Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-01-08
2001-10-02
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S096000, C345S087000
Reexamination Certificate
active
06297793
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid-crystal display device and, more particularly, to a voltage conversion circuit that converts an output voltage from a data driver used when a liquid-crystal display device is driven so as to invert dots.
2. Description of the Related Art
Generally speaking, dot inversion driving, which is one of the methods for driving a liquid-crystal display device, is a driving method which features high display quality, such as high contrast or low crosstalk. In this driving method, however, when a standard liquid crystal is used, a driving voltage of ±5 V is used to invert each dot, and therefore, a data driver capable of outputting about 10 V is required. In this type of data driver IC suitable for use with a high voltage, finer patterning of internal elements is difficult, hampering improvements in speed, reduction of size, and lowering of cost of the liquid-crystal display device.
Therefore, instead of using a data driver IC suitable for use with a high voltage, capable of outputting about 10 V, by using, for example, a combination of a data driver IC of a single +5 V output and a switched capacitor circuit, it is possible to easily form a polarity inversion circuit capable of outputting a driving voltage of −5 V, which is of a polarity opposite to that of the data driver IC.
The switched capacitor circuit, as shown in, for example,
FIG. 8
, comprises transistors M
1
, M
2
, M
3
, and M
4
, and a capacitor Cb. On/off control of the transistors M
1
and M
4
is performed in accordance with a clock signal &phgr;a, and on/off control of the transistors M
2
and M
3
is performed in accordance with a clock signal &phgr;b. That is, the transistors M
1
and M
4
are turned on when the clock signal &phgr;a is at a “high” level and turned off when the clock signal &phgr;a is at a “low” level. Also, the transistors M
2
and M
3
are turned on when the clock signal &phgr;b is at a “high” level and turned off when the clock signal &phgr;b is at a “low” level. As shown in
FIG. 9
, these clock signals &phgr;a and &phgr;b have a phase difference of 180° at the same cycle, and are formed such that they do not reach a “high” level simultaneously. Reference numeral C
2
denotes a data-line parasitic capacitor.
Therefore, in
FIG. 9
, in the period in which the clock signal &phgr;a is at a “high” level and the clock signal &phgr;b is at a “low” level, the transistors M
1
and M
4
are turned on, causing the side connected to the transistor M
1
of the capacitor Cb to be charged to a Vin level and the side connected to the transistor M
4
of the capacitor Cb to be charged to a Vsc level. Next, when the clock signal &phgr;a reaches a “low” level and the clock signal &phgr;b reaches a “high” level, the transistors M
2
and M
3
are turned on, causing the side connected to the transistor M
2
of the capacitor Cb to reach a Vsc level. As a result, Vsc−Vin is output as a Vout level from the side connected to the transistor M
3
. That is, when Vin=+5 V and Vsc=0 V, Vout=Vsc−Vin =−5 V is output, making it possible to invert the output voltage of the data driver IC. In actuality, there is an example in which this switched capacitor circuit is used to generate an inverse-polarity constant voltage for driving a STN liquid crystal.
However, in the above-described conventional switched capacitor circuit, there is a problem in that the output impedance is large, and it is difficult to directly drive a load by itself. Therefore, in order to solve this problem, the switched capacitor circuit is used only to generate an inverse-polarity constant voltage, and in order to prevent an increase in the output impedance of the switched capacitor circuit, a method is conceived in which a buffer capacitor is provided separately. However, if a buffer capacitor is added, a problem arises in that response speed is decreased, and it is not appropriate to use the buffer capacitor for image signals.
As another method of decreasing output impedance, generally speaking, a method is conceivable in which an impedance conversion circuit by an operational amplifier is used. However, it is not possible to form this circuit, for example, from an amorphous Si thin-film transistor (hereinafter referred to as an aSi-TFT), which is often used as a switching element of an active-matrix-type liquidcrystal display device. The reason for this is that, in an aSi-TFT, the mobility of carriers is small and a predetermined response speed cannot be obtained.
Both methods require that a buffer capacitor, an impedance conversion circuit, or the like, be mounted separately on a substrate of a liquid-crystal display device. This is an obstacle to size reduction and power consumption reduction of the liquid-crystal display device.
SUMMARY OF THE INVENTION
An object of the present invention, the achievement of which will solve the above-described problems, is to provide a liquid-crystal display device which is capable of realizing dot inversion driving with fast response speed without using a data driver IC suitable for use with a high voltage and which is capable of achieving a reduction in size and a reduction in power consumption of the entire device including a data driver.
To achieve the above-mentioned object, according to a first aspect of the present invention, there is provided a liquid-crystal display device, comprising: a first voltage application circuit for applying to a data line an output voltage as it is from a data driver; a second voltage application circuit for converting the output voltage from the data driver into an inverse output voltage which is of a polarity opposite to that of the output voltage from the first voltage application circuit and for applying it to the data line alternately with the output voltage from the first voltage application circuit; and a third voltage application circuit for applying an auxiliary voltage to the data line in such a manner for the inverse output voltage as to be rapidly applied to the data line before the inverse output voltage is applied to the data line.
In the liquid-crystal display device in accordance with the first aspect of the present invention, there is provided a first voltage application circuit which applies to a data line an output voltage as it is from a data driver, and a second voltage application circuit which converts the output voltage from the data driver to an inverse output voltage having a polarity opposite thereto and which applies it to the data line alternately with the output voltage. Therefore, it is possible to supply an output voltage having both positive and negative polarities to the data line and to realize dot inversion driving. Furthermore, since a third voltage application circuit which applies an auxiliary voltage is provided, the inverse output voltage can be rapidly applied to the data line. Therefore, according to the liquid-crystal display device of the present invention, dot inversion driving having sufficient speed and accuracy is made possible.
In a specific example of the voltage application circuit, a first voltage application circuit can be formed of one transistor inserted, for example, between a data driver and a data line, and this transistor may be operated in accordance with a clock signal.
A second voltage application circuit generates an inverse output voltage and can be formed from, for example, a switched capacitor circuit described above.
A third voltage application circuit may include a power source, connected to a connection intermediate point between the second voltage application circuit and the data line, for supplying an auxiliary voltage; a transistor, connected to this power source, for applying an auxiliary voltage supplied from the power source to the data line in an ON state; and clock means, connected to this transistor, for supplying an on/off clock signal. For example, if a writing period for writing an inverse output voltage to a data line is divided into a first h
Brinks Hofer Gilson & Lione
LG. Philips LCD Co. Ltd.
Saras Steven
Spencer William C.
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