Liquid crystal display device and a driving method therefor

Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix

Reexamination Certificate

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Reexamination Certificate

active

06344842

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to techniques for inverting the polarity of an electric signal which is applied to liquid crystal so as to correspond to a plurality of frames or fields, when a liquid crystal display device is driven.
2. Description of the Related Art
In general, a thin-film-transistor (TFT) color liquid crystal display device includes a TFT substrate on which gate lines (scanning lines) and data lines (signal lines) are arranged, and another color filter substrate on which a common electrode is disposed. In this TFT liquid crystal display device, basically, a scanning signal is applied to the gate lines, while a corresponding display signal is applied to the source lines, so that matrix operation is performed, and pixels realize a high-quality image by performing a charge holding operation.
“Driving by scanning” means that a horizontal line of pixels is selected and a display signal is applied to the selected line. In driving a liquid crystal display device, the application of a direct current signal causes ions in liquid crystal to gather on one substrate, often resulting in deterioration of its liquid crystal display. To prevent this phenomenon, the liquid crystal display device is driven by positively and negatively inverting the applied display signal so as to correspond to each field.
Further, as disclosed in Japanese Patent Publication No. 55-6916, there is a known technique for driving a liquid crystal display device with an alternating current having symmetrically positive and negative polarity by inverting the polarity of an electric signal to be applied to the liquid crystal so that direct current components of the signal are not included in the applied signal.
As mentioned above, when the TFT color liquid crystal display device is driven with an alternating current by positively and negatively inverting the display signal, corresponding to each field, a signal sent from a signal line is inverted and the inverted signal is inputted to pixels. Conventionally, there are several methods for inverting the signal sent from the signal line. The simplest method is “inversion-to-field” in which a signal sent from a signal line is inverted in units of fields, corresponding to pixels.
Other methods which are employed are “inversion-to-gate-line” in which a cycle of the inversion corresponds to each scanning line, and “inversion-to-data-line” in which a cycle of the inversion corresponds to each signal line. In addition, in order to eliminate cross-talk and insufficient writing more efficiently than these two methods, “driving-by-inversion-to-dot” in which a signal is inverted corresponding to adjacent dots is also employed.
By referring to
FIGS. 7 and 8
, both the occurrence mechanism of a sticking phenomenon and the occurrence mechanism of flickers in a liquid crystal device will be described below.
FIG. 7A
shows the most general schematic structure of a liquid crystal cell that constitutes a liquid crystal display device. A condition in which liquid crystal
1
is encapsulated between a pair of transparent substrates is shown. On both electrodes with respect to the liquid crystal
1
, electrode layers
2
and
3
, insulating films
4
containing SiNx thereon, and orientation films
5
containing polyimide thereon are generally formed.
FIG. 7A
also shows a condition in which a voltage is not applied to the liquid crystal cell, and ions which are inevitably present in the liquid crystal
1
are dispersed at random.
When a direct current voltage is applied to the liquid crystal cell as shown in
FIG. 7B
, this voltage causes the ions present in the liquid crystal
1
to be polarized and absorbed to the surfaces of the orientation films
5
. In other words, since the SiNx-contained insulating films
4
and the polyimide-contained orientation films
5
(both are insulating films) are present on the electrode layers
2
and
3
in the liquid crystal cell, which is conventional, the ions are absorbed to form capacitors C
SINx·PI
between the surfaces of the orientation films
5
and the surfaces of the electrode layers
2
and
3
.
When the capacitors C
SINx·PI
is formed, an asymmetric voltage V
AS
(voltage generated by sticking) is generated by the absorbed ions. As shown in
FIG. 7C
, in the liquid crystal cell a relationship expressed as V
AS
=Q
ion
/C
SINx·PI
is established, where Q
ion
represents the charges of the absorbed ions.
In a condition in which the above-described sticking occurs and the absorbed ions are fixed, as shown in
FIG. 7C
, when the voltage between the electrode layers
2
and
3
is expressed as V
0
, a relationship expressed as V
0
=V
AS1
(asymmetric voltage to the electrode layer
2
)+V
AS2
(asymmetric voltage to the electrode layer
3
)+V
Lc
(actually applied voltage to the liquid crystal
1
) is established. If an applied voltage from the exterior is set to zero with this condition, a voltage expressed as V
LC
=V
AS1
+V
AS2
=V
AS
is applied to the liquid crystal cell as shown in FIG.
7
D. This is the result of the sticking, which affects display quality, as a residual image in display. Further, if the liquid crystal cell is driven with an alternating current, the voltage of V
AS
is a factor in the occurrence of flickers.
The occurrence mechanism of flickers will be described below.
To avoid sticking caused by driving the liquid crystal with a direct current as described above, the liquid crystal is driven with an alternating current as shown in FIG.
8
A. In a condition in which the liquid crystal is driven with an alternating current while the asymmetric voltage (V
AS
) is being generated, even when the voltage ±V
0
is applied, in connection with the signal applied to one pixel as shown in
FIG. 8B
, the positive polarity has a relationship expressed as V
0
−V
AS
=|V
LC
| and the negative polarity has a relationship expressed as V
0
+V
AS
=|VLC|, so that the applied voltage has different magnitude in polarity.
When the electro-optic characteristics of the liquid crystal is as shown in
FIG. 8C
, in other words, when the relationship of transmittance (T) with respect to applied voltage (V) is represented by a curve shown in
FIG. 8C
, transmittance of T(V
0
) must be obtained with respect to the applied voltage (V
0
). However, this case has asymmetric voltage V
AS
, the transmittance differs depending upon the polarity. The transmittance in the positive and negative polarities are expressed as follows:
transmittance in positive polarity T(V
0
−V
AS
);
transmittance in negative polarity T(V
0
+V
AS
).
Consequently, flickers occur, and the amplitude of a change in the transmittance is expressed as the following relationship: &Dgr;T=T(V
0
−V
AS
)−T(V
0
+V
AS
)
Based on the described background, the liquid crystal is driven by inverting the signal. Such driving by inversion needs voltage amplitude which is twice as much as necessary voltage amplitude for normally driving the liquid crystal, resulting in large power consumption. For example, if a voltage necessary for driving the liquid crystal is 5 V, a signal needs to have 10 V (namely, ±5 V) in positive and negative polarity.
In addition, power consumption P caused by such a driving signal is generated by charging or discharging the liquid crystal cell serving as a capacitor at the inverting frequency of the signal. Thus, the following relationship is established:
P∝f, V
2
where P represents power consumption; f, the inverting frequency of the signal; and V, voltage. As expressed in the above relationship, P is proportional to f, and P is proportional to V
2
.
Therefore, power consumption necessary for applying the source voltage (V
sig
) when the polarity of the signal is inverted needs to be not less than four times greater than power consumption when the polarity is not inverted, so that a problem of large power consumption occurs. Further, if the inverting frequency is consid

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