Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-08-02
2003-06-10
Hjerpe, Richard (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S096000, C345S690000
Reexamination Certificate
active
06577293
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display, in more particular, to a method for driving the source lines of a liquid crystal display, which reduces the consumption power thereof.
2. Discussion of Related Art
A liquid crystal display (LCD) draws growing attentions as a display device for displaying video signals and studies and researches for this device are being actively carried out. In general, the LCD is roughly divided into a liquid crystal panel part and a driving part. The liquid crystal panel includes a lower glass plate on which pixel electrodes and thin film transistors (TFTs) are arranged in matrix form, a upper glass plate on which a common electrode and a color filter layer are formed, and a liquid crystal layer filled between the upper and the lower glass plates.
The driving part includes a video signal processor for processing video signals externally inputted, a controller for receiving a composite synchronous signal outputted from the video signal processor, dividing it into horizontal and vertical synchronous signals and controlling timing in response to mode (NTSC, PAL or SECAM) selecting signal, a source driver for supplying a signal voltage to the source lines of the liquid crystal panel in response to the output signal of the controller, and a gate driver for sequentially applying driving voltages to the scanning lines of the liquid crystal panel in response to the output signal of the controller. There have been actively performed researches for reducing the consumption power of the liquid crystal display constructed as above.
A conventional circuit and method for driving the source of a LCD is explained with reference to the attached drawings.
FIG. 1
shows the configuration of a conventional TFT-LCD. Referring to
FIG. 1
, the TFT-LCD includes a liquid crystal panel
10
having pixels each of which is located at each of points where a plurality of gate lines GL and a plurality of source lines SL intersect each other, a source driver
20
for providing each pixel with a video signal through the source lines SL, and a gate driver
30
for selecting a certain gate line GL of the liquid crystal panel
10
to turn on plural pixels. Here, each pixel consists of a TFT
1
whose gate is connected to the gate line GL and whose drain is connected to the source line SL, a storage capacitor Cs connected to the source of the TFT
1
in parallel, and a liquid crystal capacitor Clc.
FIG. 2
shows the configuration of the source driver of the conventional TFT-LCD. In this drawing, a 384-channel 6-bit driver is illustrated as an example of the source driver. That is, each of R, G, and B data is 6-bit and the number of the column lines is equal to 384. Referring to
FIG. 2
, the source driver includes a shift register
21
, a sampling latch
22
, a holding latch
23
, a digital/analog converter
24
, and an output buffer
25
.
The shift register
21
shifts the horizontal synchronous signal pulse HSYNC in response to a source pulse clock HCLK, to output a latch enable clock to the sampling latch
22
. The sampling latch
22
samples and latches digital R, G, and B data by column lines in response to the latch enable clock outputted from the shift register
21
. The holding latch
23
simultaneously receives the R, G, and B data latched by the sampling latch
22
in response to a load signal LD to latch the R, G, and B data. The digital/analog converter
24
converts the digital R, G, and B data stored in the holding latch
23
into analog R, G, and B data. Then, the output buffer
25
amplifies signal current corresponding to the R, G, and B data to output it to the source line of the liquid crystal panel.
The source driver constructed as above samples and holds the digital R, G, and B data during one horizontal period, converts it into the analog R, G, and B data, and current-amplifies it. Here, when the holding latch
23
holds R, G, and B data corresponding to the nth column line, the sampling latch
22
samples R, G, and B data corresponding to the (n+1)th column line.
FIG. 3
shows the gate driver of the conventional TFT-LCD. Referring to
FIG. 3
, the gate driver includes a shift register
31
, a level shifter, and an output buffer
33
. The shift register
31
shifts the vertical synchronous signal pulse VSYNC in response to a gate pulse VCLK, to sequentially enable the scanning lines. The level shifter
32
sequentially level-shifts a signal applied to the scanning lines to output it to the output buffer
33
. By doing so, the plural scanning lines connected to the output buffer
33
are sequentially enabled.
A method for driving the conventional TFT-LCD constructed as above is explained below.
First of all, the sampling latch
22
of the source driver
20
. sequentially receives video data corresponding to a single pixel and stores video data corresponding to the source lines SL. The gate driver
30
outputs a gate line selection signal GLSS to select one of the plural gate lines GL. Then, the TFT
1
connected to the selected gate line GL is turned on so as to apply the video data stored in the holding latch
23
to the drain thereof, thereby displaying the video data on the liquid crystal panel
10
.
Subsequently, the above-described operation is repeated to display video data on the liquid crystal panel
10
.
At this time, the source driver
20
provides VCOM, positive and negative video signals to the liquid crystal panel
10
to display the video data thereon.
FIG. 4
shows the voltage range of the video signals of FIG.
1
. Referring to
FIG. 4
, the positive and the negative video signals are alternately supplied to the pixels every time frame is changed, in order not to directly apply DC voltage to the liquid crystal during operation of the TFT-LCD and, for this, the electrode of the TFT-LCD upper plate is provided with the VCOM that is the medium voltage between the positive and negative video signals. In case where the positive and negative video signals are alternately applied to the pixels on the bases of the VCOM, however, light transmission curves of the liquid crystal do not agree with each other, generating flicker.
Accordingly, for the purpose of reducing the generation of flicker, four inversion modes are employed as shown in
FIGS. 5A
,
5
B,
5
C and
5
D. They are frame inversion, line inversion, column inversion and dot inversion modes.
FIG. 5A
shows the frame inversion mode in which the polarity of a video signal is modulated only when the frame is changed, and
FIG. 5B
shows the line inversion mode in which the video signal polarity varies every time the gate line GL is changed. Furthermore, the
FIG. 5C
shows the column inversion mode in which the video signal polarity varies when the source line and the frame are changed, and
FIG. 5D
shows the dot inversion in which the polarity changes whenever each source line SL and gate line GL are changed and the frame is changed. The picture quality is good in the order of the frame inversion, line inversion, column inversion, and dot inversion, and the number of times of polarity change becomes larger in proportion to the picture quality, to result in the increases in power consumption. This is explained below in detail with reference to the dot inversion mode for driving the conventional LCD shown in FIG.
6
.
FIG. 6
shows the waveform of a video signal applied to odd-numbered source lines SL or even-numbered source lines SL of the liquid crystal panel
10
. This illustrates that the polarity of the video signal of the source lines SL is modulated at every gate line change on the basis of the VCOM.
Here, it is assumed that the entire TFT-LCD panel displays the same gray color, the variation width (V) of the video signal of the source lines SL becomes twice that of the VCOM plus positive video signal or that of the VCOM plus negative video signal. Accordingly, the conventional dot inversion consumes a large amount of power because the polarity of the video signal changes from positive to negative or from negative to
Dinh Duc Q
Hjerpe Richard
NTek Research Co., Ltd.
Schweitzer Cornman Gross & Bondell LLP
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