Selection of Pen Segment LCD Driver Chip
- Time of issue:2018-12-07 00:00
Selection of Pen Segment LCD Driver Chip
- Time of issue:2018-12-07 00:00
The driver IC of the pen segment code display screen is generally HT 1621, which can drive 128 segments, and 1622 is required for more than 128 segments. You can also choose to use a single-chip microcomputer, which is only for simple content and low price.
Liquid crystal display is due to the application of an electric field on the display pixel, and this electric field is generated by the synthesis of the potential signals on the two electrodes before and after the display pixel. It is very easy to establish a DC electric field on the display pixel, but the DC electric field will This leads to the chemical reaction of the liquid crystal material and the aging of the electrodes, thereby rapidly reducing the display life of the liquid crystal. Therefore, an AC drive electric field must be established, and the DC component in this AC electric field is required to be as small as possible. Usually, the DC component is required to be less than 50mV. In practical applications, due to the use of digital circuit drive, this AC electric field is established by pulsed voltage signals.
The strength of the AC electric field on the display pixel is represented by the effective value of the AC voltage. When the effective value is greater than the threshold voltage of the liquid crystal, the pixel is in the display state; when the effective value is less than the threshold voltage, the pixel does not produce electro-optical effects; when the effective value is at the threshold Near the voltage, the liquid crystal will show a weak electro-optical effect, which will affect the contrast of the liquid crystal display device.
The driving of the liquid crystal display is used to adjust the phase, peak value, frequency, etc. of the potential signal applied to the electrode of the liquid crystal device to establish a driving electric field to realize the display effect of the liquid crystal display device. There are many driving methods for liquid crystal display, and the commonly used driving methods include static driving method and dynamic driving method.
(1) Static drive method
The static driving method is the most basic method to obtain the best display quality, and it is suitable for driving pen-segment liquid crystal display devices. In the electrode structure of this type of liquid crystal display device, when multiple digits are combined, the back electrodes BP of each bit are connected together. In the circuit of the static driving method, the pulse signal of the oscillator is directly applied to the back electrode BP of the liquid crystal display device after frequency division; and the pulse signal of the segment electrode is synthesized by the display selection signal and the timing pulse.
When a certain display pixel is selected for display, the phase difference of the pulse voltage of the two electrodes on the display pixel is 180, and a 2V voltage pulse sequence is generated on the display pixel to make the display pixel exhibit display characteristics; when a certain display pixel is non-display When selected, the pulse voltages of the two electrodes on the display pixel have the same phase, and the synthesized voltage pulse on the display pixel is 0V, thereby achieving the display effect. This is the static driving method. In order to improve the contrast, the voltage of the pulse should be adjusted appropriately.
(2) Dynamic drive method
When there are many display pixels on a liquid crystal display device (such as a dot matrix liquid crystal display device), in order to save a huge hardware drive circuit, the production and arrangement of the electrodes of the liquid crystal display device are processed, and the matrix structure is implemented. The back electrodes of a group of display pixels are all connected and drawn together, called row electrodes; the segment electrodes of a group of display pixels in the vertical direction are all connected and drawn together, called column electrodes. Each display pixel on the liquid crystal display device is uniquely determined by the position of its column and row. Correspondingly, a raster scanning method similar to CRT is adopted in the driving mode. The dynamic driving method of liquid crystal display is to cyclically apply selection pulses to the row electrodes, and at the same time, all the column electrodes that display data give corresponding selective or non-selected driving pulses, so as to realize the display function of all display pixels in a certain row. This row scanning It is carried out sequentially line by line, and the cycle period is very short, which makes the LCD display stable display. People call the scanning driving mode of liquid crystal display as dynamic driving mode.
The selection time of each row in a frame is equal. Assuming that the number of scanning lines in a frame is N and the scanning time is 1, then the selection time occupied by one line is 1/N of the time of one frame. In the driving method of liquid crystal display, this value, that is, the reciprocal of the number of line scans in a frame, is called the duty ratio of the liquid crystal display driving (duty), which is represented by d. Under the same voltage, the increase in the number of scanning lines will decrease the duty cycle of the liquid crystal display, which will result in a decrease in the effective value of the variable electric field voltage and reduce the display quality. Therefore, with the increase of the display screen and the increase of display lines, in order to ensure the quality of the display, it is necessary to appropriately increase the driving voltage or adopt a dual-screen electrode arrangement structure to increase the effective value of the electric field voltage or increase the duty cycle.
In the dynamic driving mode, the display mechanism of the display pixel at a certain position of the liquid crystal display device is realized by the combination of the row selection voltage and the column display data voltage, that is, to make a position such as (I, J) point display, it needs to be displayed in the first position. The selection voltage is applied to column I and row J at the same time to maximize the variable electric field at this point. But at this time, except for the points (I, J), the other points on the I column and the J row are also subjected to a certain voltage. These points are called semi-selected points. If the effective voltage on the semi-selected points is greater than the threshold Unwanted display appears on the screen when the voltage is applied, which reduces the contrast. This phenomenon is called "cross effect".
The method to solve the "cross effect" in the dynamic driving method is the average voltage method, which averages the voltages at the half-selected point and the non-selected point, and moderately increases the voltage at the non-selected point to offset part of the voltage at the half-selected point, making the half-selected point The voltage drop on the spot increases the contrast of the display. As the voltage appears on average, it is called the bias ratio (bias), represented by b