Classification of LED display
LED screen is divided into indoor LED screen and outdoor LED screen according to the use environment; The LED screen is divided into single primary color LED screen, double primary color LED screen and full-color LED screen according to the display color; LED screen can be divided into 16, 32, 64, 128 and 256 gray LED screens according to gray level; LED screen is divided into text LED screen, graphic LED screen, synchronous video LED screen and TV video LED screen according to display performance.
System composition architecture of LED display
See Figure 1 for the system composition architecture of LED display screen.
Display unit: This is the main part of the LED display screen, which is composed of luminous 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: it is used to buffer, transform and reorganize the input RGB digital video signal, and generate various control signals.
Switching power supply: the purpose is to change 220V AC into various DC to supply various circuits.
Transmission cable: the display data and various control signals generated by the main controller are transmitted to the screen body by twisted pair cable.
Scanning controller: the function of the circuit board is data buffer to generate various scanning signals and duty cycle gray control signals.
Special display card and multimedia card (video card): in addition to the basic functions of computer display card, the special display card for LED full-color screen also outputs digital RGB signals, line, field, blanking and other signals to the main controller. In addition to the above functions, the multimedia card can also change the input analog video signal into digital RGB signal (i.e. video acquisition).
Other signal sources and their external devices: including computers, televisions, Blu ray, DVDs, VCDs, video cameras, etc.
Product introduction of LED driver for LED display screen
As a high-tech product, LED display has attracted great attention. The intelligent full-color display, which is controlled by computer and integrates light and electricity, has been applied in a wide range of fields. The pixel points adopt LED light-emitting diodes, and many light-emitting diodes are arranged in a dot matrix to form an LED array and then an LED screen. Through different LED driving modes, images with different effects can be obtained. Therefore, the advantages and disadvantages of LED driver chip play an important role in the display quality of LED display screen. LED driver chips can be divided into general chips and special chips.
General purpose chips are generally used for low-end products of LED display screens, such as indoor single and dual color screens. The most commonly used general chip is 74HC595, which has the functions of 8-bit latch, serial shift register and three state output. Each channel can output 35mA non constant current at most.
Because LED is a current characteristic device, that is, on the premise of saturated conduction, its brightness changes with the change of current, not with the change of voltage at both ends. The biggest feature of the special chip is to provide constant current source output, ensure the stable driving of LED and eliminate the flicker of LED. With the characteristics of high output current and constant current, it is suitable for occasions requiring high current and high image quality, such as outdoor full-color screen, indoor full-color screen, etc.
Key indicators of LED 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 each channel outputting constant current 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 at the same time.
Number of constant current output channels: the constant current source output channel has two specifications: 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 LED driver board (PCB), especially for LED driver boards with small point spacing.
Accurate current output: one is the current error value between channels of the same chip; The other is the output current error between different chips. Accurate current output is a key parameter, which has a great impact on the display uniformity of LED display. The larger the error, the worse the display uniformity, and it is difficult to achieve 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 index affecting the update rate of display screen. As a large-size display device, the display refresh rate should be above 85Hz to ensure a stable picture (no scanning flicker). High data shift clock is the basis for the display screen to obtain high refresh rate pictures. At present, the shift clock frequency of mainstream constant current source driver chips is generally above 15MHz 25MHz.
LED driver chip for LED display screen
Smt5026, a series of LED driver chips for LED display screen, was launched in early 2008. Smt5026 has built-in CMOS shift register and latch function, which can convert serial input data into parallel output data format. The functional block diagram is shown in Figure 2.
Main features of smt5026:
The output channel and current output size of 16 channel constant current source do not change due to the change of load voltage at the output end;
Constant current output current range: 5mA 90mA;
Extremely accurate matching degree of constant current output current value: the maximum error between channels is By adjusting the external resistance, the constant current output current value (IOUT = 5 90mA) can be set;
Up to 25MHz data transmission clock frequency, with Schmitt trigger input device to strengthen the anti noise function;
Working voltage: 3.0V 5.5V;
RoHS compliant, 100% lead-free package;
Packaging form: ssop24-300-1.0, ssop24-150-0.635, compatible with mainstream products in the market.
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