Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-03-25
2001-03-13
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S211000, C345S213000, C345S205000, C345S206000
Reexamination Certificate
active
06201523
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flat panel display device and, more particularly, an active matrix type liquid crystal display device.
2. Description of the Related Art
A flat panel display device which is typified by a liquid crystal display device is employed in various fields to profitably employ characteristics of light weight, small thickness, and low power consumption. In particular, the liquid crystal display device in which a liquid crystal layer is employed as an optical modulation layer is widely employed as a display device for OA devices, home electrical appliances, etc. Especially an active matrix type liquid crystal display device in which switch elements are provided to pixels one by one is spread rapidly as the display device of the OA device.
In the liquid crystal display device, when the voltage of the video signal applied to the liquid crystal layer is unipolar, a direct current (DC) component is always applied to the liquid crystal layer. Problems such as deterioration of the liquid crystal layer are caused if such condition is continued for a long time. In order to prevent such problem, the polarity inversion driving method is executed, i.e., polarity of the voltage applied to the liquid crystal layer is inverted every frame period.
Meanwhile, a video signal is applied to the liquid crystal panel at a timing which is in synchronism with a reference clock signal being input from an external device. For this reason, if the reference clock signal is in a non-conformity state due to any reason, the DC component is applied to the liquid crystal layer over a long time. This situation causes deterioration of the liquid crystal and in turn causes abnormality in image display.
Therefore, such a technology has been proposed that a clock monitoring circuit is provided in the liquid crystal display device and thus the DC component can be prevented from being applied to the liquid crystal layer over a long time when the reference clock signal is in a non-conformity state . However, according to such configuration in the prior art, there are problems such that the number of parts to be incorporated is increased and it is difficult to use the parts in common. As a result, it is difficult to achieve improvement in productivity and a lower cost of the device.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a flat panel display device which is capable of preventing application of a DC voltage to a liquid crystal layer when a reference clock signal is in a non-conformity state and also achieving improvement in productivity and a lower cost of the device.
In order to attain the above object, according to the present invention, there is provided a flat panel display device comprising a display panel in which display pixels each of which includes a pixel electrode, an opposing electrode, and an optical modulation layer interposed between these electrodes are arranged in a matrix fashion; a first D/A converter circuit for receiving a digital video signal, a clock signal, a first voltage, and a second voltage lower than the first voltage, and then converting the digital video signal into a first analog video signal based on the clock signal; a second D/A converter circuit for receiving the digital video signal, the clock signal, the second voltage, and a third voltage lower than the second voltage, and then converting the digital video signal into a second analog video signal based on the clock signal; a driving control portion for outputting the digital video signal and the clock signal to the first D/A converter circuit and the second D/A converter circuit; and a driving circuit portion for outputting signal voltages to respective pixel electrodes in the display panel based on the first analog video signal and the second analog video signal; wherein the first D/A converter circuit and the second D/A converter circuit have a same circuit configuration which includes a clock monitoring portion for monitoring non-conformity state of the clock signal, and an output selector portion for setting both the first analog video signal and the second analog video signal to a predetermined intermediate voltage based on an output of the clock monitoring portion.
According to the above configuration, when the input clock signal is in a non-conformity state, output potentials of the first D/A converter circuit and the second D/A converter circuit are set to become substantially equal to the opposing electrode potential. Therefore, such situation that the DC voltage which is not preferable for the liquid crystal is applied for a long time can be eliminated, and thus deterioration in characteristics of the liquid crystal due to application of the DC voltage can be prevented.
Moreover, because the clock monitoring portions are built in the D/A converter circuits respectively, the number of parts can be reduced rather than the conventional liquid crystal display device in which the clock monitoring circuit is arranged independently on the outside of the display device. In addition, constituent parts of the first D/A converter circuit and the second D/A converter circuit can be employed commonly. Accordingly, improvement in productivity and a lower cost of the device can be achieved.
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Akiyama Ichiro
Yamanaka Satoru
Hjerpe Richard
Kabushiki Kaisha Toshiba
Nguyen Kimnhung
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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