Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-08-24
2003-04-29
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S100000
Reexamination Certificate
active
06556182
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device used for a personal computer, a work station or the like, and in particular to a technique useful for a video signal line driver circuit (a drain driver) in a liquid crystal display device capable of a multi-gray scale display.
Active matrix type liquid crystal display devices which have an active element (a thin film transistor, for example) for each pixel and switch the active elements are widely used as display devices for notebook personal computers and the like. In the active matrix type liquid crystal display device, each pixel electrode is supplied with a video signal voltage (a gray-scale voltage) via an active element, no cross talk between pixels occur, and therefore a multi-gray scale display is produced without the need for a special driving scheme for prevention of the cross talk, unlike a liquid crystal display device of the so-called simple matrix type.
As one of the active matrix type liquid crystal display devices, there is known a TFT (Thin Film Transistor) type liquid crystal display module having a TFT type liquid crystal display panel (a TFT-LCD), drain drivers disposed at the top side of the liquid crystal display panel and gate drivers disposed at the lateral side of the liquid crystal display panel.
The TFT type liquid crystal display module includes, in its drain drivers, a gray-scale voltage generating circuit for generating a plurality of gray-scale voltages, decoders for selecting a gray-scale voltage in accordance with a display data from among the plurality of gray-scale voltages generated by the gray-scale voltage generating circuit, amplifiers for amplifying the gray-scale voltage selected by the decoders so as to output a video signal voltage in accordance with the display data to a corresponding one of drain signal lines, and a bias circuit for controlling a current of a constant current source in the amplifiers. Such a technique is disclosed in Japanese Patent Application No. Hei 11-47885 (filed on Feb. 25, 1999, but not laid-open on the filing date of the present application), for example.
SUMMARY OF THE INVENTION
The gray-scale voltage generating circuit in the drain driver includes a voltage-dividing resistor circuit for dividing voltages between a plurality of gray-scale reference voltages supplied from a power supply circuit so as to generate a plurality of gray-scale voltages.
Each of the drain drivers is formed in one semiconductor integrated circuit (a semiconductor chip), and the voltage-dividing resistor circuit is formed of a tapped resistive element, a plurality of gray-scale voltage lines for outputting gray-scale voltages, an interlayer insulating film for insulating the gray-scale voltage lines from the tapped resistive element, and a plurality of connections for connecting the gray-scale voltage lines with the taps of the tapped resistive element via contact holes formed in the interlayer insulating film.
A resistance of the resistive element between two adjacent taps of the resistive element is determined by (a length of the resistive element between the two taps of the resistive element)/(a width W of the resistive element)×(a sheet resistance of the resistive element).
In conventional drain drivers, the above-mentioned connections for connecting the gray-scale voltage lines-with the taps of the tapped resistive element are disposed in a current path of a current flowing through the resistive element. In this case, the length L of the resistive element between the two adjacent taps varies with manufacturing variability of dimensions of the contact holes or the like, and consequently, a problem arises in that the resistance of the resistive element between two adjacent taps varies such that a gray-scale voltage generated in the voltage-dividing resistor circuit varies and the quality of a display image by the liquid crystal display panel is degraded.
The area of the contact holes had to be made small because the contact holes are disposed in a current path of a current flowing through the resistive element and therefore the area of the contact holes is limited. As a result, there has been a problem that the resistance at the connections for connecting the gray-scale voltage lines with the taps of the resistive element is increased and time delay is caused in transfer characteristics of gray-scale voltages from the voltage-dividing resistor circuit to a succeeding amplifier.
Recently, there are demands for a larger-sized display panel (a larger-sized TFT-LCD), higher resolution, a higher-quality display image and lower power consumption on the TFT active matrix type liquid crystal display device, and also there is a demand for reduction of power consumption on the liquid crystal display devices because necessity for their long-period operation powered by batteries is becoming greater as notebook personal computers spread.
In this case, for the purpose of improving the quality of a display image, the greater the voltage range of gray-scale voltages applied across the liquid crystal layer, that is, the voltage range of output voltages outputted from the drain drivers, the better for improvement of response speed of the liquid crystal and display contrast. In view of this, the power supply voltage VDD for the drain drivers is selected to be high.
In general, each of the amplifiers of the drain drivers comprises a high-voltage amplifier for amplifying positive-polarity gray-scale voltages and a low-voltage amplifier for amplifying negative-polarity gray-scale voltages. These high-voltage and low-voltage amplifiers are formed by differential amplifiers and current values of constant-current sources each for a respective one of the differential amplifiers are determined by one bias circuit. The bias circuit had to be formed by high-breakdown-voltage MOS transistors (hereinafter referred to merely as high-voltage MOS transistors) because the power supply voltage VDD for the drain drivers is high.
Generally, in the high-voltage MOS transistors, the thickness of the gate insulator oxide is made thick enough to ensure a high breakdown voltage (a high withstand voltage) and a region for relaxing electric fields is necessary, and consequently, variations in threshold voltages and the like of high-voltage MOS transistors are greater than those of low-breakdown-voltage MOS transistors (hereinafter referred to merely as low-voltage MOS transistors). As a result, current values of the currents supplied from the bias circuits to the constant-current sources of the differential amplifiers forming the amplifiers of the drain drivers vary from drain driver to drain driver, and in the liquid crystal panel incorporating about ten drain drivers there is a problem that there is a possibility that display brightness varies from drain driver to drain driver and the quality of a display of the liquid crystal display panel is degraded.
The present invention solves the above-mentioned problems with the prior art, and it is an object of the present invention to provide a technique for improving the quality of a display image of the liquid crystal display panel in the liquid crystal display device.
It is another object of the present invention to provide a technique for preventing occurrence of variations in respective gray-scale voltages generated by gray-scale voltage generating circuits in the liquid crystal display device.
It is yet another object of the present invention to provide a technique for making uniform the current values of the currents of the constant-current sources of the amplifiers of the drain drivers in each of the drain drivers by making it possible to use low-voltage MOS transistors in the bias circuits, in the liquid crystal display device.
The above objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.
The following explains a summary of representative configurations of the present invention.
To accomplish the above objects, in accordance with an
Agata Kentaro
Goto Mitsuru
Kotera Koichi
Nakayasu Yozo
Yamashita Yuji
Hjerpe Richard
Nguyen Kevin M.
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