Analysis of various problems in the display of full-color LED display screens

April 07, 2022

The display quality of the LED display has always been closely related to the constant current drive chip, such as ghosting, dead pixel cross, low gray cast, dark first scan, high contrast coupling, etc., and line drive has always been a simple scanning requirement. Too much attention. With the development of small pitch, LED display screens also put forward higher requirements for row driving, from pure P-MOSFET to realize row switching, to higher integration and more powerful multi-function row driving. The design and selection of line drivers also face six major challenges such as ghost elimination, lamp bead reverse voltage, short circuit caterpillar, open cross, lamp bead VF value is too large, and high contrast coupling.
When the scan screen is switched, it takes a while for the PMOS switch to be turned on and off and the charge discharge on the parasitic capacitance Cr of the row line. Therefore, at the moment when the VLED and OUT are turned on in the next line, the unreleased charge of the VLED in the previous line is The conduction path. When Row(n) is turned on, the row parasitic capacitance 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 beads, resulting in a dim LED light.
Therefore, the charge on Cr needs to be discharged in advance at the time of line change. Generally, a line tube with integrated blanking function can quickly discharge the charge of the parasitic capacitor Cr when switching is performed by adding a pull-down circuit. The lower the pull-down potential, that is, the lower the blanking voltage VH, the faster the charge on the parasitic capacitance will be discharged, and the better the effect of eliminating ghost images. Generally, VH Lamp bead reverse voltage
The reverse impulse voltage of the lamp beads greatly affects the service life of the lamp beads. The dead pixels caused by the back pressure have always been the pain points of the LED display, especially the small pitch.
When the output channel is closed, the parasitic capacitance at the channel will be continuously charged due to the freewheeling effect of the parasitic inductance, forming a very high voltage glitch. At this time, it forms a reverse voltage loaded on the lamp bead with the output of the line tube, so the blanking voltage of the line tube affects the reverse voltage of the lamp bead at the same time. 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. Actually tested by the manufacturer, the back pressure below 1.4V can greatly reduce the dead pixels caused by the back pressure. Therefore, the blanking voltage cannot be too low for the lamp bead back pressure problem. Not lower than VCC-2V.
Short circuit caterpillar
When the LED is short-circuited, there will be a long bright phenomenon, which is generally called a short-circuit caterpillar. When the LED lamp bead in the middle is short-circuited, the LED lamp bead in the same column 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 lighting value of the LED lamp bead, a normal column will be formed. Bright caterpillar.
The biggest difference between the short-circuit caterpillar and the open cross is that as long as the screen is in the scanning state, the short-circuit caterpillar will appear regardless of whether the LED lamp bead displays an image, and the open-circuit caterpillar will only have an open cross problem when the open lamp bead is lit. Usually by increasing the line tube blanking voltage, the voltage difference is smaller than the LED forward voltage VF, that is, VLED-VHVCC-1.4V can completely solve the short-circuit caterpillar problem. When VCC-2V<vh<vcc-1.4v, there is only one red led below the short-circuit point Is faintly lit.< p=”">

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