LED screen scan screen ghost generation, scanning is divided into line scan and column scan, we mainly introduce the ghost generated by line scan. The scanning line is driven by the system at different time. When driving the next line, the previous line appears dark and bright. We call it “ghost”. The reason is that: when the last line is turned off, there is no place to release the residual power in the circuit, which is only released in the form of LED light, forming a ghost. Ghost phenomenon is particularly prominent in small space LED display.
since we know the cause of ghost, then we have a way to eliminate it. Since the electricity is released through the LED lamp, then we connect a resistance to the ground on the line line to discharge the line line, so it’s ok? Yes, it’s a good way to get rid of the ghost problem, but there’s another phenomenon: the caterpillar phenomenon. First of all, what is called caterpillar phenomenon is that if one of the lights breaks down in the scanning screen, the other lights in the column of this lamp are always highlighted. What is the reason for this? We analyze it from Figure 1 below
The breakdown of a lamp in
, such as D4, is equivalent to that of other lights in this section of the path train, such as D8, D12, D16, and the current on the broken down lamp D4 and resistance R1 will form a path to the ground, causing the whole column to highlight. In the case of LED lamp quality can not be guaranteed, this will be a fatal blow to the LED display.
it seems that we can see that these two phenomena are contradictory relations. However, is there a way to eliminate these two phenomena at the same time? The answer is yes.
first of all, we must not add resistance on the line. We have to find a kind of component that can quickly discharge the residual power of the line after the line is turned off, and then restore the off state.
based on the above considerations: we use an IC device called 4051. The use of requires the system to provide a signal “od”, that is, the vanishing image we see in the debugging interface (this signal is provided in the scanning board). The longer the elimination time is, the better the effect is. Because the connection between the IC line and the ground is in the electronic switch state, the caterpillar phenomenon basically disappears. What should be noted here is that because the shadow elimination takes up the system clock, it will have a certain impact on the data of the LED display screen, and it will also increase the difficulty of PCB drawing!
now some LED display manufacturers use the above ideas to develop a new IC integrated in the line driver IC for shadow elimination and caterpillar removal. It uses the clock signal generated by itself to control the connection time between the line and the ground, so as to remove the hidden lines and the caterpillars. More importantly, this use reduces the number of LED display driver IC, and can improve the data volume of the whole LED display screen. However, because this kind of IC is integrated circuit, its driving current ability is weak, and the former driving tube is simple MOSFET, the driving current ability of this kind of IC is larger, so the number of lamps that this kind of IC can carry is less than before. The ghost image produced by
column scan is mainly caused by the difference of conduction voltage drop of LED display screen. There is a special LED display driver chip to remove the ghost shadow.