Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-11-30
2002-09-24
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000, C345S098000, C345S087000, C345S204000, C345S205000, C345S206000, C345S209000, C345S210000, C345S214000, C345S096000
Reexamination Certificate
active
06456267
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display based on an active matrix system, and more particularly to a liquid crystal display in which MOS transistors on a single crystal silicon substrate or Thin-Film Transistors employing poly crystal silicon are used.
In order to clarify the present invention, a conventional active matrix driving system will be explained below: Incidentally, concerning an active matrix panel technology known up to the present time, it is explained in detail in Shunsuke Kobayashi. “Color Liquid Crystal Display” (published from Industrial Library Ltd. in 1990). Moreover, concerning a technology for preventing a flicker caused by leakage resistance of liquid crystal, it is described in JP-A-6-118912.
A liquid crystal display based on the active matrix system, in which MOS (Metal-Oxide Semiconductor) transistors on a single crystal silicon substrate or Thin-Film Transistors (TFT) employing poly crystal silicon are used, comprises a display unit and a driving circuit unit. The display unit is a unit in which transistors are located at the intersections of data signal lines and scanning signal lines arranged in a matrix-like structure. The driving circuit unit controls voltages for the data signal lines and the scanning signal lines.
In a transistor in the display unit, the gate is connected to a scanning signal line, the drain to a data signal line, and the source to a liquid crystal capacitor. Usually, a holding capacitor is added in parallel with the liquid crystal capacitor. Here, when the gate electrode comes into a selection state, the transistor is brought into conduction, thereby allowing an image signal on the data signal line to be written into the liquid crystal capacitor and the holding capacitor. When the gate electrode is changed into a non-selection state, the transistor has a high impedance, thus holding the image signal written in the liquid crystal capacitor.
The driving circuit unit comprises a scanning circuit for controlling the voltages for the scanning signal lines and a signaling circuit for controlling the voltages for the data signal lines. The scanning circuit applies a scanning pulse to each of the scanning signal lines once every one frame time. Usually, a timing of the scanning pulse toward each of the scanning signal lines is shifted in sequence from an upper side of a panel to a lower side thereof. A time of 1/60 second is often employed as the one frame time. In a panel of 640×480 dots, i.e. a representative pixel configuration, since 480 scannings are performed during the one frame time, a time width for the scanning pulse becomes equal to about 35 &mgr;s. A shift register is commonly used in the scanning circuit, and an operating rate of the shift register is equal to about 28 kHz.
Meanwhile, the signaling circuit applies, to each of the data signal lines, a liquid crystal driving voltage the value of which is equivalent to driving liquid crystal of pixels by a single row to which the scanning pulse is applied. In a pixel to which the scanning pulse is applied, a voltage of a gate electrode of the transistor, which is connected to a scanning signal line, becomes high, and thus the transistor is switched to ON state. At this time, the liquid crystal driving voltage is applied to the liquid crystal from a data signal line by way of a drain and a source of the transistor, thus charging a pixel capacitor comprising the liquid crystal capacitor and the holding capacitor. Repetition of this operation allows a signal voltage corresponding to an image, every frame time and repeatedly, to be applied to a pixel capacitor over the entire surface of the panel.
The liquid crystal driving voltage applied to the liquid crystal, by inverting the polarity thereof every frame time, is converted into an alternating voltage. When a frame frequency is equal to, as usual, 60 Hz, a liquid crystal driving frequency becomes equal to 30 Hz, i.e. one-half of the frame frequency. Also, the liquid crystal-driving voltage converted into the alternating voltage with positive and negative polarities is distorted by crosstalk, which is caused by the gate voltage when the transistor is switched from ON state to OFF state, or by the leakage resistance of the liquid crystal.
At the liquid crystal driving frequency of 30 Hz, the distortion of the liquid crystal-driving voltage causes people to feel and see a flickering light called flicker. In order to prevent the flicker from being perceived, it can be considered that a period of the liquid crystal driving voltage (a specific period of the voltage applied to pixel electrodes and having different polarities) is made shorter so that the flicker becomes imperceptible to human eyes. However, it is difficult to fabricate, with a stable yield, an active element for driving pixel electrodes in the conventional liquid crystal display. Also, as a method of making the flicker difficult to recognize visually with human eyes, there exists a driving method in which polarities of driving voltages applied to adjacent pixels are inverted. This is a method of applying signal voltages, the polarities of which are obtained by mutually inverting polarities of signal electrodes of the pixels which are adjacent in a right-to-left direction and those of signal electrodes of the pixels which are adjacent in an up-and-down direction.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a liquid crystal display which produces no flicker.
It is another object of the present invention to provide a liquid crystal display which, by miniaturizing transistors therein and lowering a withstanding voltage thereof, has a large aperture ratio and consumes less electric power.
In a liquid crystal display based on the conventional active matrix driving system, there occur the following problems:
First of all, a serious problem is the occurrence of the flicker. In the above-described liquid crystal display based on the active matrix driving system, the liquid crystal-driving voltage, by inverting the polarity thereof every frame time, is converted into the alternating voltage. As a result, when the frame frequency is equal to, as usual, 60 Hz, the liquid crystal driving frequency becomes equal to 30 Hz, i.e. one-half of the frame frequency. At the liquid crystal driving frequency of 30 Hz, the flickering light called flicker becomes perceptible. In order to prevent the flicker from being perceived, the driving method, in which polarities of driving voltages applied to adjacent pixels are inverted, is employed. This is the method of applying signal voltages, the polarities of which are obtained by mutually inverting polarities of signal electrodes of the pixels which are adjacent in a right-to-left direction and those of signal electrodes of the pixels which are adjacent in an up-and-down direction. The polarities of the signal electrodes, in the case of the above-mentioned panel of 640×480 dots, are inverted every one scanning time period, i.e. 35 &mgr;s. Accordingly, a driving frequency for the signal electrodes becomes equal to 14.4 kHz, i.e. about 500 times as great as the liquid crystal driving frequency. This situation brings about a decrease in the design flexibility.
Also, in the above-described driving method, when displaying a specific pattern such as a checkered pattern obtained by simultaneously displaying pixels to which voltages with an identical polarity are applied, the flicker becomes so conspicuous as to be recognized visually with human eyes.
A second problem is a high withstanding voltage of the transistors. In the above-described liquid crystal display based on the active matrix driving system, sampling of the voltage, the polarities of which are inverted every frame time by the transistors in the display unit, is performed, thereby controlling the liquid crystal-driving voltage. This requires that a withstanding voltage of the transistors in the display unit should be two times or more of an effective voltage for driving the liquid crystal, thus resulti
Mikami Yoshiro
Minemura Tetsuro
Nagae Yoshiharu
Sato Hideo
Tsumura Makoto
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Hjerpe Richard
Zamani Ali
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