The display quality of led display has always been closely related to the input voltage, such as ghost, bad dot cross, low gray color, first scan dark, high contrast coupling and other problems, and line drive as a simple scanning requirement has been less attention. With the development of small pitch, LED display has also put forward higher requirements for row drivers, from simple P-MOSFET to achieve row switching, to more integrated and more powerful multi-function row drivers. The design and selection of line drive also face six challenges: upper ghost elimination, lamp bead reverse voltage, short-circuit caterpillar, open-circuit cross, lamp bead VF value is too large, and high contrast coupling.

1. Ghosting

When switching on the scanning screen, due to the opening and closing of the PMOS tube switch and the charge release on the parasitic capacitor Cr on the line takes a period of time, the unreleased charge of the VLED on the next row of scanning and the opening of OUT will have a conduction path. When turned on, the line parasitic capacitor Cr is charged to the VCC potential. When switching to Row(n+1), a potential difference is formed between Cr and OUT, and the charge is discharged through the lamp bead, resulting in LED dimming. Therefore, the charge on Cr needs to be discharged in advance at the time of line feed, and the line tube with the integrated blanking function is usually integrated, and the charge of the parasitic capacitor Cr is quickly discharged by adding a pull-down circuit when switching. the lower the blanking voltage VH, the faster the charge on the parasitic capacitor will be discharged, and the better the effect of eliminating upper ghosting, usually VH<VCC-1V can eliminate upper ghosting.

2, lamp bead reverse voltage

The reverse impulse voltage of the lamp bead largely affects the service life of the lamp bead, and the bad point caused by the reverse voltage has been the pain point of the LED display screen, especially the small pitch. When the output channel is closed, the parasitic capacitor at the channel will be charged continuously due to the continuous flow of parasitic inductance, forming a high voltage burr. At this time, the reverse voltage loaded on the lamp bead is formed with the output of the tube, so the blanking voltage of the tube also affects the reverse voltage of the lamp bead.When the voltage at the constant current output channel is fixed, the higher the blanking voltage of the line tube, the smaller the reverse voltage of the lamp bead. Usually the nominal reverse voltage of the lamp bead is 5V, the actual test by the manufacturer, the reverse voltage below 1.4V can greatly reduce the bad point caused by the reverse voltage, so the blanking voltage for the lamp bead reverse voltage problem can not be too low, generally not less than VCC-2V.

3. Short-circuited Caterpillar

When the LED shorted, there will be a long line of bright phenomenon, generally known as the short-circuit caterpillar. When the middle LED lamp bead is short-circuited, the LED lamp bead in the same row will form A path as shown in the figure below when scanning to the row. If the pressure difference between VLED and point A is greater than the LED lamp bead lighting value, a column of steady on caterpillars will be formed. The biggest difference between the short-circuit caterpillar and the open-circuit cross is that the short-circuit caterpillar will appear as long as the screen is in the scanning state, regardless of whether the LED lamp beads display images.

4. Open the cross

When the bead is open in the scanning screen and the point is lit, the channel OUT1 voltage is pulled down to below 0.5V. If the scanning line potential blanking voltage VH is 3.5V, a conduction path will be formed for the column of lamp beads, forming an open caterpillar.When the lamp bead is open, the channel OUT1 voltage is pulled down to below 0.5V or even 0V, and affects the column parasitic capacitor Cr through the parasitic capacitor C1 and C2. When the Cr potential is pulled down, the LED in the same row as the lamp bead appears dark. Reducing the blanking voltage of the blanking tube can effectively solve the problem of open cross, that is, blanking voltage VH<1.4V. Some lines in the industry also use the adjustable blanking voltage to reduce the blanking voltage to 1.4V to solve the open cross, but this will bring about an increase in LED back pressure to accelerate LED lamp bead damage and short-circuit caterpillars.

Author: Sily

This article from:http://www.ledtransparency.com/news.html