(a) Field of the Invention
The present invention relates to a liquid crystal display, an apparatus for driving a liquid crystal display, and a method of generating gray voltages for a liquid crystal display.
(b) Description of the Related Art
A typical liquid crystal display (“LCD”) includes a pair of transparent glass substrates facing each other to define a narrow gap therebetween and a liquid crystal layer with dielectric anisotropy filled in the gap. A plurality of field-generating electrodes opposite each other are provided on the inner surfaces of the respective glass substrates. The field-generating electrodes are applied with voltages to generate an electric field in the liquid crystal layer. The LCD displays a desired image by controlling the voltages applied to the field-generating electrodes to adjust the transmittance of light passing through the liquid crystal layer.
Among the LCDs, a thin-film transistor (“TFT”) LCD using TFTs as switching elements is widely used. A typical TFT LCD has a plurality of pixels arranged in a matrix, a plurality of gate lines extending in a row direction, and a plurality of data lines extending in a column direction. Each pixel includes a TFT connected to one of the gate lines and one of the data lines and a liquid crystal capacitor having a pixel electrode, a common electrode opposite thereto and a liquid crystal layer therebetween.
An electric field is generated by the voltage difference between the pixel electrodes and the common electrode, and the field direction is periodically inversed in order to prevent the deterioration of the characteristics of the LCD. If not, continuous application of unidirectional electric field causes precipitation of ionic impurities in the liquid crystal layer onto the pixel electrodes and the common electrode, thereby causing electrochemical reactions in the electrodes. The field-direction is inversed by reversing the polarity of the voltages applied to the pixel electrodes (referred to as “data voltages” hereinafter) with respect to the voltage applied to the common electrodes (referred to as “common voltage” hereinafter).
The inversion in an LCD reverses the polarity of the data voltages by frame (“frame inversion”), by row (“line inversion”), and by pixel (“dot inversion”).
The dot inversion includes one dot inversion and two-to-one dot inversion. The dot inversion reverses the polarities of the pixels adjacent to each other in the row direction. In the one dot inversion, the adjacent pixels in the column direction have the opposite polarities. On the other hand, the polarity of the pixels in the column direction is reversed every two rows in the two-to-one inversion.
In the dot inversion, voltages across liquid crystal capacitors (referred to as “pixel voltages”) in a row are dropped when liquid crystal capacitors in the next row are charged, since parasitic capacitors between the liquid crystal capacitors in the adjacent rows generate AC currents. In particular, the voltage difference of the pixels in adjacent two rows with the same polarity in the two-to-one dot inversion induces brightness difference therebetween. For example, the upper one of two adjacent pixels with the same polarity in the column direction, when applied with the same data voltage, has larger pixel voltage than the lower one.
On the contrary, voltage delay caused by a slew rate decreases the pixel voltage of the upper pixel larger than the lower pixel. For example, it is assumed that the same data voltage is applied to the upper and the lower pixels. The data voltage flowing through the data line experiences RC delay when charging the upper pixel since the voltage difference from the previous data voltage with different polarity is large. That is, the large voltage difference makes it to take time to reach the expected value. However, the data voltage hardly experiences the RC delay when charging the lower pixel since the data voltages for the upper and the lower pixels are the same. Therefore, the pixel voltage of the upper pixel has a smaller value than the lower pixel.