I. Overview
The large screen has always been an important part of the development of the LED field. It is an important part of the large-scale entertainment, sports events and square theme display. Since the birth of the blue LED, the full-color screen has maintained a high-speed development trend. In the decorative display market LED will play a positive role, and the market expansion is obvious. China is a global LED display production country, completely dominated from LED chips, driver ICs, controllers, screen manufacturing and other links.
The origin of 16-bit shift constant current IC: The two-color screen is mainly based on display text, and the single-chip scanning is convenient. Due to the increase of the number of LEDs, in order to save O/I resources, 74HC595 shift scanning is adopted. In order to be more suitable for LED applications, the constant current setting function is integrated on this basis, the current driving capability is increased, and 16-bit devices are packaged in accordance with the requirements and cost requirements, and are widely used in current full-color screens.
The current drive capability is continuously reduced. The TLC5940 has a single drive current capability of 120mA. Later, the TB62726, ST2221, and MBI5026 current drive capability were reduced to 80-90mA. Currently, the current drive capability of 45mA is basically used, such as MBI5024 and CYT62726. The main reason for the decrease in capability is that LED devices have higher and higher luminous intensity. In order to improve image quality, static screen design is increasingly used, and the demand for driving current capability is reduced. From the perspective of IC cost, the chip size can be reduced to reduce the cost. For this purpose, a 25mA static screen driver chip CYT62727 is designed.
At present, 80% of the world's LED screens are purchased and produced in mainland China. The 16-channel constant current device design is widely used. It will continue in the short term and will not disappear in the next five years. The main control technology is mature and the products have been serialized. Unless the system control technology and chip driver design have a huge leap and the cost is further reduced, the status quo will not change. In recent years, many companies have continuously introduced new architectures, which will be recognized by the market in the future. The biggest problem is that through the control technology, if the color consistency of the LED is changed, the new technology has no big breakthrough before, and it is not attractive to the application, and the purchase intention is not strong.
In recent years, mainland chip design companies will definitely occupy the LED display market. For example, Silan Mingxin is the mainstream LED chip supplier. For reasons of excellent quality assurance, a good direct sales model is the market magic weapon. . Future driver ICs also require a direct sales model. Display companies are mostly local companies in mainland China, and Taiwanese ICs need further improvement in terms of distribution period, delivery speed and trust.
In the driving application technology, the color correction technology needs to solve the bottleneck, the display attenuation consistency problem is prominent, wavelength correction and brightness correction are the next key breakthrough targets, firstly to solve the brightness consistency problem, and then the wavelength consistency Correction. This is a worldwide problem and a technical problem that needs to be solved urgently.
The development of control technology has reached the forefront of the world, but it has stagnated in the new mode control concept. The reason is that controller manufacturers and IC design manufacturers are not well coordinated, and each is relatively independent. The driving technology and control technology cannot be well connected. The development of 16-channel compatibility design is high, and new control technologies are rarely introduced, let alone technological innovation. The 16-channel constant current device has been used on LED screens for more than a decade. Until now, it has only reduced current, and it has to deal with fiercely competitive prices. There is no technical breakthrough. A number of companies have launched a series of ICs, which have not been well applied and are caused by the lack of control technology. In today's rapid development of computer technology, it can replace the simplified controller scale, but the technology migration also needs the support of IC design manufacturers, the market lack of control technology and chip driver integration solution provider.
In 2009, the LED display industry has a domestic market of more than 30 billion, and there are more than 30 companies with an annual output value of over 100 million. There are more than 10 million companies, and thousands of companies are engaged in the display manufacturing industry. LED displays continue to grow at a rate of 15%, and technology is maturing. China's large-scale event projects continue to drive LED growth. For example: Olympics, World Expo, Asian Games, Shanghai Disney, subway, high-speed rail and other engineering events.
Second, the screen principle design
The text display, as long as the content is clear and has sufficient brightness, will basically meet the needs of customers. But for the evaluation of the display quality of the image display, the problem is much more complicated. It is generally a subjective way to evaluate the display quality of the display image. The so-called subjective evaluation is a man-made method of judging, and judging by the quality of the image display. such. The evaluation results are not only related to the quality of the image itself, but also related to the subjective factors of the observer. It is difficult to say that it is a standard of fairness and certainty. Despite this, there is still no good way to do so. Subjective methods are still the most effective and practical method before the emergence of objective measurement methods.
(1) Maximum number of displayed colors
The color of each pixel of the display is composed of three primary colors of RGB (red, green, blue). Low-end liquid crystal display panels, each base color can only represent 6-bit color, that is, 26 = 64 colors. Through simple calculations, we can know that the maximum number of colors that each individual pixel can represent is 64 × 64 × 64 = 2.62K colors; the high-end liquid crystal display panel uses FRC technology to make each primary color can represent 8-bit color, that is, 28 = 256 colors, the maximum number of colors that a pixel can represent is 256 × 256 × 256 = 16 KK colors. The display panel displays a richer color and a layered look. At present, the liquid crystal display on the market has adopted both display panels, so you can pay attention to it.
The illuminance unit of the surface is lux; multiply this value by the effective display area of ​​the screen to obtain the illuminance intensity of the entire screen at the optimal viewing angle, assuming that the illuminating intensity of each pixel in the screen is constant in the corresponding space, then This value can be considered as the luminous flux of the entire screen. Generally, the outdoor LED display must have a brightness of 4000 cd/m2 or more to have an ideal display effect under daylight. Ordinary indoor LED, the maximum brightness is about 700~2000cd/m2.
The luminous intensity of a single LED is in cd, and is equipped with a viewing angle parameter. The luminous intensity has no relationship with the color of the LED. The luminous intensity of a single tube varies from a few mcd to five thousand mcd. The luminous intensity given by the LED manufacturer refers to the point at which the LED illuminates at a current of 20 mA, with the highest luminous intensity at the best viewing angle and at the center position. The shape of the top lens and the position of the LED chip from the top lens when packaging the LED determine the LED viewing angle and intensity distribution. Generally speaking, the larger the LED viewing angle is, the smaller the maximum luminous intensity is, but the luminous flux accumulated on the entire three-dimensional hemisphere is unchanged.
When a plurality of LEDs are relatively tightly discharged, the illuminating spheres are superimposed on each other, resulting in a relatively uniform distribution of luminous intensity of the entire illuminating plane. When calculating the luminous intensity of the display, it is necessary to multiply the maximum luminous intensity value provided by the manufacturer by 30% to 90% according to the LED viewing angle and the emission density of the LED as the average luminous intensity of the single tube.
Generally, the luminous life of LEDs is very long, and manufacturers generally indicate that they are more than 100,000 hours. Under the optimal conditions set, the brightness decay period of LEDs should be paid attention to, the brightness decay period and the material process of LED production. Manufacturers have a lot to do with, and generally, brands with slower brightness decay should be used when economic conditions permit.
In actual use, light intensity calculations often use data units that are easier to map or use. For the LED display screen, the active illuminant generally adopts cd/m2 as the unit of luminous intensity, and the observation angle is an auxiliary parameter, which is equivalent to the screen body.
Most of the screens use straight-line elliptical LEDs, and most of the domestic packaging technologies can meet the needs of screen design. LED chips are mostly packaged with Cree or domestic LED light-emitting chips such as Silan Mingxin and Xiamen Sanan.