LCD static drive

The basic idea is to continuously apply an electric field or no electric field between a pair of corresponding electrodes.

As shown in Figure 1:

Its driving circuit principle

Figure 1. LCD static drive diagram

Figure 2. Driving circuit schematic

Driving waveform

According to this electrical signal, the segment waveform is either in phase or in reverse phase with the common waveform. When in phase, there is no electric field on the liquid crystal, and the LCD is in a non-selected state. When in reverse phase, a rectangular wave is applied to the liquid crystal. When the voltage of the rectangular wave is much higher than the liquid crystal threshold, the LCD is in a selected state.

Figure 3. Static waveform

￡2. Multi-channel drive

Basic idea

The electrodes are arranged in a matrix along the X and Y directions (as shown in Figure 4), and a gating waveform is applied to the X electrode in sequence, and a gating or non-gating waveform synchronized with the X electrode is applied to the Y electrode, and this cycle is repeated. Through this operation, the pixels at the intersection of the X and Y electrodes can be independently selected or non-selected.

The time required to drive the X electrode from the first row to the last row is the frame period Tf (frequency is the frame rate), and the ratio of the time Tr used to drive each row to the frame period is the duty cycle: Duty=Tr/Tf=1/N.

Figure 4. Electrode array

Voltage averaging

From the basic idea of ​​multi-channel driving, it can be seen that voltage is applied not only to the selected pixels, but also to the non-selected pixels. The ratio of the waveform voltage when non-selected to the waveform voltage when selected is the bias ratio Bias=1/a. In order to make the display state between the selected pixels and the non-selected pixels consistent, the selected point voltage Von and the non-selected point voltage Voff must be consistent. In order to make the pixels selected under the action of the selected voltage, and not selected under the action of the non-selected voltage, the photoelectric performance of the LCD must have a threshold characteristic, and the steeper the better. However, due to the limitations of materials and modes, the steepness of the LCD electro-optical curve is always limited. Therefore, it is required that V _on and V _off be pulled as far apart as possible, that is, the larger V _on /V _off, the better. According to theoretical calculations, when Duty and Bias satisfy the following relationship, V _on /V _off takes the maximum value. The a that satisfies the following formula is the optimal bias value for the number of driving channels N.

Six-level drive

In semiconductor integrated circuits, a six-level method as shown in FIG5 is generally used to achieve the optimal bias voltage.

Figure 5. Six-level level

Figure 6. Schematic diagram of six-level circuit

The circuit to achieve six-level voltage levels generally uses a Bias circuit as shown in Figure 6.

When driving with six-level voltage, the voltage levels given to the COM electrode and SEG electrode are as follows:

For the 6-level level above, when a<5, degeneration will occur. For example:

• When a=4, V3=V4
• When a=3, V2=V4, V3=V5
• When a=2, V1=V4, V2=V5, V3=V6

After degeneration, the level distribution relationship in the above table still holds true.