LED” target=”_blank”>LEDThe Display screen is a new type of display product developed rapidly in the world in the late 1980s. It is characterized by high reliability, high brightness, long service life, strong environmental adaptability, high cost performance, low power consumption, impact resistance, and stable performance. It has rapidly grown into a mainstream product of flat panel display. China’s LED display industry started in the early 1990s and developed rapidly; since the beginning of the 21st century, the LED display industry has faced good market development opportunities; On the other hand, the advancement of technology has provided innovative technical support for the market expansion of LED display products and the creation of new application fields. On the other hand, the opportunities of the Olympic Games and World Expo have accelerated the development of the industry. According to the market analysis report from 2007 to 2010, the compound annual growth rate of China’s LED display market will reach 15.1%.
The biggest feature of the LED display is that its manufacturing is not limited by the area, and can reach more than tens or even hundreds of square meters. It is used in various indoor/outdoor public places to display text, graphics, images, animations, video images and other information. It has strong advertising rendering power and shocking power. Its high brightness, full color, convenient and fast error detection and free adjustment of LED brightness are the development trend of the market.
Classification of LED display
LED screens are divided into indoor LED screens and outdoor LED screens according to the use environment; LED screens are divided into single-primary LED screens, dual-primary LED screens and full-color LED screens according to display color; , 64, 128, 256-level gray 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 of LED display
The system composition structure of the LED display screen is shown in Figure 1.
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: the function is to buffer the input RGB digital video signal, transform grayscale, reorganize, and generate various control signals.
Switching power supply: The purpose is to convert 220V alternating current into various direct currents and provide them to 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 cables.
Scanning controller: The function of this circuit board is data buffering, generating various scanning signals and duty cycle grayscale control signals.
Special display card and multimedia card (video card): In addition to the basic functions of a computer display card, the special display card for LED full-color screen 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 (ie, video capture).
Other signal sources and their external devices: including computers, TVs, Blu-rays, DVDs, VCDs, 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 integrates light and electricity using computer control has been used in a wide range of fields. The pixel points use LED light-emitting diodes, and many light-emitting diodes are arranged in a dot matrix to form an LED array, which in turn forms 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 special-purpose 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 the 74HC595, which has an 8-bit latch, a serial-parallel shift register, and a three-state output. Each channel can output a non-constant current of up to 35mA.
Because the LED is a current characteristic device, that is, under the premise of saturated conduction, its brightness changes with the change of the current size, and does not change with the change of the voltage across it. The biggest feature of the special chip is to provide a constant current source output, to ensure the stable driving of the LED, and to eliminate the flickering of the LED. It has the characteristics of large output current and constant current, and is suitable for occasions requiring large current and high image quality, such as outdoor full-color screens, indoor full-color screens, etc.
Key indicators of LEDs include:
Maximum output current: At present, the maximum output current of the current mainstream constant current source LED driver chips is about 90mA per channel. The maximum value of the constant current output by each channel at the same time is more meaningful for the display screen, because in the white balance state, each channel is required to output a constant current 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 the LED driver board (PCB), especially for the dot pitch 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 very critical 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 make the screen 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 indicator that affects the update rate of the display screen. 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 driver chips is generally above 15MHz to 25MHz.
LED driver chip for LED display
In early 2008, SMT5026, a series of LED driver chips for LED displays, was launched. 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:
16 constant current source output channels, the current output size does not change due to the change of the output load voltage;
Constant current output current range value: 5mA ~ 90mA;
Extremely accurate constant current output current value matching: the maximum error between channels is <±1.5%, and the maximum error between chips is <±3.0%;
By adjusting the external resistance, the constant current output current value can be set (Iout=5～90mA);
Up to 25MHz data transmission clock frequency, with Schmitt trigger input device, strengthen the anti-noise function;
Fast output current response, minimum response time ≤200ns;
Working voltage: 3.0V～5.5V;
RoHS compliant, 100% lead-free package;
Package form: SSOP24-300-1.0, SSOP24-150-0.635, compatible with mainstream products on the market.