Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-07-21
2003-03-18
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000, C345S099000, C345S100000
Reexamination Certificate
active
06535189
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device capable of a multi-gray scale display and used as a displaying means for a personal computer, a work station or the like.
Liquid crystal display devices are widely used as a display device for office automation equipment such as a personal computer. Liquid crystal display devices are divided roughly into a simple matrix type which forms pixels using intersections of intersecting stripe-shaped electrodes and an active matrix type which is provided with an active element such as a thin film transistor (TFT) at each pixel and switches the active element ON or OFF.
The active matrix type liquid crystal display device has a TFT-type liquid crystal panel, a scanning-signal line driver circuit (sometimes referred to as a gate driver) for supplying a scanning voltage to each of scanning signal lines (gate lines) of the liquid crystal panel, a video-signal line driver circuit (sometimes referred to as a drain driver) for supplying video signal voltages to video-signal lines (drain lines), a display control device for supplying various kinds of control signals and display data provided from a host computer such as a personal computer to the gate driver and the drain driver as display signals, and an internal power supply circuit.
FIG. 24
is a block diagram for explaining a rough configuration of a liquid crystal display device to which the present invention is applied. A liquid crystal panel
281
of the liquid crystal display device is an active-matrix type liquid crystal panel using thin-film transistors (a TFT LCD), and a plurality of drain drivers
282
and a plurality of gate drivers
283
are disposed along the top side of the liquid crystal panel
281
.
The liquid crystal panel
281
comprises 1024×768 pixels, for example, each of which comprises three-color sub-pixels, red (R), green (G) and blue (B) sub-pixels.
A display control device
285
receives a display data (a video signal) in three colors of red (R), green (G) and blue (B), and control data including a clock signal, a display timing signal and a synchronizing signal from a host computer such as a personal computer via an interface connector
284
.
The display control device
285
generates data in a display format of the liquid crystal panel based upon the control signal, and supplies them to the drain drivers
282
via data bus, and simultaneously with this, supplies timing signals such as a display start timing clock, a line clock and a pixel clock (a carry signal, CL
1
and CL
2
) to the drain drivers
282
.
An internal power supply circuit
286
generates reference voltages (V
9
to V
0
) for generating gray scale display voltages and supplies them to the drain drivers
282
, and supplies a scanning voltage (a gate voltage) to the gate drivers
283
.
Each of the drain drivers
282
is allotted to a group comprised of a given number of video signal lines (drain lines), and outputs a carry signal to a succeeding one of the drain drivers
282
when the count reaches the above given number.
The drain drivers
282
each include a gray-scale generating circuit for generating a gray-scale voltage based upon a display data, and an amplifier for amplifying the generated gray-scale voltage and supplying a video signal voltage corresponding to the display data to a corresponding one of the drain lines.
In a liquid crystal display device of the TFT type, it is necessary to reverse the polarity of a video signal voltage applied to a drain line with respect to a voltage (hereinafter VCOM) applied to a counter electrode which opposes pixel electrodes from frame to frame, so as to prevent “burning” of the liquid crystal layer. For this polarity reversal, there are a VCOM AC driving method which reverses polarities of both two voltages applied to a pixel electrode and a counter electrode, respectively, and a dot-polarity inversion drive method which changes greatly a voltage applied to a pixel electrode with a fixed voltage applied to the counter electrode.
Such prior art techniques for the liquid crystal display devices are disclosed in Japanese Patent Application Laid-open No. Hei 9-281930 (laid-open on Oct. 31, 1997 and corresponding to U.S. Pat. No. 5,995,073 issued on Nov. 30, 1999), for example.
SUMMARY OF THE INVENTION
Recently, there is a tendency for the TFT active matrix type liquid crystal display device to be made larger in size of the liquid crystal panel, increase image resolution, improve image quality, and reduce power consumption. Further, it is desired that a useless space and the border areas around a display area are minimized to achieve aesthetic qualities of the display device.
It is essential that the cost of the liquid crystal display devices is brought down as their market matures, and there is a demand for reduction of the areas of chips of the drain drivers as well as the reduction of the border areas around a display area.
As liquid crystal panels used for a monitor spread as a large-screen display device superseding a cathode ray tube, there has been a demand for liquid crystal display devices capable of higher resolution and a larger number of gray scales. It is essential that the liquid crystal panels for a monitor can display 256 gray scales, while liquid crystal panels for notebook personal computers displayed 64 gray scales.
As for resolution also, the number of pixels in the liquid crystal monitor panel is changing from the XGA (extended video graphics array) specification to the SXGA (super XGA) specification and the UXGA (ultra XGA) specification and consequently, electrical loads on the liquid crystal panels tends to increase, and a time for writing in gray-scale voltages corresponding to a line in the liquid crystal panel is made shorter because a display speed of one picture is fixed. At the present time, the larger the screen size and the higher the resolution, the higher the gray-scale voltages, to retain the brightness equal to that by the conventional liquid crystal panel.
In the above situation, the increases in resolution, the number of gray-scales and operating voltages lead to the increases in IC chip size and consequently, the cost is increased.
A conventional decoder system of a so-called tournament type requires the same number of decoder circuits as that of gray scales, which is a great factor in the increase in chip size caused by the increase in the number of gray-scales, and this makes it difficult to reduce the border areas around a display area. The term tournament type comes from an analogy that exists between selection of one of many gray-scale voltages and a tournament in which many contestants compete for championship in series of elimination contests.
FIG. 25
is a circuit of a low-voltage circuit portion of a drain driver employing the conventional tournament type decoder system. The dot-polarity inversion method requires a high-voltage circuit portion of the drain driver for forming a pair with the low-voltage circuit portion. The high-voltage circuit portion is identical in configuration with the low-voltage circuit portion in
FIG. 25
, except that NMOS transistors serving as switching elements in
FIG. 25
are interchanged with PMOS transistors, and its explanation is omitted.
In the low-voltage circuit portion in
FIG. 25
, three circuits CKTB, CKTC and CKTD identical with a circuit CKTA connected to a terminal A as shown in
FIG. 25
are connected to terminals B, C and D, respectively and the circuits CKTA, CKTB, CKTC and CKTD are supplied with four groups of gray-scale voltages V
000
to V
063
, gray-scale voltages V
064
to V
127
, gray-scale voltages V
128
to V
191
and V
192
to V
255
, respectively.
All the tournament type decoders CKTA, CKTB, CKTC and CKTD connected to the terminals A, B, C and D, respectively, are identical in configuration, and therefore the following explains only the tournament type decoder CKTA connected to the terminal A and supplied with the gray-scale voltages V
000
to V
063
Akiyama Ken'ichi
Goto Mitsuru
Isami Hironobu
Kotera Koichi
Yamashita Yuji
Antonelli Terry Stout & Kraus LLP
Anyaso Uchendu O.
Hitachi ULSI Systems Co. Ltd.
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