Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-09-02
2002-02-19
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
Reexamination Certificate
active
06348909
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a grey-scale LCD driver which is able to realize multi-gradation of a liquid crystal display device by varying a voltage applied to a liquid crystal, and more particularly, to a grey-scale LCD driver which is able to realize multi-gradation by shaping a rectangular-waveform voltage used for a driving voltage into a ramp voltage through an integrating circuit and sampling the ramp voltage at selected bit timings.
2. Description of the Prior Art
Generally, liquid crystal display (LCD) devices are display units which display images by controlling the amount of light transmission utilizing dielectric anisotropy of liquid crystals, and are widely used as display units of laptop personal computer, word processor, and portable television receiver.
In the structure of the LCD device, there are a simple-matrix structure which controls arrangement of liquid crystal materials sandwiched between two stripe electrodes with a voltage generated at intersections of the stripe electrodes formed in a matrix shape, and an active-matrix structure which improves contrast, drive duty, and multi-gradation by adding thin film transistors to the simple matrix LCD device as a switching means for driving.
As shown in
FIG. 4
, a thin film transistor (TFT) active-matrix LCD device comprises an upper substrate
31
, a lower substrate
51
, and a liquid crystal layer
72
sandwiched between the upper substrate
31
and the lower substrate
51
.
The upper substrate
31
comprises a polarizer
36
attached on an upper surface thereof, a color filter
34
sequentially disposed on a lower surface thereof, and a common electrode
32
disposed on the color filter
34
. The common electrode
32
is made of Indium Tin Oxide (ITO).
The lower substrate
51
comprises thin film transistors TFT, an insulating layer
54
, and pixel electrodes
52
electrically connected to the thin film transistors TFT through contact holes. Aligning films
75
a
and
75
b
rubbed in a predetermined direction are disposed on the common electrode
32
and the pixel electrodes
52
, respectively. Therefore, the liquid crystal molecules of the liquid crystal layer
72
are twisted according to the rubbing direction of the aligning films
75
a
and
75
b
.
In the TFT active-matrix LCD device having above-mentioned structure, the thin film transistors TFT are turned on by signals applied to gate electrodes of the thin film transistors TFT, which results in that electrical signals applied to drain electrodes are applied to the pixel electrodes
52
through source electrodes. And then, the liquid crystal molecules sandwiched between the pixel electrodes
52
and the common electrode
32
are twisted in a direction different from a polarizing direction, which results in that the pixels are displayed.
In the LCD device, recently, the research and development to realize full-color closed to a natural color have been conducted. For example, Korean Patent Publication No.
96-3961
discloses three methods for realizing multi-gradation.
These three methods are a voltage-level driving method which displays multi-gradation by applying different levels of voltages, a frame driving method which displays multi-gradation by changing a effective voltage by varying a time for applying voltages in an unit frame, and a complex method which is in combination of the voltage-level driving method and the frame driving method.
These various methods are based on digital data from a controller, and the digital data is converted to analog data, which controls the voltage applied to the liquid crystal and time for applying the voltages to the liquid crystal layer. This leads the LCD device to realize multi-gradation of a unit pixel.
Hereinafter, a conventional grey-scale LCD driver for realizing multi-gradation will be explained in detail with reference to FIG.
5
.
Referring to
FIG. 5
, the thin film transistor TFT of TFT LCD device is turned on by scan signals Vg applied to the gate electrode and transmits electrical signals applied from a switching element Q
1
of the grey-scale LCD driver to the source electrode through the drain electrode. At this time, since the source electrode of the thin film transistor TFT is electrically connected to the pixel electrodes
52
on the lower substrate
51
shown in
FIG. 4
, a liquid crystal capacitor is formed between the pixel electrodes
52
of the lower substrate
51
and the common electrode
32
of the upper substrate
31
.
Further, maintaining capacitor (not shown) connected in parallel with the liquid crystal capacitor is formed separately on the upper and lower substrate, in order to remove residual images by compensating DC voltage level shift which degrades the display quality at still images.
The conventional grey-scale LCD driver comprises a data storing section for storing n bits of digital data A
0
~A
n-1
assigned for producing a grey-scale in synchronization with a clock signal CLK of a shift register, a counter
20
for generating counting data B
0
~B
n-1
in synchronization with the clock signal CLK, a comparator
30
for comparing the digital data A
0
~A
n-1
stored in the data storing section
10
with the counting data B
0
~B
n-1
supplied from the counter
20
and outputting a comparing signal Comp if the digital data A
0
~A
n-1
are equal to the counting data B
0
~B
n-1
, a ramp voltage generating section
50
for generating a ramp voltage, a transistor Q
1
for supplying the ramp voltage to the drain electrode of the thin film transistor TFT connected to the liquid crystal capacitor C depending on a signal from a sampling control section
40
, and a sampling control section
40
for controlling the transistor Q
1
in accordance with a control signal supplied from a controller (not shown) in order to sample the ramp voltage. Where, the transistor Q
1
serves as a switching element.
The counter
20
divides selected time of a line into the number more than the number of gradations and counts them.
Now, the operation of the above-mentioned conventional grey-scale LCD driver will be explained.
The comparing signal Comp supplied from the comparator
30
is at inactive status, that is, a low level during the digital data A
0
~A
n-1
stored in the data storing section
10
are not equal to the counting data B
0
~B
n-1
supplied from the counter
20
, and then if the digital data A
0
~A
n-1
are equal to the counting data B
0
~B
n-1
supplied from the counter
20
, the comparing signal becomes a high level and activates the sampling control section
40
. At this time, the sampling control section
40
outputs the sampling signal at a bit timing by means of the control signal Crt Sig supplied from the controller (not shown) and turns on the transistor Q
1
.
Therefore, the ramp voltage outputted from the ramp voltage generating section
50
passes through the transistor Q
1
and the thin film transistor TFT and is charged in the liquid crystal capacitor C. And then, if the comparing signal Comp becomes the low level, the transistor Q
1
is turned off. Therefore, the operation for charging the liquid crystal capacitor C is stopped.
The sampling bit timing is determined in accordance with the digital input data A
0
~A
n-1
, and the liquid crystal capacitor C is charged to the voltage having a selected level within levels of multi-gradation, and then the selected level of multi-gradation is displayed.
Namely, the voltage for displaying a desired level of gradation is determined in accordance with the bit timing to be sampled, that is, position of a slope of the surface of the waveform of the ramp voltage to be sampled.
The number of bits of the counter
20
determines the number of gradations, which are realized by means of the LCD device. For example, if the number of bits is 8, it is possible that 256 gradations are realized (2
8
=256).
Conclusively, in order to display the number of gradation, the conventional grey-scale LCD driver controls time for charging the liquid crystal capacitor C in accordance with the valu
Bushnell , Esq. Robert E.
Hjerpe Richard
Laneau Ronald
Samsung SDI & Co., Ltd.
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