Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-06-02
2003-04-15
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S098000
Reexamination Certificate
active
06549186
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor-liquid crystal display (TFT-LCD) and, more particularly, to a TFT-LCD, if which source lines of the liquid crystal panel are driven with a low power through charge sharing, to reduce the consumption power of a TFT-LCD driving circuit.
2. Discussion of Related Art
In general, a TFT-LCD is being widely used as a screen or a desk-top computer, TV, computer's monitor because it has the most excellent properties in a variety of LCDs, such as high image quality similar to that of CRT, high-speed response and soon. A conventional TFT-LCD, as shown in
FIG. 1
, includes a liquid crystal panel
10
having a plurality of pixels each of which is located at the point where each of a plurality of gate lines GL intersects each of a plurality of source lines SL, a source driver
20
for supplying a video signal to each of the pixels through a corresponding source line SL of the liquid crystal panel
10
, and a gate driver
30
for selecting a gate line GL of the liquid crystal panel
10
to turn or plural pixels. Each pixel consists of a thin film transistor
1
whose gate is connected to a corresponding gate line GL and whose drain is connected to a corresponding source line SL, and a storage capacitor Cs and a liquid crystal capacitor Clc which are connected to the source of the thin film transistor
1
in parallel.
The operation of the conventional TFT-LCD constructed as above is described below with reference to the attached drawings. A sampling register (not shown) of the source driver
20
sequentially receives video data items each of which corresponds to one pixel and stores them which correspond to the source lines SL, respectively. The video data items which are stored in the sampling register are transferred to the holding register by the signal of the controller. The gate driver
30
outputs a gate line selection signal GLS, to select a gate line GL among the plural gate lines GL. Accordingly, the plural thin film transistors connected to the selected gate line are turned on to allow the video data stored in the holding register of the source driver
20
to be applied to their drains, thereby displaying the video data on the liquid crystal panel
10
.
Here, the source driver
20
supplies VCOM, a positive video signal and a negative video signal to the liquid crystal panel
10
, to thereby display the video data thereon. That is, in the operation of the convention TFT-LCD, as shown in
FIG. 2
, the positive video signal and the negative video signal are alternately supplied to the pixels whenever a frame changes in order not to directly apply DC voltage to the liquid crystal. For this, the intermediate voltage between the positive and negative video signals, VCOM, is applied to an electrode formed on an upper plate of the TFT-LCD. When the positive and negative video signals are alternately provided to the liquid crystal on the basis of VCOM, however, light transmission curves of the liquid crystal do not accord with each other, resulting in flicker.
To reduce the generation of flicker, there is employed one of a frame inversion, line inversion, column inversion and dot inversion shown in
FIGS. 3A
to
3
D, respectively. The frame inversion of
FIG. 3A
is a mode that the polarity of the video signal changed only when the frame is changed. The line inversion of
FIG. 3B
is a mode that the video signal's polarity is varied whenever the gate line GL changes. The column inversion shown in
FIG. 3C
converts the polarity of the video signal whenever the source line SL changes, and the dot inversion of
FIG. 3D
converts it whenever the source line SL, gate line GL and frame change. The image quality is satisfactory in the order of the frame inversion, line inversion, column inversion and dot inversion. A higher image quality requires higher power consumption because the number of the generation of polarity conversions increases in proportional to the image quality. This is explained below with reference to the dot inversion shown in FIG.
4
.
FIG. 4
illustrates the waveforms of an odd-numbered source line SL and an even-number source line SL, applied to the liquid crystal panel
10
, showing that the video signals of the source lines SL change their polarities on the basis of VCOM whenever the gate line GL changes. Here, when it is assumed that the entire TFT-LCD panel displays gray color, the video signal swing width V of the source lines SL is twice the sum of VCOM and the swing width of Lhe positive video signal or the sum of VCOM and the swing width of the negative video signal. The consumed power at the output terminal of the TFT-LCD when the capacitance of the source line SL is C
L
is calculated by the following formula.
E=C
L
·V
2
That is, the dot inversion consumes a large amount of power because he video signal changes its polarity from (+) to (−) or from (−) to (+) on the basis of VCOM whenever the gate line GL changes.
Furthermore, the conventional TFT-LCD consumes a larger quanity of power to increase the generation of heat in case where its TFT is configured of a polysilicon TFT. Accordingly, the characteristic of the liquid crystal and the property of the TFT are deteriorated due to the heat generated. To solve this problem, there is proposed a method for driving the TFT-LCD an which, in order to supply a desired amount of voltage to the liquid crystal of each pixel, wish the voltage of the common electrode being fixed, the source driver supplies both ends of the liquid crystal with a voltage higher than the common electrode voltage in the nth frame, and supplies them with a voltage lower than the common electrode voltage in the (n+1)th frame, the voltages, respectively applied to the pixels placed above the same column line and the pixels placed therebelow, having their polarities different from each other, and the voltages, respectively applied to the pixels placed at the left side of the same row line and the pixels located at the right side thereof, having their polarities different from each other even in the same nth frame.
This TFT-LCD is driven in such a manner that charge sharing is performed with charge sharing time set for every row line for charge sharing, and then a voltage corresponding to video data is applied to each pixel. Since the voltage polarity of odd-numbered pixels of the (M−1) th low line is different from that of even-numbered pixels thereof, odd-numbered source lines are connected to even-numbered source lines through a switching element before a desired amount of voltage corresponding to the video data is applied to the pixels of the Mth row line. By doing so, the source line to which the voltage higher than the common electrode voltage is applied to and the source line to which he voltage lower than the common electrode voltage is applied maintain the maximum voltage at the common electrode through charge sharing. With this charge sharing, the source driving circuit reduces the voltage swing width by half in comparison with that of the conventional circuit, decreasing the power consumed for driving the TFT-LCD. The conventional TFT-LCD using charge sharing, however, connects the odd-numbered source lines SL to the even-numbered source lines SL using a transfer gate for a period of blanking time, to move a part of the charges of the source lines charged with the positive video signal to the source lines charged with the negative video signal to allow them to share the charges. Accordingly, the consumption power is reduced by 50% at most. Furthermore, the conventional TFT-LCD requires a plurality of source covers in order to realize a higher resolution of VGA class<SVC-A class<XGA class<SXGA class<UXGA class. This narrows the line pitch, bring about reliability problems.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a TFT-LCD using multi-phase charge sharing that substantially obviates one or more of the problems due to limitations and disad
Patel Nitin
Schweitzer Cornman Gross & Bondell LLP
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