terminal
300
and the non-inverted input terminal of the differential operational amplifier
304
, the fourth switch
301
having one end connected to the other end of the second switch group
308
and the one end of the third switch
309
and the other end connected to a third reference voltage input terminal
302
, and a fifth switch
311
connected in parallel to the second capacitor group
306
.
In this liquid crystal drive circuit, two values are selected from gamma-compensated analog gradation voltages of for example 8 to 10 gradation levels, which are supplied from an external circuit of the drive circuit, and the two selected values of the analog gradation voltages are supplied to the second and third reference voltage input terminals
301
and
302
, respectively, and on the other hand, the first to fifth switch groups and switches
307
to
311
are selected turned on, so that an analog gradation voltage is further divided with the result that one level of multi-gradated gradation data is outputted from the output terminal
303
as an analog image data. In addition, the polarity of the voltages applied to the second and third reference voltage input terminals
301
and
302
is inverted in order to carry out the AC drive. Incidentally, the inversion of the polarity of the reference voltage generates a large load when the liquid crystal drive circuit is operated. Therefore, the above referred Japanese publication discloses that a control circuit is provided for selectively operating each of the above mentioned switches. This control circuit receives a digital image data, a sample/hold input clock and a frame input clock, and inverts the polarity of the voltage outputted from the output terminal, on the basis of the voltage on the first reference voltage input terminal
300
, in accordance with the image data and the clocks. However, the detail will be omitted.
However, in the liquid crystal drive circuit shown in
FIG. 7
, since the output voltage is determined by a ratio between the first capacitor group
305
and the second capacitor group
306
, if the value of this ratio varies, the output voltage is deviated from a set value. For example, in the process of a fabrication of the liquid crystal drive circuit, when a reticle is prepared or when a capacitor is actually shot onto a wafer, if a difference occurs in capacitance between the first capacitor group
305
and the second capacitor group
306
, by changing from one circuit to another, from one chip to another, from one wafer to another, and from one lot to another, an error occurs in the output voltage as mentioned above, with the result that a display unevenness attributable to the output voltage difference occurs in an image displayed in the liquid crystal.
This output voltage difference can be specifically expressed by the following mathematical equations. Here, in order to simplify the calculation, it is assumed that the circuit shown in
FIG. 7
is a 2-bit switched capacitor type D/A converter. When the value of the first capacitor group
305
is deviated from the value of the second capacitor group
306
by a capacitance value &Dgr;&agr; in a capacitance increasing direction, the voltage value of a positive side is expressed by the equation (1):
Vout
(positive, &agr;)={
Vref
(4+&Dgr;&agr;)}/(4+&Dgr;&agr;) −4
V
2
/(4+&Dgr;&agr;)−{&khgr;(
V
1
+V
2
)}/(4+&Dgr;&agr;) (1)
In a similar condition, the voltage value of a negative side is expressed by the equation (2):
Vout
(negative, &agr;)=&Dgr;&agr;/(4+&Dgr;&agr;) +4
V
2
/(4+&Dgr;&agr;)+{&khgr;(
V
1
+V
2
)}/(4+&Dgr;&agr;) (2)
where Vref is a first reference voltage supplied to the first reference voltage input terminal
300
, V
1
is a second reference voltage supplied to the second reference voltage input terminal
301
, and V
2
is a third reference voltage supplied to the third reference voltage input terminal
302
.
In the case of driving the liquid crystal panel, the AC drive is carried out by alternately outputting the voltage expressed by the equation (1) and the voltage expressed by the equation (2). However, if the capacitance value difference expressed by &Dgr;&agr; occurs in each of the above equations, the amplitude of the voltage on the basis of the potential of the opposing electrode in the liquid crystal panel increases as shown in
FIG. 8
, or alternatively decreases, so that the output signal having an error is outputted. Therefore, an actually displayed color is expressed as an effective value=[(1)−(2)]/2. This effective value is expressed by the equation (3). Incidentally, the equation (3) is exp
Awad Amr
Choate Hall & Stewart
NEC Electronics Corporation
LandOfFree
Liquid crystal driving method and liquid crystal driving... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid crystal driving method and liquid crystal driving..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal driving method and liquid crystal driving... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3149072