(a) Field of the Invention
The present invention relates generally to an apparatus of driving a liquid crystal display, and more particularly to a driving apparatus for a liquid crystal display for optionally adjusting white gray voltages and black gray voltages supplied to the liquid crystal display.
(b) Description of the Related Art
A liquid crystal display (LCD) is a display apparatus for obtaining desired images by applying electric field to liquid crystal material having dielectric anisotropy and injected between two panels, and adjusting the intensity of the electric field so that the amount of light passing through the panels is controlled. Such an LCD is representative of a flat panel display device handy to carry. As a kind of LCD, a TFT-LCD employing thin film transistors (TFTs) as switching devices is mainly being used.
Although most of the LCDs have used a twisted nematic (TN) mode, recently an LCD in an optically compensated bend (OCB) mode for improving a slow response time and a narrow viewing angle is briskly being studied.
The OCB mode LCD uses a bend alignment of liquid crystal molecules. However, since the bend alignment is broken under application of a voltage lower than a specific threshold voltage Vc, the OCB mode LCD can be driven by a voltage larger than the threshold voltage Vc.
However, since a voltage to transmittance (VT) curve for the OCB mode LCD is unstable, it is difficult to secure a proper driving because of serious variation of characteristics of products depending on process deviation, for example, thickness of a liquid crystal layer and alignment deviations.
In other words, in the case of a normally white mode OCB LCD, it is difficult to adjust the luminance since a significant luminance difference is generated even by a minute voltage variation due to a sharp gradient of the VT curve in a white region corresponding to low voltages. On the other hand, since the VT curve in a black region corresponding to high voltages has several inflection points, there occurs a gray inversion that the higher the voltage is, the higher the luminance is. Since such a phenomenon produces significant differences in gray characteristics even by minute process deviations of products, there occurs a problem that the luminance is different even for the same gray voltage.