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Technology

The driving principle and precautions of TN liquid crystal display

  • Categories:Technology
  • Author:
  • Origin:
  • Time of issue:2019-06-28 00:00
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(Summary description)

The driving principle and precautions of TN liquid crystal display

(Summary description)

  • Categories:Technology
  • Author:
  • Origin:
  • Time of issue:2019-06-28 00:00
  • Views:
Information
TN liquid crystal display adopts static driving mode. The so-called static driving refers to the continuous application of driving voltages on the displayed pixel electrodes and common electrodes at the same time until the end of the display time. Since the driving voltage is maintained during the display time, it is called static driving. The following takes the most commonly used pen-segment TN liquid crystal display as an example.
 
The pen-segment TN liquid crystal display realizes display through segmented display pixels. Segmented display pixel refers to the display pixel in a long bar shape, also known as pen segment shape. In digital display, a seven-segment electrode structure is often used, that is, each digit is composed of a "8"-shaped common electrode and seven segment electrodes forming a "8"-shaped pattern, which are respectively arranged on two substrates, as shown in Figure 1. Shown.
 
 
 
Figure 1 Electrode arrangement diagram of seven-segment pen-segment liquid crystal display
 
The driving voltage of each pen segment is AC 3~5V, and the frequency is 32Hz, 167Hz, 200Hz. When working, a continuous square wave with a duty ratio of 1/2 is continuously added to the back electrode (COM). A continuous square wave with the opposite phase, the same amplitude, and the same frequency is applied to the pen segment of the back electrode, then a positive and negative alternating voltage twice the amplitude of the square wave is applied to the displayed pen segment , It should be greater than the threshold voltage of the liquid crystal display device 14h; and apply a waveform with the same phase, amplitude and frequency as the voltage waveform on the back electrode on the pen segment that does not need to be displayed, then no electric field can be formed on the pen segment , Of course it cannot be displayed. Figure 2 shows the principle and waveform diagram of a liquid crystal display drive circuit with segment electrodes.
 
 
 
Figure 2 Principle and waveform diagram of a liquid crystal display drive circuit with a segment electrode
 
Figure 2 (a) is an exclusive OR gate circuit. The input terminal A is a square wave oscillation pulse spoon generated by the oscillation circuit and is directly connected to the COM terminal of the LCD screen. The input terminal B can be connected to high and low (ON/OFF) levels to control the on and off of the electrode. The output terminal C of the XOR gate is connected to the front electrode (a, b, c, d, e, f or g) of the pen segment end of the liquid crystal display.
 
The AC drive waveform at both ends of the liquid crystal display (LCD) can be obtained from the XOR gate truth table shown in Figure 2(b), as shown in Figure 2(c). It can be seen that when the voltage phase of the two electrodes on the field is the same, the potential difference between the two electrodes is zero, and this field is not displayed; when the voltage phase of the two electrodes on this field is opposite, the potential difference between the two electrodes is For a square wave voltage with twice the amplitude, the field appears black.
 
The driving of liquid crystal display is very different from that of LED. For LEDs, adding a constant on or off voltage at both ends of the LED can control its brightness or darkness. And LCD, because its two poles cannot add a constant DC voltage, which brings complexity to the drive. Generally, a constant alternating square wave signal should be added to the common electrode of the LCD (usually the back electrode), and the required zero voltage or double amplitude alternating voltage is generated between the two electrodes of the LCD by controlling the voltage change of the front electrode. In order to achieve the control of the display screen on and off.
 
Figure 3 shows the electrode configuration and static driving circuit diagram of the seven-segment liquid crystal display. The seven segments share a back electrode COM, the front electrodes a, b, c, d, e, f, g are independent of each other, and each segment is driven by an exclusive OR gate.
 
 
 
Figure 3 Electrode configuration and static driving circuit diagram of a seven-segment liquid crystal display
 
At present, there are many LCD drive integrated chips on the market, and multiple LCD drive circuits can be integrated together, which is very convenient to use.
 
The pen-segment static drive has two characteristics: ①The driving of each electrode is independent of each other and does not affect each other; ②During the display period, the driving voltage is maintained to make the liquid crystal fully driven. Compared with the dynamic drive described below, the static drive has the advantages of good contrast, high brightness, and fast response. The disadvantage of static driving is that each pen-shaped electrode requires a control element. When the number of digits displayed is too large, the number of corresponding driving elements and lead terminals will be too much. Therefore, its application is limited and only suitable for bits. A few pen segment electrodes are displayed.
 
note:
 
For the LCD screen, the following points must be noted:
 
(1) When driving the LCD screen, it is not advisable to apply DC voltage, otherwise it will cause electrolysis and electrode aging of the liquid crystal, which will greatly reduce the service life of the LCD screen. Therefore, the LCD screen must be driven by AC voltage and limited AC The DC component in the composition is not more than tens of millivolts.
 
(2) Since the change of the optical performance of the liquid crystal under the action of an electric field depends on the elastic deformation of the liquid crystal as an elastic continuum, the response time is long, so the effect of the alternating driving voltage does not depend on its peak value, and the frequency is less than 10 Hz. Next, the change in the transmittance of the liquid crystal is only related to the effective value of the applied voltage.
 
(3) The liquid crystal cell is a capacitive load, and the resistance of the liquid crystal is negligible in most cases. It is non-polar, that is, the effect of positive pressure and negative pressure is the same.

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