Introduction of led display driver and key indicators
As a high-tech product, LED display has attracted people’s attention. The intelligent full-color display that integrates light and electricity using computer control has been used in a wide range of fields. The pixel points use LED light-emitting diodes, and many light-emitting diodes are arranged in a dot matrix to form an LED array, which in turn forms an LED screen. Through different LED driving methods, images with different effects can be obtained. Therefore, the quality of the LED driver chip plays an important role in the display quality of the LED display. LED driver chips can be divided into general-purpose chips and special-purpose chips.
General-purpose chips are generally used in low-end LED display products, such as indoor single-color and double-color screens. The most commonly used general-purpose chip is the 74HC595, which has an 8-bit latch, a serial-parallel shift register, and a three-state output function. Each channel can output a non-constant current of up to 35mA.
Because LED is a current characteristic device, that is, under the premise of saturated conduction, its brightness changes with the change of the current size, not with the change of the voltage across it. The biggest feature of the special chip is to provide a constant current source output, to ensure the stable driving of the LED, and to eliminate the flickering of the LED. It has the characteristics of large output current and constant current, and is suitable for occasions requiring large current and high image quality, such as outdoor full-color screens, indoor full-color screens, etc.
Key indicators of LEDs include:
Maximum output current: At present, the maximum output current of the current mainstream constant current source LED driver chips is about 90mA per channel. The maximum value of the constant current output by each channel at the same time is more meaningful for the display screen, because in the white balance state, each channel is required to output a constant current current at the same time.
Number of constant current output channels: There are two specifications of constant current source output channels: 8-bit and 16-bit. Now 16-bit is the mainstream. Its main advantage is to reduce the chip size and facilitate the wiring of the LED driver board (PCB), especially for the dot pitch Smaller LED driver boards are more beneficial.
Accurate current output: one is the current error value between channels of the same chip; the other is the output current error value between different chips. Accurate current output is a very critical parameter, which has a great influence on the display uniformity of the LED display. The larger the error, the poorer the display uniformity, and it is difficult to make the screen white balance. At present, the bit-to-bit current error of mainstream constant current source chips is generally within ±3%, and the chip-to-chip current error is within ±6%.
Data shift clock: It determines the transmission speed of display data and is a key indicator that affects the update rate of the display. As a large-size display device, the display refresh rate should be above 85Hz to ensure a stable picture (no scanning 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 mainstream constant current source driver chips is generally above 15MHz~25MHz.