The birth of the LED display has directly changed the whole world. The appearance of the large screen allows us to feel the brilliant colors, the large screen and the high-definition picture quality. LED displays currently occupy 80% of the outdoor market. The performance of the LED display is closely related to the LED driver chip.

1. Type of driver chip:

LED driver chips can be divided into general-purpose chips and special-purpose chips. The so-called general-purpose chip itself is not specially designed for LED, but some logic chips (such as serial-parallel shift register) with some logic functions of LED display screen. The special-purpose chip refers to the driver chip specially designed for the LED display according to the luminous characteristics of the LED. LED is a current characteristic device, that is, under the premise of saturation conduction, its brightness changes with the change of current, rather than by adjusting the voltage across it. Therefore, one of the biggest features of the dedicated chip is to provide a constant current source. The constant current source can ensure the stable driving of the LED and eliminate the flickering of the LED, which is the prerequisite for the LED display to display high-quality images. Some special-purpose chips also add some special functions for the requirements of different industries, such as brightness adjustment, error detection and so on. This article will focus on the dedicated driver chip.

2. General purpose chip

General-purpose chips are generally used for low-end LED display products, such as indoor monochrome screens, dual-color screens, etc. The most commonly used general-purpose chip is the 74HC595. The 74HC595 has an 8-bit latch, a serial-parallel shift register, and a three-state output. Each channel can output a maximum current of 35mA (non-constant current). General IC manufacturers can produce such chips. Products from Motorola (Onsemi), Philips and ST are commonly used in the display industry, among which Motorola's products have better performance.

3. Dedicated chip

The special chip has the characteristics of large output current and constant current, and is more suitable for occasions with large current and high image quality requirements, such as outdoor full-color screens, indoor full-color screens, etc.

The key performance parameters of the dedicated chip are the maximum output current, the number of output channels of the constant current source, the current output error (bit-bit, chip-chip) and the data shift clock.

4. 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 around 90mA. Constant current is the most fundamental feature of special-purpose chips, and it is also the basis for obtaining 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 to the display screen, because in the white balance state, each channel is required to output a constant current at the same time. Generally, the maximum constant current output current is less than the allowable maximum output current.

5. Number of output channels of constant current source

The number of output channels of constant current source mainly has two specifications: 8 (8-bit source) and 16 (16-bit source). Now 16-bit source basically occupies the mainstream: such as TLC5921, TB62706/TB62726, MBl5026/MBl5016, etc. The main advantage of the 16-bit source chip is that it reduces the chip size and facilitates the wiring of the LED driver board (PCB), especially for the PCB with a small dot pitch.

6. 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 very critical parameter, which has a great influence on the uniformity of the display screen. The larger the error, the worse the uniformity of the LED display screen, and it is difficult to achieve the white balance of the screen body. At present, the inter-bit current error of mainstream constant current source chips is generally less than ±6%, and the inter-chip current error is less than -+15%.

7. Data shift clock

The basic functions of the LED display driver chip include the function of the serial shift register, so as to realize the cascade and transmission of display data, and build a large-size LED display with multiple display points. The data shift clock determines the transmission speed of the display data and plays a crucial role in the update rate of the LED display data. As a large-size display device, the LED 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 chips is generally above 15MHz.

The constant current of the LED chip is the basic premise to ensure the color stability of the LED display. The high-precision constant current source is the excellent picture quality of the LED display screen.