1. Understanding LED driver chips
LED display screens are mainly large-size flat-panel displays composed of display units composed of light-emitting diodes (LEDs) and their driver chips.
The performance of driver chips plays a vital role in the display quality of LED display screens. In recent years, with the vigorous development of the LED market, many powerful IC manufacturers, including Toshiba (TOSHIBA) and Sony (SONY) in Japan, Texas Instruments (T1) in the United States, MBl and SITl in Taiwan, etc., have begun to produce LED-specific driver chips.
2. Types of driver chips
LED driver chips can be divided into general-purpose chips and special-purpose chips. The so-called general-purpose chips are not designed specifically for LEDs, but are some logic chips with some logical functions of LED display screens (such as serial-parallel shift registers).
Special-purpose chips refer to driver chips designed specifically for LED display screens according to the light-emitting characteristics of LEDs.
LED is a current characteristic device, that is, under the premise of saturated conduction, its brightness changes with the change of current, rather than by adjusting the voltage at both 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 and eliminate the flickering of the LED, which is the premise for the LED display to display high-quality images.
Some dedicated chips also add some special functions to meet the requirements of different industries, such as brightness adjustment, error detection, etc.
This article will focus on the dedicated driver chip.
2.1 General purpose chips
General purpose chips are generally used for low-end products of LED display screens, such as indoor monochrome screens, dual-color screens, etc.
The most commonly used general purpose chip is 74HC595. 74HC595 has 8-bit latch, serial-parallel shift register and three-state output. Each channel can output a maximum current of 35mA (non-constant current).
General IC manufacturers can produce such chips. Products from manufacturers such as Motorola (Onsemi), Philips and ST are commonly used in the display industry, among which Motorola's products have better performance.
2. 2 Dedicated chips
Dedicated chips have the characteristics of large output current and constant current, and are more suitable for occasions with large current and high picture quality requirements, such as outdoor full-color screens, indoor full-color screens, etc.
The key performance parameters of dedicated chips include maximum output current, number of constant current source output channels, current output error (bit-bit, chip-chip) and data shift clock, etc.
The maximum output current of the current mainstream constant current source chip is mostly defined as the maximum output current of a single channel, generally around 90mA.
Constant current is the most fundamental characteristic of dedicated chips and the basis for obtaining high picture quality. The maximum value of the constant current output by each channel at the same time (i.e. the maximum constant output current) is more meaningful to the display 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.
- Constant current source output number
The constant current source output number 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 16-bit source chip is that it reduces the chip size and facilitates the wiring of LED driver board (PCB), especially for PCB with small dot pitch.
There are two types of current output error: 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 error of output current between different chips.
Current output error is a very critical parameter, which has a great impact on the uniformity of the display.
The larger the error, the worse the uniformity of the display, and it is difficult to achieve white balance of the screen.
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%. o
The basic functions of LED dedicated driver chips all include the function of serial shift register, so as to realize the cascading 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 display data and plays a vital role in the update rate of display data on 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 chips is generally above 15MHz.