Active matrix LCD device

Details Active matrix LCD device Active matrix LCD device

2000-11-28

2003-01-07

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

06504523

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (LCD) device and, more particularly, to a driving technique for driving the active-matrix LCD panel in the LCD device.
2. Description of the Related Art
An in-plane switching (IPS) mode active-matrix LCD (AM-LCD) panel uses a lateral electric field for driving the LC layer, the lateral electric field being parallel to the interface between the LC layer and the substrate surface. In the IPS mode AM-LCD panel, the LC layer should have a lower specific resistivity in order to prevent the residual image of the previous image from being displayed on the screen for a long time (See JP-A-7-159786, for example). On the other hand, in a general AM-LCD panel, irrespective of either the IPS mode or a twisted nematic (TN) mode of the LCD panel, a mode switching function is generally used for switching between the display modes in resolution or for switching between two set of input sections.
For example, in the mode switching function between the display modes, either a higher resolution mode (SXGA) for displaying 1280×1024 pixels or a lower resolution mode (VGA) for displaying 640×480 pixels is selected on the screen by the user.
In the AM-LCD panel, the image signal is not supplied to the LCD panel for several seconds during the mode switching operation between the display modes or between the input sections. If the screen continues for displaying image during this switching period, large noise appears on the screen to degrade the image quality. Thus, it is generally employed in the AM-LCD to temporarily stop displaying the image on the screen during the switching operation.
Immediately after the LCD panel is restarted for image display following to the temporary stop of the image display, there is a phenomenon wherein a flicker appears on the screen for about 2 to 20 seconds. The cause of the flicker is considered as follows.
In a LCD device, an alternately driving technique is generally used for preventing degradation of the LC layer such as burning. In other words, the potential of the pixel electrode alternates in the polarity thereof with respect to the potential of the common electrode (or common electrode potential). For temporarily stopping the image display on the screen during the switching operation, it may be considered to equalize the pixel electrode potential with the common electrode potential at the ground potential, for example. However, this causes a difference in DC potential between both the electrodes.
The difference in the DC potential (DC potential difference) is caused by the fact that the common electrode immediately assumes the ground potential whereas the pixel electrode does not assume the ground potential during the switching operation due to a large discharge period of the pixel electrode. This is because the pixel electrode is grounded via the data line and the TFT in the pixel, which delay the discharge (or drainage) of the electric charge from the pixel electrode.
The DC potential difference reverses from pixel to pixel in the case of a dot reversible driving technique of the LCD panel, wherein the image signal is reversed in the polarity thereof pixel by pixel in both the column and row directions and also frame by frame in each pixel. The DC potential difference causes attachment of electric charge having an opposite polarity with respect to the polarity of the potential difference and an amount corresponding to the potential difference onto each of the pixel electrodes or the common electrodes. The attached electric charge generally remains on the electrode as residual electric charge after the LCD panel is restarted for image display from the temporary stop in the mode switching operation. The residual electric charge is superposed with the current image signal and lightens or darkens the screen every frame, thereby causing a flicker until the residual electric charge eventually disappears. The flicker has become more noticeably with the reduction of the specific resistivity of the LC layer.
The mechanism of the flicker problem will be described with reference to drawings.
FIG. 1
shows an equivalent circuit diagram of each pixel in the pixel array of the LCD panel. The pixel includes a parallel branch including LC layer capacitor CL and LC layer resistor RL and connected through a capacitor C
1
to the pixel electrode
210
and through a capacitor C
2
to the common electrode
212
. The capacitors C
1
and C
2
are formed by the LC layer and the pixel electrode
210
and the common electrode
212
, respectively, sandwiching therebetween a passivation layer
213
as shown in
FIG. 2A
(and
FIG. 2B
) The pixel electrode
210
is connected to the source of TFT
206
, the drain of which is connected to a corresponding data line. The gate of TFT
206
is connected to a corresponding gate line
202
, whereas the common electrode
212
is connected to the common electrode line
204
.
When a DC voltage is applied between the pixel electrode
210
and the common electrode
212
during the switching operation for resolution mode or selection of input signals, with the pixel electrode
210
having a positive polarity with respect to the common electrode
212
, positive electric charge in the LC layer moves toward the common electrode
212
as schematically shown by an arrow in FIG.
2
A. The positive electric charge, after reaching the vicinity of the common electrode
212
, forms a residual electric charge and applies an electric field in the direction from the common electrode
212
to the pixel electrode
210
as shown by an arrow in FIG.
2
B.
FIG. 3
shows a timing chart of potentials of electrodes and nodes in the LCD panel of
FIG. 1
for a switching operation, wherein the input of the data line is switched from an input signal A to an input signal B. In
FIG. 3
, gate line potential, data lien potential, pixel electrode potential and common electrode potential are represented by V
G
, V
D
, V
PI
(V
PI
′) and Vcom. After the input signal A having a positive polarity is input through the data line, the potential V
G
of the gate line
202
rises from a Vgoff level (−10 volts, for example) to a Vgon level (19 volts, for example) at time instant t
1
, whereby the TFT
206
is turned on to deliver the input signal A to the pixel electrode, which holds the voltage level until the power supply is switched off at t
6
. In
FIG. 3
, only a bias voltage applied to the data line is depicted as the potential V
D
of the data line, with the signal voltage superposed thereon being omitted
The potential V
G
of the gate line
202
rises at t
2
from V
gon
level to V
goff
level, which is maintained on the gate line
202
until t
6
. The common electrode potential Vcom assumes a constant level (4.5 volts, for example) until t
6
. The input signal A is switched off by a data driver at t
3
from the data line
200
, although the voltage level V
D
is maintained on the data line
200
until t
4
, at which a an absence signal is delivered indicating that the input signal is not supplied from the data driver.
The time period T
1
between time t
3
at which the input signal A is switched off and time t
4
at which the absence signal is delivered corresponds to the time needed for the video signal processor receiving the input video signal to judge the absence of the input signal, and may be 40 milliseconds, for example. The time period T
2
between time t
4
and time t
6
at which the power supply for the LCD panel is switched off corresponds to the time needed for switching off the power supply after the judgement of the absence of the input signal, and may be 5 milliseconds, for example. The time period T
3
between time t
6
and time t
7
at which the input signal B is switched on corresponds to a waiting time for waiting a new input signal, and may be 300 milliseconds, for example.
The power source for the data driver is switched off at time t
4
at which the absence signal is delivered indicating the absence of the

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