|LED display screen's several drive solutions|
As a high-tech product, LED display has attracted people's attention. The large-screen intelligent LED display with computer control and integration of light and electricity has been applied to many fields. The LED display screen adopts LED light-emitting diodes, and many light-emitting diodes are arranged in a dot matrix to form an LED array, thereby forming an LED display screen. Different effect images can be obtained by different LED driving methods. Therefore, the advantages and disadvantages of the driver chip play an important role in the display quality of the LED display.
LED driver chips can be divided into general-purpose chips and dedicated chips. General-purpose chips are generally used in low-end products of LED displays, such as single- and dual-color LED display screens in indoors. The most commonly used general-purpose chip is the 74HC595, which has an 8-bit latch, a serial-to-parallel shift register, and a three-state output function. Each channel can output a maximum of 35 mA (not constant current). General IC manufacturers can produce such chips. Since the LED is a current-characteristic device, that is, under the premise of saturation conduction, its brightness changes with the magnitude of the current, and does not change with the voltage across it. Therefore, one of the biggest features of a 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 phenomenon of the LED. The following will focus on the dedicated driver chip for LED displays.
Main parameters and development status of LED display chip
The LED display MBI chip has the basic characteristics of large output current and constant current, and is suitable for occasions requiring high current and high image quality, such as outdoor full color screen and indoor full color screen. The key performance parameters of the LED display chip are the maximum output current, the number of constant current source output channels, the current to output error (bit to bit, chip to chip) and the data shift clock.
Maximum output current
At present, the maximum output current of the mainstream constant current source chip is defined as a single maximum output current, generally about 90 mA. Constant current is the basic characteristic of a dedicated chip and the basis for high image quality. The maximum output current of each channel at the same time (ie, the maximum constant output current) is more meaningful to the display, because in the white balance state, each channel is required to simultaneously output a constant current. Generally, the maximum constant current output current is less than the maximum allowable output current.
Constant current output channel
There are two types of constant current source output channels: 8-bit (8-channel constant source) and 16-bit (16-channel constant source). Now 16-bit source is dominant. Its main advantage is that it reduces chip size and facilitates LED driver board (PCB). Wiring, especially for LED driver boards with small dot pitches.
Current output error
The current output error is divided into two types, one is the current error between the bits, that is, the error between each output of the same chip; the other is the current error between the chips, that is, the error of the output current between different chips. Current output error is a critical parameter that has a large impact on the uniformity of the display. The larger the error, the worse the uniformity of the display screen, and it is difficult to achieve white balance of the screen. At present, the bit-to-bit current error of the mainstream constant current source chip is generally within +60%, and the chip to chip inter-chip current error is within ±15%.
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. As a large-size display device, the display refresh rate should be above 85 Hz 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 mainstream constant current source driver chip shift clock frequency is generally above 15 MHz.