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LED driver IC determines the ghost, bad cross, low grey bias color

LED display quality has always been closely related to the constant current driver chip, such as ghosting, bad point cross, low grey bias color, first scan bias dark, high-contrast coupling and other issues, while the line drive as a simple scanning requirements has not been Too much emphasis. With the development of a small pitch, the LED display also imposes higher requirements on the line driver, from simple P-MOSFET line switching to more integrated and more powerful multi-function line driver. The line-driven design and selection also face six major challenges: ghost elimination, lamp reverse voltage, short-circuited caterpillars, open crosses, large lamp VFs, and high-contrast coupling.


Ghosting:

When the scan screen is switched, since the opening and closing of the PMOS transistor switch and charge discharge on the parasitic capacitance Cr of the row line require a period of time, the unreleased charge of the previous row scanning VLED has the moment when the next row scans the VLED and the OUT turn on. Conduction path. When Row(n) is on, the parasitic capacitance Cr of the row 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 to generate a hidden LED.



Therefore, at the time of line feed, the charge on Cr needs to be discharged in advance, and the line tube of the blanking function is usually integrated. By adding the pull-down circuit, the charge of the parasitic capacitor Cr is quickly discharged when switching is performed. The lower the pull-down potential, that is, the lowering of the blanking voltage VH, the faster the charge discharge on the parasitic capacitance is, and the better the effect of eliminating the ghosting effect is. Generally, VH


Reverse Voltage:

The reverse voltage of the LED lamp affects the service life of LED lamp at a large extent. The dead point caused by the back pressure has always been the pain point of the LED display screen, especially the small pitch.


short-circuited caterpillars:

When the LED is short-circuited, there will be a constant light phenomenon, commonly known as a short-circuit caterpillar. When the middle led lamp are short-circuited, the same row of LED lamp will form the path as shown in the figure below when scanning to this row. If the pressure difference is larger than the LED lamp lighting value between VLED and point A, there will be form a row constant bright caterpillars.


The biggest difference between short-circuited caterpillars and an open cross is that short caterpillar will appear as long as the screen is scanned, regardless of whether the LED lamp are display image or not, and the open caterpillar will only appear on the open circuit cross when the opening lamp is lit.Usually, by raising the hidden voltage of the pipeline, the pressure difference is less than LED forward voltage VF, that is, VLED VHVCC-1.4V can solve the problem of short circuiting caterpillars completely. When VCC-2V


Open Cross:

When the lamp bead opens in the scan screen, the channel OUT1 voltage is pulled below 0.5V when the point is lit. If the scanning line potential blanking voltage VH is 3.5V, a conduction path will be formed for the array of lamp to form an open caterpillar.


When the LED lamp opens, the channel OUT1 voltage is pulled down to below 0.5V or even 0V, and the parasitic capacitance Cr is affected by the parasitic capacitance C1 and C2. When the Cr potential is pulled down, the LED of the same line with the opening lamp appears bright.

Lowering the hidden voltage of the hidden line can effectively solve the problem of open crosses, namely, the hidden voltage VH<1.4V. Some pipe lines in the industry also use adjustable hidden voltage to reduce the hidden voltage to 1.4V to solve the open cross, but this will bring LED back pressure to increase the damage of LED lamp and short caterpillar.


large lamp VFs:

The column constant brightness due to the large VF value of LED lamp is also a problem that puzzles users.Usually, the nominal voltage VF of the green light is 2.4~3.4V. Normally, the green anode and the cathode voltage difference of 1.8V can make it light up, and if the blanking voltage VH of the line tube is too high, it will cause the column to stay on.


With the LED lamp forward voltage VF1=3.4V as the column, VOUT and VLED1 are turned on simultaneously when scanning to the next lamp.Channel terminal voltage: VOUT=VLED1-VF1

The other lines of the column lamp voltage at both ends: VΔ=VH-VOUT=VH-VLED1+VF1

If V△>1.8V, it may cause the column to be on, ie, VH-VLED1+VF1>1.8V, where VLED=VCC (line voltage drop is neglected), so VH>VCC-1.6V is not good for solving large lamp VF. Caused the problem of permanent lights.


High-contrast coupling:

The high-contrast coupling means that in a low-lighting background, the highlight image is superimposed, and the areas where the low-bright image and the highlight image are in the same area appear biased color or dark phenomenon. As shown in picture below shows , the dotted line is a superimposed highlight image. The high-contrast coupling phenomenon is caused by row channels interfering with each other in the column channels. By designing the clamp voltage, the voltage remains at a certain level after the end of the discharge, so that the line blanking voltage is lowered, and the high-contrast coupling can be improved to some extent. However, this design method will cause problems such as dark side of the short circuit, reddish gray, and large lamp VF. Improving the high-contrast coupling from the line-drive perspective can reduce the blanking voltage, but it will lead to large lamp back pressure and short-circuit caterpillar problems.



Selection of hidden voltage for row pipe:

To sum up, the choice of the hidden voltage of the pipeline is facing the challenge of the above six problems, and there are some challenges. The voltage of the blanking can not be too high or too low. Normally, the open crosses are eliminated by constant current driving detection, because the low hidden voltage will reduce the long-term reliability of the bulbs. The following table summarizes the appropriate range of blanking voltages under various conditions.

Project

Ghosts

Lamp back pressure

Short- circuited caterpillar

Open Cross

large lamp VFs

High-contrast coupling

Selection of hidden voltage

The lower the better

The higher the better

The higher the better

The lower the better

The lower the better

The lower the better

Target Value

VH1V

VHVCC2V

VHVCC1.4V

VH<1.4V

VH1.6V

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Therefore, comprehensive treatment of various application problems, the blanking voltage in the 3V ~ 3.4V (VCC = 5V) is a more reasonable choice. It can meet the needs of users for various types of scanning module design, so as to reasonably solve a variety of application problems.



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