Technology | What are the causes of ghost images, dead pixel crosses, and low gray color cast on LED displays?(2)

Open the way to cross

When the lamp bead open circuit appears in the scanning screen and the point is lit, the voltage of channel OUT1 is pulled down to below 0.5V. If the scanning row potential blanking voltage VH is 3.5V, a conduction path will be formed for the column of lamp beads, forming an open-circuit caterpillar.

When the lamp bead is open, the channel OUT1 voltage is pulled down to below 0.5V or even 0V, and the column parasitic capacitance Cr is affected by the parasitic capacitances C1 and C2. When the Cr potential is pulled down, the LED in the same row as the open lamp bead appears Faintly bright.

Lowering the blanking voltage of the blanking tube can effectively solve the problem of open crosses, that is, the blanking voltage VH<1.4V. Some line tubes in the industry also use adjustable blanking voltage to reduce the blanking voltage to below 1.4V to solve the open cross, but this will increase the LED back pressure and accelerate the LED lamp bead damage and short-circuit caterpillars.

Lamp beads VF value is too large

 

The column is always on due to the large VF value of the lamp beads is also a problem that plagues user applications. Normally, the nominal forward voltage VF of the green light is 2.4~3.4V. Normally, the voltage difference between the anode and the cathode of the green light is 1.8V to make it light, and the blanking voltage VH of the line tube is too high, which will cause the column to light up.

Taking the lamp bead forward voltage VF1=3.4V as the column, when scanning to the second lamp bead, VOUT and VLED1 are turned on at the same time, the channel end voltage: VOUT=VLED1－VF1, the voltage at the other row lamp beads in the column: V△=VH －VOUT=VH－VLED1＋VF1, if V△>1.8V, it may cause the column to be always on, that is, VH－VLED1＋VF1>1.8V, where VLED=VCC (ignoring the voltage drop of the line tube), so VH>VCC－1.6V is not good for Solve the problem of column constant light caused by too large VF value of lamp beads.

High contrast coupling

High-contrast coupling refers to superimposing a bright picture under a low-brightness background, and the area where the low-bright picture and the high-bright picture are in the same color cast and darken. The high-contrast coupling phenomenon is caused by the interference of the column channels through the row tubes. The high-contrast coupling can be improved to a certain extent by reducing the row tube blanking voltage by designing the clamping voltage method, that is, keeping it at a certain level after discharge. However, this design method will cause problems such as dark short-circuit columns, low gray and reddish colors, and large lamp beads VF values. From the perspective of row driving, the high-contrast coupling can be improved by lowering the blanking voltage, but it will cause the lamp beads to have a large back voltage and short-circuit caterpillars.

Tube blanking voltage selection

In summary, the selection of the line tube blanking voltage faces the challenges of the above six major problems, and there are certain challenges respectively, and the blanking voltage cannot be too high or too low. Usually open crosses are eliminated by constant current drive detection, because too low blanking voltage will reduce the reliability of the lamp beads for long-term use. The following table summarizes the suitable range of blanking voltage in various situations.

Therefore, to deal with various application problems comprehensively, a blanking voltage of 3V~3.4V (VCC=5V) is a more reasonable choice. It can meet the design requirements of various scanning modules of users, so as to reasonably solve a variety of application problems.

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Technology | What are the causes of ghost images, dead pixel crosses, and low gray color cast on LED displays?(1)

Sunrise LED Display manufacturer