Publisher: Supplier of LED Display Time: 2020-09-14 Views: 2785
The display quality of the LED display has always been closely related to the constant current driver chip, such as ghosting, dead pixel cross, low gray color cast, dark first scan, high contrast coupling, etc., and the line drive as a simple scanning requirement has always been too much attention.
The display quality of the LED display has always been closely related to the constant current driver chip, such as ghosting, dead pixel cross, low gray color cast, dark first scan, high contrast coupling, etc., and the line drive as a simple scanning requirement has always been too much attention. With the development of small pitches, LED displays also put forward higher requirements for line driving, from simple P-MOSFET to realize line switching, to multi-functional line driving with higher integration and stronger functions. The design and selection of the line driver also faces six challenges: ghost elimination, lamp reverse voltage, short-circuit caterpillar, open cross, large lamp VF value, and high contrast coupling.
1. Upper ghost: When the scanning screen is switched, it takes a while for the PMOS switch to be turned on and off and the discharge of the charge on the parasitic capacitance Cr of the row line. Therefore, at the moment when the next row scans VLED and OUT is turned on, the previous row scans VLED. The undischarged charge has a 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 bead, resulting in a dim LED.
Therefore, it is necessary to discharge the charge on Cr in advance at the time of line feed. Usually, the line tube with integrated blanking function can quickly discharge the charge of the parasitic capacitor Cr when switching by adding a pull-down circuit. The lower the pull-down potential, that is, the blanking voltage VH is set, the faster the charge on the parasitic capacitance is discharged, and the better the effect of eliminating ghosting is. Usually VH
Second, the reverse voltage of the lamp bead: the reverse impulse voltage of the lamp bead greatly affects the service life of the lamp bead, and the dead pixels caused by the back pressure have always been the pain point of the LED display, especially the small spacing.
When the output channel is closed, due to the freewheeling effect of the parasitic inductance, the parasitic capacitance at the channel will continue to be charged, forming a high voltage glitch. At this time, a reverse voltage loaded on the lamp bead is formed 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. In fact, after the manufacturer's test, the reverse voltage below 1.4V can greatly reduce the dead pixels caused by the reverse voltage. Therefore, the blanking voltage should not be too low for the lamp reverse voltage problem. Generally, Not lower than VCC-2V.
3. Short-circuit caterpillar: When the LED is short-circuited, there will be a long light phenomenon, which is generally called short-circuit caterpillar. When the middle LED lamp bead 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 voltage difference between VLED and point A is greater than the lighting value of the LED lamp bead, a column of constant voltage will be formed. Bright caterpillar.
The biggest difference between the short-circuit caterpillar and the open-circuit 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 or not, while the open-circuit caterpillar will only appear when the open-circuit lamp bead is lit. Usually, the short-circuit caterpillar problem can be completely solved by increasing the blanking voltage of the line tube to make the voltage difference less than the LED forward voltage VF, that is, VLED-VHVCC-1.4V.
4. Open-circuit cross: When the lamp bead is open-circuited in the scanning screen, when the point is lit, the voltage of channel OUT1 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-circuit caterpillar.
When the lamp bead is open, the voltage of the channel OUT1 is pulled down to below 0.5V or even 0V, and the parasitic capacitance Cr of the column is affected by the parasitic capacitances C1 and C2. When the potential of Cr is pulled down, the LED in the same row as the open lamp bead appears. Hidden bright.
Lowering the blanking voltage of the blanking line tube can effectively solve the problem of the open cross, that is, the blanking voltage VH<1.4V. Some line tubes in the industry also use the method of adjustable blanking voltage to reduce the blanking voltage to below 1.4V to solve the open-circuit cross, but this will increase the LED back pressure, accelerate the damage of the LED lamp bead and short-circuit the caterpillar.
5. The VF value of the lamp bead is too large: The column always on due to the large VF value of the lamp bead is also a problem that plagues user applications. Usually the nominal forward voltage VF of the green light is 2.4~3.4V. Under normal circumstances, the anode and cathode voltage difference of the green light is 1.8V to make it light, and the blanking voltage VH of the row tube is too high, which will cause the column to be always on.
Taking the lamp bead forward voltage VF1=3.4V as a column, when scanning to the secondary lamp bead, VOUT and VLED1 are turned on at the same time, the channel terminal voltage: VOUT=VLED1-VF1, the voltage across other rows of lamp beads in this 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 (the row tube voltage drop is ignored), so VH>VCC-1.6 V is not conducive to solving the problem of always-on column caused by the large VF value of the lamp bead.
6. High-contrast coupling: High-contrast coupling refers to superimposing a high-brightness image on a low-brightness background, and the area where the low-brightness image and the high-brightness image are in the same line appears color cast and dark. As shown in Figure 9, the dotted line is the superposition high bright screen. The high-contrast coupling phenomenon is caused by the mutual interference of the column channels through the row tubes. By designing the clamping voltage, that is, maintaining a certain level after the discharge is completed, thereby reducing the blanking voltage of the row tubes, the high-contrast coupling can be improved to a certain extent. However, this design method will bring problems such as darker short-circuit column, low gray and reddish, and large VF value of the lamp bead. To improve the high-contrast coupling from the perspective of row driving, the blanking voltage can be reduced, but it will cause the back pressure of the lamp bead to be too large and the problem of short-circuit caterpillars.
To sum up, it can be seen that the selection of the blanking voltage of the line tube faces the challenges of the above six 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 long-term reliability of lamp beads. Comprehensively dealing with various application problems, the 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 various application problems.