Detailed explanation about LED screen driver solution

There are two kinds of LED screen drive methods: constant current drive and constant voltage drive. Constant current drive is better and more expensive (more circuit boards and chips are added to keep the current constant and increase the service life and stability of the LED display)

Different LED screen driving methods can obtain images with different effects. Therefore, the advantages and disadvantages of the driving chip play a very important role in the display quality of the LED display screen.

LED driver chips can be divided into general chips and special chips. General-purpose chips are generally used for low-end products of LED screens, such as indoor single and dual-color screens. The most commonly used general-purpose chip is 74HC595, with 8-bit latch, serial-to-parallel shift register and tri-state output function.

Each channel can output a maximum of 35 mA (not a constant current). General IC manufacturers can produce such chips.

Because the LED is a current characteristic device, that is, under the premise of saturation conduction, its brightness changes with the change of the current size, not with the change of the voltage across the two ends. Therefore, one of the biggest features of dedicated chips is to provide a constant current source. The constant current source can ensure the stable driving of the LED screen and eliminate the flicker of the LED screen.

The following will focus on the dedicated driver chip for LED screen.

The main parameters and development status of the dedicated driver chip

The dedicated driver chip has the basic characteristics of high output current and constant current, and is more suitable for occasions requiring high current and high image quality, such as outdoor full-color LED display and indoor LED display. The key performance parameters of the dedicated driver chip are the maximum output current, the number of constant current source output channels, current output error (bit to bit, chip to chip), and data shift clock.

1) Maximum output current

At present, the maximum output current of mainstream constant current source chips is mostly defined as the maximum output current of a single channel, generally about 90 mA. Constant current is the basic characteristic of a dedicated chip and the basis for high image quality. The maximum value of the constant current output by each channel at the same time (that is, the maximum constant output current) is more meaningful for the LED screen, because in the white balance state, each channel is required to output constant current at the same time. Generally, the maximum constant current output current is less than the maximum allowable output current.

2) Constant current output channel

The number of constant current source output channels has 8-bit (8-channel constant source) and 16-bit (16-channel constant source) specifications. Now the 16-bit source is the mainstream. Its main advantage is to reduce the chip size and facilitate the LED driver board (PCB ) Wiring, especially for LED driver boards with a small dot pitch.

3) Current output error

There are two types of current output errors, one is the inter-bit current error, that is, the error between each output of the same chip; the other is the inter-chip current error, that is, the output current error between different chips. The current output error is a critical parameter, which has a great influence on the uniformity of the LED screen. The greater the error, the worse the uniformity of the display screen, making it difficult to achieve white balance for the screen body. At present, the bit-to-bit current error of mainstream constant current source chips is generally within + 60%, and the chip-to-chip current error is within ± 15%.

4) Data shift clock

The data shift clock determines the transmission speed of the display data and is a key indicator that affects the update rate of the display screen. As a large-sized display device, the display refresh rate should be above 85 Hz in order to ensure a stable picture (no scan flicker). A higher data shift clock is the basis for the display to obtain a high refresh rate picture.

At present, the shift clock frequency of the mainstream constant current source driver chip is generally above 15 MHz.

In the production of upstream epitaxial wafers and chips for LEDs, the United States, Japan, and the European Union still have great technological advantages, while Taiwan, China, has become an important LED production base in the world. Although China's LED epitaxial wafer and chip production technology is still far from the international advanced level, domestic chip and epitaxial wafer production is still concentrated in low-end products, but the huge domestic demand for LED downstream manufacturers Great development opportunities. Although the solutions of various chips are used to drive LED display screens, due to the different functions of various chips, the characteristics of the driving solutions are also different. The following introduces the 16-bit constant current LED screen driver chip currently dominant in China, and analyzes and compares them from an application perspective.

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