Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-09-01
2003-05-20
Nguyen, Chanh (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S089000
Reexamination Certificate
active
06567062
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus and a driving method therefor.
2. Description of the Related Art
The active matrix type liquid crystal display apparatus controls a transmittance (luminance) of each pixel by a RMS value of a voltage applied thereto. In this liquid crystal display apparatus, as illustrated in
FIG. 2
, one pixel includes one MOS type transistor. Moreover, the gate is connected to a gate electrode that the pixels in a transverse direction include in common, and the drain is connected to a drain electrode that the pixels in a longitudinal direction include in common. Also, the source is connected to a common electrode that all the pixels include in common and that is positioned on the side opposite to the source with a liquid crystal cell located in between. As illustrated in
FIG. 3
, the driving method for the display apparatus is as follows: An active state (in
FIG. 3
, “high”) of a scan line signal, which indicates a scan line to be scanned, is applied to each of the gate electrodes in time-division. In accordance with gray-scale information of display data on the line the scan line signal of which is switched into the active state, a one-level gray-scale voltage is selected out of a plurality of levels, then being applied to the drain electrode. Also, a voltage becoming the reference is applied to the common electrode. This procedure holds, in the respective liquid crystal cells in the line sequence, a gray-scale voltage to be applied at the end of a gate-on state. Namely, it becomes possible to control the applied RMS voltage (luminance) to each pixel in correspondence with display data.
Also, as another driving method, there exists a method disclosed in JP-A-10-54998. In this method, as illustrated in
FIG. 4
, one pixel includes two MOS transistors. For example, in the first MOS transistor, the gate is connected to the first gate electrode that the pixels in a longitudinal direction include in common, and the drain is connected to a drain electrode that all the pixels include in common, and the source is connected to a drain of the second MOS transistor. Also, in the second MOS transistor, the gate is connected to the second gate electrode that the pixels in a transverse direction include in common, and the source is connected to a common electrode that all the pixels include in common and that is positioned on the side opposite to the source with a liquid crystal cell located in between. As illustrated in
FIG. 5
, the driving method is as follows: An active state (in
FIG. 5
, “high”) of a scan line signal, which indicates a scan line to be scanned, is applied to each of the second gate electrodes in time-division. In accordance with gray-scale information of display data on the scan line, a gray-scale voltage control signal with a pulse-width corresponding to the gray-scale information is applied to the first gate electrode. Furthermore, a gray-scale voltage, which is in synchronization with a scanning time-period for one line and has, for example, a ramp waveform, is applied to the drain electrode. Also, a voltage becoming the reference is applied to the common electrode. This procedure holds, in the respective liquid crystal cells in the line sequence, a gray-scale voltage level to be reached at the end of a state where the first and the second gates becomes gate-on simultaneously. Accordingly, as is the case with the former method, it becomes possible to control the applied RMS voltage to each pixel in correspondence with display data.
In the method described earlier, as the number of gray-scales (the number of colors) to be displayed is increased, the number of levels of a gray-scale voltage to be prepared is increased. This condition has resulted in increases in the numbers of gray-scale voltage generating output amplifiers and of gray-scale voltage selecting switches, thereby bringing about a problem of a rising in the cost.
Also, for example, if the above-described method is applied to the liquid crystal display apparatus where peripheral driving circuits and the pixels are formed integrally, it turns out that the above-described output amplifiers and selecting switches are also formed in portions of the peripheral driving circuits. This has resulted in a problem that a variation in the characteristics of these elements gives rise to a deterioration in the picture quality.
Also, in the method described later, the pulse-width of the gray-scale voltage control signal makes it possible to control the transmittance of each liquid crystal cell. This brings about an advantage that, even if the number of gray-scales is increased, there is little increase in the circuit scale. Moreover, since all the peripheral circuits can be configured using digital circuits, there exists an effect of suppressing the above-described variation. In this method, however, two MOS transistors are located within one pixel. This condition causes new problems to occur, such as a decrease in the pixel transmittance and a decrease in the yield.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide the active matrix type liquid crystal display apparatus and the driving method therefor, the liquid crystal display apparatus making it possible to prevent the deterioration in the picture quality caused by the inconsistency in the characteristics of the circuit elements.
In solving the above-described problems, at first, let's consider the operation of the MOS transistor in the pixel: In the case where the MOS transistor is of, for example, N type, if an electric potential of the gate is higher than that of the source by the amount of a fixed value or more, the gate is switched into the ON state and thus an electric current is caused to flow between the drain and the source. As a result, a voltage between the drain electrode and the common electrode is applied to the liquid crystal cell. Meanwhile, if the electric potential of the gate is lower than those of the source and the drain, the gate is switched into the OFF state and thus no electric current is caused to flow between the drain and the source. As a result, the voltage that has been applied to the liquid crystal cell at the time of the gate-on is held thereto.
Taking advantage of this operational characteristic in the present invention, gates in pixels on a scan line to be scanned are switched ON and gates in pixels existing on non-scan lines other than the scan line are switched OFF, thereby allowing the line sequence scanning to be executed.
Meanwhile, in the above-described method disclosed in JP-A-10-54998 as well where the gray-scale voltage control signal with the pulse-width in accordance with the gray-scale information is applied to the gate electrode, it is required to perform the control of applying the gray-scale voltage only to the pixels existing on the scan line to be scanned. For this purpose, the second MOS transistor is employed, which allows this control to be implemented.
However, even if the second MOS transistor is not employed, in the following manner for example, it is possible to apply the gray-scale voltage only to the pixels existing on the scan line: The common electrodes are separated in such a manner that they correspond to each of the transverse lines. Then, an electric potential at which the gray-scale voltage control signal is “high” and the gates are switched into the ON state is provided to the common electrodes existing on the scan line to be scanned. Moreover, an electric potential that is higher than the electric potential at which the gray-scale voltage control signal is “high” is provided to the drain electrodes and the common electrodes existing on the non-scan lines other than the scan line.
In view of the above-described points, the present invention implements an active matrix type liquid crystal display apparatus utilizing the pulse-width, and a driving method therefor.
Namely, the liquid crystal display apparatus according to the present inventi
Akimoto Hajime
Furuhashi Tsutomu
Kudo Yasuyuki
Manba Norio
Mikami Yoshiro
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