An LCD is a display apparatus in which polarized liquid crystal, a macromolecule substance, is sealed in between two transparent electrodes. Information is displayed on the LCD by applying a desired voltage between the two electrodes to change the orientation of the liquid crystal molecules according to the applied voltage to control the light transmittance between the electrodes on a pixel basis. To fabricate an LCD, therefore, a pixel part which consists of transparent electrodes and liquid crystal sealed therebetween as well as a driver for controlling the voltage to be applied to the pixel are required.
FIG. 1 shows an equivalent circuit of an LCD. The liquid crystal sandwiched between two electrodes is represented as a pixel capacitor 1. In many cases, an auxiliary capacitor 9 is formed on a panel in order to provide sufficient capacitance. The auxiliary capacitor 9 has a constant capacitance. The pixel capacitor 1 and the auxiliary capacitor 9 are connected to a switching transistor 2 which is driven by a gate line 3. The source electrode of the switching transistor is connected to a source line 4. An address is assigned to the gate line 3 and the source line 4, respectively. When the address (Sm, Gn) is specified, the voltage on the source line 4 is provided to the pixel capacitor 1 which consists of two electrodes and liquid crystal sealed in between them, and the auxiliary capacitor 9 described above through the switching transistor 2 which is driven by the gate line 3. This voltage causes the orientation of the liquid crystal molecules to change to control light transmittance. An electrode which is opposed to a pixel electrode 5 is commonly called the "counter electrode" 6.
In general, the tilt angle of liquid crystal molecules is roughly proportional to an applied voltage. In recent years, as the display quality has been refined, eight or 16 levels of voltage are applied, instead of simple two levels, and the different brightness levels are represented according to the different voltage levels. That is, the voltage applied to the source line is not constant. Instead, the voltage varies according to data which is to be displayed by a particular pixel.
The alignment of the liquid crystal molecules may be caused by applying a dc voltage to them. However, it is known that the liquid crystal sealed in the cell deteriorates in a very short time or is burnt if a dc voltage is applied. To apply a level of voltage to the liquid crystal cell, therefore, an ac voltage is generally used. That is, usually, voltages which have the same absolute value and opposite polarity and corresponds to certain gray scale are applied alternately in order to display gray scale.
There are two types of such a driving method using an alternating voltage which are conventionally used. The first method uses a high voltage driver. This method applies a potential to the pixel electrode by using an alternating voltage while retaining the voltage applied to the counter electrode at a constant level, as shown in FIG. 2. The potential applied to the cell is high, typically between 10 and 20V. This method presents a number of problems in terms of manufacturability. For example, it is difficult to develop a driver which achieves both a high voltage and high speed. Furthermore, it is not easy to integrate a high voltage circuit which provides multiple levels of output. The second method, as shown in FIG. 3, applies a relatively low voltage (about 5V) to the pixel electrode while applying a high alternating voltage to the counter electrode, and combines these voltages applied to the pixel and the counter electrode in order to achieve an alternating voltage drive effect. This method, however, requires that the counter electrode with large load be driven by a high alternating voltage, thus, the power consumption of the LCD panel is very large. Furthermore, this method is not practical because, as the pixel size becomes smaller, it is difficult to include wiring for driving the counter electrode by alternating voltage, especially in the case where an auxiliary capacitor 9 is included in the cell.
As described above, although the counter electrode potential may be maintained at a constant level using a high voltage driver, it is difficult to achieve high speed using such a driver, and such a driver is costly. If a low withstand voltage driver is used, an alternating voltage must be applied to the counter electrode in order to accomplish alternative driving of the cell. The application of this voltage will consume more electric power and increase the complexity of wiring, and the complex wiring will increase the cost. Therefore, it is desirable to overcome these disadvantages.