LED display and performance index analysis

LED display screen is a new type of display product that has developed rapidly in the world since the late 1980s. It has rapidly grown into a mainstream product of flat panel display with its high reliability, high brightness , long service life, strong environmental adaptability, high cost performance, low power consumption, impact resistance and stable performance. China's LED display screen industry started in the early 1990s and has developed rapidly. Since the beginning of the 21st century, the LED display screen industry has faced good market development opportunities. On the one hand, demand has continued to expand, and there is a strong demand for e-government, government transparency, and public information display. On the other hand, technological progress has provided innovative technical support for the expansion of the LED display screen product market and the creation of new application fields. On the other hand, the opportunities of the Olympic Games and the World Expo have accelerated the development of the industry. According to the market analysis report, from 2007 to 2010, the average annual compound growth rate of China's LED display screen market will reach 15.1%.

The biggest feature of LED display screen is that its manufacturing is not limited by area, and it can reach dozens or even hundreds of square meters. It is used in various public places indoors and outdoors to display text, graphics, images, animations, video images and other information, and has strong advertising rendering 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 Displays

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

LED display system architecture

Display unit: This is the main part of the LED display screen, which is composed of luminous materials and driving circuits. Indoor screens are unit display panels of various specifications, and outdoor screens are unit boxes.

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

Switching power supply : Its purpose is to convert 220V AC into various DC power and provide it to various circuits.

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

Scan controller: The function of this circuit board is data buffering, generating various scan 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 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 (i.e. video acquisition).

Other signal sources and their external devices: including computers, TVs, Blu-ray , DVDs, VCDs, camcorders, etc.

LED driver for LED display Product introduction

As a high-tech product, LED display screen has attracted great attention. The intelligent full-color display screen that integrates light and electricity and uses computer control has been applied in a wide range of fields. Its 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. Different LED driving methods can produce images with different effects. Therefore, the quality of LED driver chips plays an important role in the display quality of LED display screens. 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-color and dual-color screens. The most commonly used general purpose chip is 74HC595, which has 8-bit latch, serial-parallel shift register and three-state output functions. Each channel can output a maximum non-constant current of 35mA.

Since LED is a current characteristic device, that is, under the premise of saturated conduction, its brightness changes with the change of current size, not with the change of voltage at both ends. The biggest feature of the dedicated chip is to provide constant current source output, ensure the stable drive of LED, and eliminate the flickering phenomenon of 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 screens, indoor full-color screens, etc.

Key indicators of LED include:

Maximum output current: The maximum output current of the current source LED driver chip is about 90mA per channel. The maximum value of the constant current output by each channel 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: There are two specifications of constant current source output channels: 8-bit and 16-bit. Now 16-bit is the mainstream. Its main advantage is that it reduces the chip size and facilitates the wiring of the LED driver board ( PCB ), especially for LED driver boards with smaller dot pitch.

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 critical parameter that has a great impact 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 a high refresh rate picture. At present, the shift clock frequency of the mainstream constant current source driver chip is generally above 15MHz~25MHz.