Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-01-21
2002-07-16
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S096000, C345S098000
Reexamination Certificate
active
06421039
ABSTRACT:
This application claims the benefit of Korean patent application No. 97-1797, filed Jan. 22, 1997, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device having an in-plane structure (IPS) where a common electrode is formed parallel to a data line in the same plane, and more particularly, to a driving method where AC voltage is applied to the common electrode of an LCD with an IPS.
2. Discussion of the Related Art
A cathode ray tube (CRT) is the most widely used display device in television sets or computer monitors, because the CRT can easily reproduce the color and has a high response speed. However, the CRT is too large, too heavy, and requires too much power for portable applications. In order to overcome these disadvantages of the CRT, a great deal of research and development has been conducted into other types of displays. Among them, a liquid crystal display (LCD) is one of the most commonly used devices.
The LCD can be used as a thin television set mounted on a wall, because the LCD does not have an electron gun, unlike the CRT. Furthermore, the LCD can be used as a portable display device in a notebook computer, because the LCD's power consumption is very low, and it can be driven by a battery.
In general, the LCD includes a liquid crystal panel
12
displaying the video data, and driver IC's
10
and
11
for controlling video data, as shown in FIG.
1
. The liquid crystal panel
12
includes, as shown in
FIG. 2
, a first substrate
25
, a second substrate
21
, and a liquid crystal layer
24
injected between the first substrate
25
and second substrate
21
. The first substrate
25
includes a plurality of scan lines
14
and a plurality of data lines
15
, with the scan lines
14
and the data lines
15
arrayed in a matrix. The first substrate
25
also includes a pixel electrode
26
(see
FIG. 2
) and a thin film transistor (TFT)
13
, formed at crossing locations of the scan lines and data lines. Both the second substrate
21
and the first substrate
25
include a common electrode
23
and a color filter layer
22
. The pixel electrode
26
and the common electrode
23
, which face each other, and which have the liquid crystal between them, act as a pixel
16
shown in FIG.
1
. The liquid crystal panel also includes a polarization plate
20
on outer sides of the first substrate
25
and the second substrate
21
.
The TFT includes a gate electrode
30
(which is usually made of chromium), a source electrode
32
, and a drain electrode
33
made of a transparent conductive material such as indium tin oxide, a semiconductor layer
34
, and a doped semiconductor layer
36
. The gate electrode
30
is connected to the scan line
14
, and the source electrode
32
is connected to the data line
15
. The drain electrode
33
is connected to the pixel electrode
26
. The TFT works as a switch, which passes a data voltage applied to the data line
15
to the drain electrode
33
when a scan voltage is applied to the gate electrode
30
through the scan line
14
. If the data voltage is applied to the drain electrode
33
, then it is applied to the pixel electrode
26
connected to the drain electrode
33
. Thus, an electric field exists due to a voltage difference between the pixel electrode
26
and the common electrode
23
. The orientation of the liquid crystal molecules between the pixel electrode
26
and the common electrodes
23
rotate in response to the electric field. Thus, the amount of light transmitted at the pixel changes. That is, there is a difference in light transmittance between the pixel having a data voltage applied to it and the pixel without the data voltage applied. By using the pixels having the difference in transmittance, the LCD functions as a display device.
In this structure of the LCD, as shown in
FIG. 2
, the liquid crystal molecules rotate their orientation in a plane parallel to the orientation of the substrates. Therefore, a transmittance is highest in the tangential direction of the panel. However, the transmittance decreases as the viewing angle from the tangential direction increases. Thus, increasing the viewing angle is a very difficult problem for this LCD structure.
An in-plane structure (IPS) is one solution for increasing the viewing angle. In the IPS, as illustrated in
FIG. 4
a
showing a plan view, a common electrode
23
of the pixel is parallel to a pixel electrode
26
and has a segment shape parallel to a data line
15
. The bus lines for connecting the common electrodes
23
and the common lines
27
are parallel to the scan line
14
. Referring to
FIG. 4
b
showing the cross-sectional view of an LCD with IPS, the LCD includes a TFT having a gate electrode
30
, a source electrode
32
and a drain electrode
33
, the pixel electrode
26
, and the common electrode
23
on the same substrate
25
. The working principle of an LCD with IPS is the same that of a non-IPS LCD. However, the direction of an electric field is different from a non-IPS LCD. As shown in
FIG. 5
, the arrangement of the liquid crystal molecules
24
is parallel to the substrate, because the electric field is formed parallel to the substrate surface. Therefore, the liquid crystal molecules cut off the light or pass the light independent of the viewing angle.
Generally, driving methods for an LCD include line inversion, column inversion, or dot inversion. In the line inversion method, as shown in
FIGS. 6
a
-
6
b
, a polarity of a voltage applied to the pixel electrodes is reversed in every scan line. In the column inversion method, as shown in
FIGS. 7
a
-
7
b
, the polarity of the voltage applied to the pixel electrodes is reversed in every data line. In the dot inversion method, as shown in
FIGS. 8
a
-
8
b
, the polarity of the voltage is reversed for every row and column, that is, for every scan and data line.
In the line or column inversion method, a flicker problem is common. The reason is that when a scan line signal is high, all TFT's connected to the scan line are turned on, and the data signals are sent to the pixel electrodes from the source electrodes, which are connected to the data lines. Then, the liquid crystal molecules are driven by the voltage difference between the pixel electrode and the common electrode. When the scan line signal is low, all TFT's connected to the scan line are turned off. When that happens, the voltage applied to the pixel electrodes
26
remains on the pixel electrodes
26
, the liquid crystal molecules remain in the same state of rotation, and display signals are maintained. However, the stored signal voltage in the pixel electrode is reduced somewhat (&Dgr;V) by the coupling capacitance (Cgs) formed between the scan lines and data lines. Thus, the liquid crystal display flickers because the voltages on the pixel electrodes
26
are not all the same.
In the dot inversion method the flicker problem does not occur because neighboring pixels have different signal values. As shown in
FIGS. 8
a
-
8
b
, if a positive signal is applied to a first pixel, a second pixel, which is a neighboring pixel, has a negative signal applied to it. At the next cycle, the first pixel has a negative signal and the second pixel has a positive signal applied to it. That is, the pixel signal has a pulse signal type, as shown in FIG.
9
. The voltage differences (&Dgr;V), which occur in positive and negative states, can be moderated by control of the common voltage. Therefore, the voltage differences are the same, and the flicker problem can be solved.
In the dot inversion method, the voltage applied to the common electrode is a DC voltage, in general. On the other hand, to solve the flicker problem, the voltage difference should be maintained the same when the signal applied to the pixel electrode is AC. Thus, the power consumption of the dot inversion method is large, because the common voltage is a DC voltage. For example, if the voltage difference is 2.5V, and the common voltage is +2.5
Ha Yong Min
Moon Beom Jin
Kumar Srilakshmi K.
LG Electronics Inc.
Morgan & Lewis & Bockius, LLP
Saras Steven
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