Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-06-25
2003-07-08
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S089000, C345S094000, C345S096000, C345S099000, C345S098000, C345S100000, C345S211000, C345S212000, C345S213000
Reexamination Certificate
active
06590552
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device, and more particularly to a driving method hat requires only a low voltage and consumes a reduced amount of power.
2. Description of the Related Art
A liquid crystal display (LCD) is formed by enclosing liquid crystal in the space between two transparent substrates, each having a transparent electrode formed thereon. As liquid crystal is electro-optically anisotropic, it exhibits optical properties in accordance with field strength if a desired voltage is applied across the electrodes to form an electric field in the liquid crystal layer. Utilizing such properties, a displayed image is formed as a collection of pixels, each presenting a desired brightness, by applying different voltages to the respective pixels. LCDs display an image thus formed by voltage control, providing various advantages such as reductions in size, thickness, and power consumption. As a result, LCDs are practically manufactured and widely used in office automation equipment, audio-visual devices, and the like.
FIG. 1
is an equivalent circuit diagram of such an LCD. A gate line
11
and a drain line
12
cross each other. At the intersection thereof, the LCD includes a thin film transistor (TFT)
13
serving as a switching element; a liquid crystal capacitor
14
and a storage capacitor
15
, each having one electrode connected to the TFT
13
; and a storage capacitor line
16
connected to the second electrode of the storage capacitor
15
. The storage capacitor line
16
is shared by all the storage capacitors
15
. The other electrode of the liquid crystal capacitor
14
is provided as a common electrode formed on a substrate opposite to the substrate on which the TFT
13
is disposed with liquid crystal interposed therebetween, and is connected to a common line
17
.
FIG. 2
illustrates waveforms of signal voltages driving the LCD shown in FIG.
1
. During an ON period, a gate voltage V
G
applied to the gate line
11
attains a high level. During this period, the TFT
13
is turned on, resulting in drain-source conduction. As a result, a source voltage V
S
becomes equal to a drain voltage V
D
applied to the drain line
12
, and is applied to said one electrode of each of the liquid crystal capacitor
14
and the storage capacitor
15
. At the beginning of an OFF period, the gate voltage V
G
falls to a low level, turning of f the TFT
13
, to thereby determine the source voltage V
S
.
At the instant the gate voltage V
G
falls from the high level to the low level, the source voltage V
S
falls by an amount &Dgr;V
S
due to capacitance coupling, and is retained as a pixel voltage V
P
. Meanwhile, the respective other electrodes of the liquid crystal and storage capacitors
14
and
15
receive the same common voltage V
com
from the storage capacitor line
16
and the common line
17
. The resulting difference in voltage between the common voltage V
com
and the pixel voltage V
P
serves as a voltage V
LC
for driving liquid crystal that is applied to the liquid crystal and storage capacitors
14
and
15
. The pixel voltage V
P
is maintained by off-resistance of the TFT
13
until the TFT
13
is turned on again to charge the capacitor to a different voltage in the next field, though it is decreased by an amount &Dgr;V
K
due to leakage current. As the storage capacitor
15
is connected in parallel to the liquid crystal capacitor
14
, exactly the same voltage is applied thereto, so that these capacitors
14
and
15
contribute to a reduction in the amounts &Dgr;V
S
and &Dgr;V
K
by increasing their combined capacitance.
Usually, the polarity of the voltage applied to the liquid crystal capacitor
14
is inverted every frame period, field period, line period, or the like, in order to prevent deterioration of the liquid crystal. This method is referred to as a common inversion driving method, in which the polarity of the common voltage V
com
is inverted at the opposite timing to the drain voltage V
D
. Consequently this method achieves a decrease in the amplitude of the drain voltage V
D
and a reduction in a power supply voltage for a drain driving circuit, contributing to a decrease in power consumption.
However, according to such a common inversion driving method, the common voltage V
com
is an alternating-current voltage signal, and is applied in common to all the liquid crystal and storage capacitors
14
and
15
. As a result, considerable wiring capacitance of the storage capacitor line
16
and of the common line
17
is necessary and a large amount of current flows during the change in voltage. This increases overall power consumption of the device, including the power consumed by the common electrode and the storage capacitor electrode.
SUMMARY OF THE INVENTION
The present invention was conceived to solve the above-described problems, and aims to provide a display device that consumes less electric power than current devices.
In order to achieve the above object, the present invention is directed to a method of driving a liquid crystal display device comprising a pixel electrode formed on a first substrate, a switching element connected to said pixel electrode, a common electrode formed on a second substrate, liquid crystal provided between said pixel electrode and said common electrode, and a storage capacitor utilizing said pixel electrode as one electrode.
This method comprises the step of applying, to the other electrode of said storage capacitor, a storage capacitor voltage that is changed from a low level to a high level immediately after said switch element is turned off during a period in which a voltage of said pixel electrode is higher than that of said common electrode, and that is changed from the high level to the low level after said switching element is turned of f during a period in which the voltage of said pixel electrode is lower than that of said common electrode.
According to one aspect of the present invention, the voltage of said common electrode is a direct-current voltage.
By thus changing the level of the storage capacitor voltage in accordance with a relationship between the voltages of the pixel electrode and the common electrode, the voltage of the pixel electrode can be shifted. As described above, the storage capacitor voltage attains a high level during a period in which the pixel electrode voltage is higher than the common electrode voltage and said switching element is off, and attains a low level during a period in which the pixel electrode voltage is lower than the common electrode voltage and said switching element is off. Consequently, a sufficiently large voltage can be applied to the liquid crystal capacitor even if an amplitude of the voltage of a display signal supplied to each switching element is decreased.
Another aspect of the present invention is directed to a method of driving a liquid crystal display device comprising a pixel electrode formed on a first substrate, a switching element connected to said pixel electrode, a common electrode formed on a second substrate, liquid crystal provided between said pixel electrode and said common electrode, and a storage capacitor utilizing said pixel electrode as its one electrode. The method according to this aspect then comprises the steps of applying a direct-current voltage to said common electrode and applying, to the other electrode of said storage capacitor, a storage capacitor voltage the level of which changes during a period in which said switching element is off.
According to a still further aspect of the present invention, said storage capacitor voltage changes from a low level to a high level during a period in which said switching element is off and the voltage of said pixel electrode is higher than that of said common electrode, and changes from the high level to the low level during a period in which the voltage of said pixel electrode is lower than that of said common electrode.
According to a yet further aspect of the present invention, the
Komiya Naoaki
Yokoyama Ryoichi
Hjerpe Richard
Zamani Ali
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