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LED display and its LED driver chip technology analysis
LED display is a new type of display product developed rapidly in the world in the late 1980s. It features high reliability, high brightness, long service life, strong environmental adaptability, high cost performance, low power consumption, impact resistance and stable performance. , quickly grew into a mainstream product for 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 one hand, demand has been expanding, e-government, government affairs, public information display and other needs. On the other hand, advances in technology have provided innovative technical support for the expansion of the LED display product market and the development of innovative applications. On the other hand, the opportunity of the Olympic Games and the World Expo has accelerated the development of the industry. According to the market analysis report, the average annual compound growth rate of China's LED display market will reach 15.1% in 2007-2010.
The biggest feature of LED display is that it is not limited by area, and can be up to tens or even hundreds of square meters. It can be used to display various information such as text, graphics, images, animations and video images in various public places indoors/outdoors. Has a strong ad rendering power and shock. Its high brightness, full color, convenient and fast error detection and free adjustment of LED brightness are the development trend of the market.
 
LED display classification
The LED screen is divided into an indoor LED screen and an outdoor LED screen according to the use environment; the LED screen is divided into a single primary color LED screen, a dual primary color LED screen and a full color LED screen according to the display color; the LED screen can be divided into 16, 32 according to the gray level. , 64, 128, 256 grayscale LED screens; LED screens are divided into text LED screens, graphic LED screens, synchronous video LED screens, TV video LED screens, etc. according to display performance.
 
LED display system composition
See Figure 1 for the system composition of the LED display.
 
Display unit: This is the main part of the LED display screen, which is composed of a luminescent material and a driving circuit. The indoor screen is a unit display panel of various specifications, and the outdoor screen is a unit cabinet.
 
Main controller: The function is to buffer the input RGB digital video signal, grayscale transform, reorganize, and generate various control signals.
 
Switching power supply: The purpose is to convert 220V AC into various DC power to provide various circuits.
 
Transmission cable: The display data generated by the main controller and various control signals are transmitted to the screen body by the twisted pair cable.
 
Scan controller: The function of this board is data buffering, which generates various scan signals and duty cycle gray control signals.
 
Dedicated display card and multimedia card (video card): LED full color screen dedicated display card not only has the basic functions of computer display card, but also outputs digital RGB signal and line, field, blanking and other signals 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 sources and their external devices: including computers, televisions, Blu-ray, DVD, VCD, camcorders, etc.
 
LED display product introduction 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 with computer control 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, thereby forming an LED screen. Different effect images can be obtained by different LED driving methods. Therefore, the advantages and disadvantages of the LED 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 for low-end products of LED displays, such as single and dual-color 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 35mA non-constant current.
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. The biggest feature of the dedicated chip is to provide constant current source output, to ensure stable driving of the LED, and to eliminate the flicker phenomenon of the LED. It has the 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.
 
Key indicators for LEDs include:
Maximum output current: The current mainstream constant current source LED driver chip maximum output current is about 90mA per channel. The maximum value of the constant current output per channel simultaneously makes more sense for the display, because in the white balance state, it is required to simultaneously output a constant current per channel.
 
Number of constant current output channels: Constant current source output channels are available in 8-bit and 16-bit formats. Now 16-bit is dominant. Its main advantage is to reduce chip size and facilitate LED driver board (PCB) wiring, especially for dot pitch. A smaller LED driver board is more advantageous.
 
Accurate current output: one is the current error value between the same chip channel; the other is the output current error value between different chips. Accurate current output is a critical parameter that has a large impact on the display uniformity of the LED display. The larger the error, the worse the display uniformity is, 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 ±3%, and the chip to chip current error is within ±6%.
Data Shift Clock: This 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 15MHz~25MHz.
 
LED driver chip for LED display
At the beginning of 2008, the LED driver chip series SMT5026.SMT5026 with LED display 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.
 
The main features of SMT5026:
16-channel constant current source output channel, current output size does not change due to load voltage change at the output;
Constant current output current range: 5mA~90mA;
Extremely accurate constant current output current value matching: maximum error between channels <±1.5%, maximum error between chips <±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 to enhance 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.
 
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