1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular, a technology effective for being applied to an in-plane switching (IPS) liquid crystal display device.
2. Description of the Related Art
Unlike a self-emitting display device, for example a cathode ray tube (CRT) or a plasma display panel (PDP), a liquid crystal display device is a non-self-emitting display device that displays a video or an image by regulating a transmitted amount of light from a light source or a reflected amount of external light, or the amounts of both the light.
Further, the liquid crystal display device has features such as thinness, light weight, and lower power consumption, and, in recent years, has been used widely in a liquid crystal display for a liquid crystal television or a personal computer, a liquid crystal display for a portable electronic appliance such as a portable telephone terminal, and the like.
The liquid crystal display device includes a liquid crystal display panel, and a driving circuit for driving the liquid crystal display panel. The liquid crystal display panel has a configuration in which a liquid crystal material is sealed between a pair of substrates, and has a display region formed of a large number of pixels. Each pixel includes a pixel electrode, a common electrode, and a liquid crystal layer, and the transmitted light amount or the reflected light amount, or the amounts of both the light are regulated by varying the alignment of a liquid crystal layer (liquid crystal molecules) with a potential difference between the pixel electrode and the common electrode. Further, the arrangement of the pixel electrodes and the common electrode is roughly classified into the case where the pixel electrodes and the common electrode are placed on different substrates and the case where the pixel electrodes and the common electrode are placed on the same substrate.
A liquid crystal display panel in which the pixel electrodes and the common electrode are placed on the same substrate is generally called an IPS system, and an effective optical axis is rotated in a plane by rotating liquid crystal molecules in an in-plane direction, to thereby regulate the transmitted light amount or the reflected light amount, or the amounts of both the light. In the liquid crystal display device having an IPS liquid crystal display panel, an alignment direction of the liquid crystal layer is substantially horizontal, and hence, a change in retardation of the liquid crystal layer due to a change in a viewing angle is small. Therefore, it is known that the IPS liquid crystal display device can achieve a wide viewing angle.
The arrangement of pixel electrodes and a common electrode in the IPS liquid crystal display panel is roughly classified into the arrangement in which the pixel electrodes and the common electrode are opposed to each other on a same side of an insulating layer and the arrangement in which the pixel electrodes and the common electrode are laminated separately via an insulating layer. In the case where the pixel electrodes and the common electrode are placed on the same side of the insulating layer, the plane shape of each of the pixel electrodes and the plane shape of the common electrode are rendered interdigital, and the pixel electrodes and the common electrode are arranged, for example, in such a manner that tooth portions of the pixel electrodes and tooth portions of the common electrode are placed alternately. On the other hand, in the method of laminating the pixel electrodes and the common electrode, for example, the plane shape of electrodes closer to the liquid crystal layer is formed in an interdigital shape and the plane shape of the other electrodes is formed in a flat plate shape.
In a liquid crystal display device such as a liquid crystal television or a liquid crystal display, generally, the sign of a potential difference between the pixel electrodes and the common electrode varies ever frame period or every predetermined number frame periods. Therefore, in a conventional liquid crystal display device, an in-plane distribution of a transmittance varies, for example, between the case where a positive voltage is applied to the pixel electrodes and the case where a negative voltage is applied thereto, and thus, a phenomenon called flicker occurs. Here, the positive voltage refers to the case where the potential of the pixel electrodes is higher than that of the common electrode, and the negative voltage refers to the case where the potential of the pixel electrodes is lower than that of the common electrode.
One of factors for the occurrence of flicker is as follows: when a thin film transistor (TFT) element is switched from on to off, an abnormal voltage is applied to the potential of the pixel electrode due to parasitic capacitance of the TFT, and a direct current voltage component (DC voltage component) is generated in the potential of the pixel electrode. Therefore, in the conventional liquid crystal display device, a fluctuation portion of the potential of the pixel electrode generated due to parasitic capacitance, i.e., a component for canceling a DC voltage component (hereinafter, referred to as a “DC offset voltage”) is applied to a gray level voltage to be applied to the pixel electrode, to thereby suppress flicker (see, for example, JP 07-020440 A).