LED display structure and drive analysis

LED screens are divided into indoor LED large screens and outdoor LED screens according to the use environment; LED screens are divided into single-color LED screens, dual-color LED screens and full-color LED screens according to display colors; LED screens can be divided into 16 according to gray levels. , 32, 64, 128, 256 grayscale LED screens, etc.; LED screens are divided into text LED screens, graphic LED screens, synchronous video LED screens, TV video LED screens, etc. according to display performance.

System composition architecture of led display

Display unit: This is the main part of the LED display screen, which is composed of luminescent materials and driving circuits. The indoor screen is the unit display board of various specifications, and the outdoor screen is the unit box.

Main controller: Its function is to buffer the input RGB digital video signal, transform and reorganize the gray scale, and generate various control signals.

Switching power supply: the purpose is to convert 220V AC power into various DC power supplies for various circuits.

Transmission cable: The display data and various control signals generated by the main controller are transmitted to the screen body through the twisted pair cable.

Scanning controller: The function of this circuit board is data buffering, generating various scanning signals and duty ratio grayscale control signals.

Special display card and multimedia card (video card): In addition to the basic functions of a computer display card, the dedicated display card for LED full-color screens also outputs digital RGB signals and signals such as line, field, and blanking to the main controller. In addition to the above functions, the multimedia card can also convert the input analog Video signal into a digital RGB signal (that is, video led screen capture).

Other signal sources and their external devices: including computers, TV sets, Blu-ray, DVD, VCD, camcorders, etc.

Introduction of LED driver products for led display

As a high-tech product, led display has attracted people’s attention. The intelligent full-color display that uses computer control and integrates light and electricity has been applied in a wide range of fields. Its pixels use LED light-emitting diodes, and many light-emitting diodes are arranged in a dot matrix to form an LED array, and then form 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 dedicated 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 74HC595, which has 8-bit latch, serial-parallel shift register and three-state output function. Each channel can output a maximum non-constant current of 35mA.

Since the LED is a device with current characteristics, that is, under the premise of saturated conduction, its brightness changes with the change of the current, not with the change of the voltage at both ends. The biggest feature of the dedicated chip is to provide a constant current source output to ensure the stable driving of the LED and eliminate the flickering phenomenon of the LED. It has the characteristics of large output current and constant current, and is suitable for occasions that require high current and high image quality, such as outdoor full-color screens and indoor full-color screens.

Key indicators for LEDs include:

Maximum output current: At present, the maximum output current of mainstream constant current source LED driver chips is about 90mA per channel. The maximum value of constant current output by each channel at the same time is more meaningful to the display screen, because in the white balance state, each channel is required to output constant current current at the same time.

Number of constant current output channels: constant current source output channels have two specifications: 8-bit and 16-bit, and now 16-bit is the mainstream. Its main advantage is that it reduces the chip size and facilitates LED driver board (PCB) wiring, especially for point spacing 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 key parameter, which has a great influence on the display uniformity of the led display. The larger the error, the worse the display uniformity, and it is difficult to achieve white balance on the screen. 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 high refresh rate images. At present, the shift clock frequency of mainstream constant current source driver chips is generally above 15MHz~25MHz.