Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-07-09
2002-04-16
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S096000
Reexamination Certificate
active
06373456
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a drive circuit of a liquid crystal display.
2. Prior Art
FIG. 3
is a diagram showing an equivalent circuit of an active matrix liquid crystal display.
In the drawing, reference numeral
1
indicates a gate line comprising a plurality of wires disposed in parallel. Numeral
2
indicates a source line orthogonal to the gate line
1
and comprising a plurality of wires disposed in parallel. Numeral
3
indicates a thin film transistor serving as a switching element. A gate electrode
4
is connected to the gate line
1
, a source electrode
5
is connected to the source line
2
, and a drain electrode is connected to a picture element electrode
6
, respectively.
Numeral
7
indicates an auxiliary capacity (Cs) connected between the picture element electrode
6
and an auxiliary electrode
8
, and numeral
9
indicates a liquid crystal capacity (Clc) connected between the picture element electrode
6
and a common electrode
10
. The common electrode
10
and the auxiliary electrode
8
are connected to a Cs line serving as a common line not illustrated.
FIG. 4
is a diagram showing a common inversion drive of common Cs line of a liquid crystal display according to a prior art.
In the drawing, reference numeral
1
indicates a gate line, numeral
10
indicates a common electrode, and numeral
11
indicates a capacity between the gate line and the common electrode. Vg indicates a gate potential which is a selection pulse signal, and Low potential of the gate potential is a constant value. Vcom indicates a common potential applied to the common electrode, and Vs is a source potential which is a gradation signal.
FIG. 5
is a diagram showing a shortage of voltage applied to the liquid crystal because of shortening of horizontal scanning period of the conventional liquid crystal display.
In the drawing, Vp is a picture element potential applied to the picture element electrode, Vcom is a common potential applied to the common electrode, and Vlc is a liquid crystal voltage applied to the liquid crystal.
FIG. 6
is a diagram showing an increase of crosstalk ratio because of shortening of horizontal scanning period of the conventional liquid crystal display.
In the conventional active matrix liquid crystal display of above arrangement, as shown in
FIG. 3
, a plurality of gate wires
1
and source wires
2
are aligned forming a matrix. And the thin film transistor
3
and the picture element electrode
6
are formed in a region surrounded by those wires. In such an active matrix liquid crystal display, in case of line common inversion drive of common Cs structure, the gate potential Vg serving as a selection pulse signal is inputted to the gate line
1
on the basis of sequential line scanning. The thin film transistor
3
formed in the picture element on the selected gate line
1
is turned on. The source potential Vs serving as a gradation signal of horizontal scanning period is applied to the source line
2
, and the common signal Vcom of horizontal scanning period is applied to the common electrode
10
and the auxiliary electrode
8
, thus the active matrix liquid crystal display is driven. However, in such a conventional drive system, as shown in
FIG. 4
, the Low potential being a non-selective potential of the gate potential Vg applied to the gate line
1
is a constant value.
FIG. 3
shows one picture element equivalent circuit of common Cs structure of the active matrix liquid crystal display. When the gate potential Vg being a selection pulse signal is inputted to the gate line
1
, the thin film transistor
3
formed in the picture element on the gate line
1
is turned on, and the source potential Vs being a gradation signal applied to the source line
2
is written in the picture element electrode
6
through the thin film transistor
3
, whereby the voltage Vlc applied to the liquid crystal and formed by a potential difference between the picture element potential Vp and the common potential Vcom is applied to the liquid crystal held between the picture element electrode
6
and the common electrode
10
. At this time, the voltage applied to the liquid crystal is subject to a field through due to last transition of the gate potential Vg, whereby a voltage Vlc′ applied to the liquid crystal during the holding period becomes Vlc′ smaller than Vlc. By establishing the Low potential of the gate potential Vg applied to the gate line
1
to be a constant value, the voltage Vlc′ applied to the liquid crystal during the holding period turns off the thin film transistor
3
and is held during the vertical scanning period.
In the conventional drive system described above, when the common potential Vcom performs an inversion for each horizontal scanning period, because of a time constant between the capacity formed through the gate line
1
, source line
2
, picture element electrode
6
and respective liquid crystals and a sheet resistance of the common electrode
10
, a delay occurs in waveform of the common potential Vcom inputted to the common electrode
10
of any picture element in the active matrix liquid crystal panel, as compared with waveform of the common potential Vcom inputted to the active matrix liquid crystal panel. As a result, in the aspect of display, there exists a problem of occurrence of shortage in voltage applied to the liquid crystal and crosstalk, because the common potential Vcom does not reach predetermined potential during the horizontal scanning period. Particularly, when the horizontal scanning period is shortened under the recent trend of large-sized and highly defined active matrix liquid crystal panel, occurrence of shortage in voltage applied to the liquid crystal and crosstalk is remarkable.
FIG. 5
shows a manner of occurring a shortage in the voltage applied to the liquid crystal because of the shortening of horizontal scanning period. When the horizontal scanning period is shortened from a horizontal scanning period A to a horizontal scanning period B, the voltage applied to the liquid crystal is lowered, and any predetermined brightness cannot be attained in the active matrix liquid crystal display.
FIG. 6
shows a mechanism of occurring a crosstalk because of shortening of horizontal scanning period, and convergence speed at the time of inversion of the common potential Vcom is different depending upon the source potential Vs which is a gradation signal applied to the source line. Accordingly, in the horizontal scanning period A, irrespective of the source potential Vs, the common potential Vcom applied to the common electrode
10
reaches a predetermined potential. On the other hand, in the horizontal scanning period B, depending upon the source potential Vs applied to the source line
2
, the common potential Vcom applied to the common electrode
10
reaches a potential of different level. In this manner, in the horizontal scanning period B, depending upon the source potential Vs applied to the source line
2
, the voltage Vlc applied to the liquid crystal is different, which results in the occurrence of crosstalk in the active matrix liquid crystal display.
To solve the above problems, it may be an idea to reduce the parasitic capacity formed between the common electrode
10
and each electrode or to minimize the delay at the time of inversion of the common potential Vcom by improving the charge supply capacity of the power source of the common electrode
10
. However, there is a restriction that if wiring width is small, wiring resistance is increased, while if improving the charge supply capacity of the power source, power consumption is increased.
SUMMARY OF THE INVENTION
The invention was made to solve the above-discussed problems and has an object of providing a liquid crystal display capable of reducing the influence on display due to the parasitic capacity between the gate line and the common electrode.
To accomplish the foregoing object, a liquid crystal display according t
Hjerpe Richard
Kabushiki Kaisha Advanced Display
Nguyen Kimnhung
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