The display quality of LED displays has always been closely related to the constant current drive chip, such as ghosting, dead pixel crosses, 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 the line driver also face six major challenges of ghost elimination, lamp bead reverse voltage, short circuit caterpillar, open cross, lamp bead VF value is too large, and high contrast coupling.
Ghost
When the scanning screen is switched, it takes a while for the PMOS tube switch to turn on and off and the charge discharge on the row line parasitic capacitance Cr. Therefore, at the moment when the next line scans VLED and OUT, the unreleased charge of the previous line scans VLED has 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.
Full color led display
Therefore, at the time of line change, the charge on Cr needs to be discharged in advance. 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 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<vcc-1v can eliminate ghost images. <p="">
Lamp beads 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, due to the freewheeling effect of the parasitic inductance, the parasitic capacitance at the channel will continue to be charged, 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 LED lamp bead lighting value, it will form a normal column. 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-VH<vf. Usually the forward voltage of the red led lamp (forward vh="">VCC-1.4V can completely solve the short-circuit caterpillar problem. When VCC-2V<vh<vcc-1.4v, there is only a red led below the short-circuit point Is faintly lit.< p=""></vf. Usually the forward voltage of red led lamp beads (forward>
Open the way to cross
When there is an open circuit of the lamp bead in the scanning screen and the point is lit, the voltage of channel OUT1 is pulled down to below 0.5V. If the scan 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 channel OUT1 voltage 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 Cr potential is pulled down, the LEDs in the same row as the open lamp bead appear Faintly bright.
Lowering the blanking voltage of the blanking line tube can effectively solve the open cross problem, 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.
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Lamp beads VF value is too large
The column is always on due to the large VF value of the lamp bead is also a problem that plagues user applications. Normally, the nominal forward voltage VF of the green light is 2.4~3.4V. Under normal circumstances, the green light anode and cathode voltage difference of 1.8V can make it light up, and the blanking voltage VH of the line tube will be too high, which will cause the column to light up.
Take 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 across the other rows of the lamp bead 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-1.6 V is not conducive to solving the problem of constant light caused by the lamp bead VF value is too large.
High contrast coupling
High-contrast coupling refers to superimposing bright pictures under a low-brightness background, and the areas where the low-bright pictures and the high-bright pictures are in the same color cast and darken. The dotted line as shown in the figure is the superimposed high-bright picture. 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 the discharge ends. However, this design method will bring about problems such as dark short-circuit columns, low gray and reddish colors, and large VF values of the lamp beads. From the perspective of row driving, the high-contrast coupling can be improved by lowering the blanking voltage, but it will bring about the problem of large back voltage of the lamp beads 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 issues, 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 needs of various scanning modules of users, so as to reasonably solve a variety of application problems.
