LED as a backlight for LCD is not a new thing, and its driving technology is very mature. However, from small-sized portable applications to large-size LCD TVs, new applications have placed more stringent requirements on LED driver circuits. At the same time, because LCD TV backlight LEDs are also faced with different modes of choice, the driver circuit design must also compare the advantages and disadvantages of various solutions.
Large size backlights put forward new requirements
LCD TVs require stricter backlight LED driver circuits
RGB backlight driver development should focus on the control system
With the decline of LED prices and the concept of green energy, the application field of LED as backlight has been rapidly expanded from portable products such as mobile phones to medium and large-sized display fields such as notebook computers, desktop monitors and LCD TVs. CCFL (Cold Cathode Lamp) backlights used in traditional medium and large size LCD monitors are facing an aggressive LED offensive. “The difference between LED backlighting and traditional CCFL lighting sources is that the former has better image quality, slimmer design, low power consumption and environmental protection. These are the reasons why LED lighting solutions have quickly adopted and replaced CCFL lighting solutions. Where is it." Li Zaipeng, director of Fairchild Semiconductor's PCIA lighting department, said. AnalogicTech CEO Richard Williams also said: "The use of LED backlights will save about 30% more energy than CCFL backlights. The use of CCFLs not only consumes more energy, but also uses mercury to pollute the environment. Currently, more and more LCD TVs are used. LED as a backlight."
However, large-size LCDs have stricter requirements for backlight LED driver circuits. Hangzhou Maoli Semiconductor Technology Co., Ltd. and the general manager of MPS Hangzhou R&D Center said in an interview with the reporter of China Electronics News: "As the size of the panel increases, the power of the LED backlight also increases rapidly, and the LED driver circuit New requirements have also been put forward. The driver circuit must be able to convert 12V or 24V voltage into a higher voltage to drive more LEDs in series; the current between each string of LEDs should be able to achieve high-precision current sharing to ensure backlight Uniform; conversion efficiency is higher, thereby reducing the heat generated in a limited space; to achieve strict open and short circuit protection."
"LED backlights have shifted from portable applications to LCD TV applications, and the changes are very large." Wu Jianxing, general manager of the power supply and power drive product line of Hangzhou Silan Microelectronics Co., Ltd. told reporters, "The change in screen size leads to changes in backlight power. Hundreds of milliwatts of mobile phones are upgraded to tens of watts or even hundreds of watts of LCD TVs. In addition, the power supply is changed. The mobile phone is powered by a lithium battery, typically 3.7V to 4.2V. In LCD TV applications, the power supply is used. It is a 12V or 24 power supply system produced by AC-DC. So LCD TV backlight and mobile phone backlight are two different product platforms. LCD TV backlight in reliability, efficiency, current matching, dimming ratio, etc. LED drivers have new requirements."
Edge-based solutions have more cost advantages
Edge and dot matrix backlight LEDs have their own advantages
Two-stage drive solution is more popular
There are also two forms of LED backlighting for LCD TVs, namely edge and dot matrix (direct). Richard Williams told reporters: "The edge-type LED backlight scheme is actually to install a number of LEDs at the edge of the LCD, that is, where the CCFL was previously arranged, to form a number of LED 'light strips'; and the dot matrix type is to form the LEDs in the form of a dot matrix. Arranged below the LCD display.†“With a dot matrix LED backlight scheme, the intelligent control of the LED current sink provides precise brightness for individual parts based on image content and provides viewers with a more pure black color, while Improve contrast for better, more natural viewing. In addition, its dynamic LED backlights meet the increasingly stringent energy standards of large flat-panel TVs," said Richard Williams. "Edged has the advantages of simple structure and low price; while dot matrix has the characteristics of good image quality and high contrast, but the driving circuit is relatively complicated, and it needs a digital interface and an image decoding chip to communicate." Any remote said, "Edge The three-stage driver circuit of the backlight has matured, and the focus of the industry is on how to reduce the cost and improve the efficiency. MPS proposed the design concept of 'drive energy partial processing'. With this design, about 80% of the energy only processed two. Once on the LED, only about 20% of the energy is processed 3 times; the efficiency of the driver is over 97%, which is 5% to 6% higher than the three-level architecture; at the same time, because the low-voltage device can be used, the cost It has also been reduced a lot. If the number of LED strings in the edge architecture is reduced from the current 4 strings to 1 string, then the two-stage drive line will be more attractive in terms of cost and drive efficiency."
Wu Jianxing also compared the advantages and disadvantages of the edge and dot matrix backlight LED solutions to reporters. "Edge-type backlight has the advantages of low cost and low power consumption; its disadvantage is that the uniformity of the backlight is not enough, and the backlight color gamut is close to the CCFL backlight. The advantage of the dot matrix type is that the backlight is uniform, the color gamut is wide, and regional dimming can be performed. The disadvantage is that the cost is higher, the power consumption is larger, and the model cannot be made ultra-thin.†Wu Jianxing said, “The edge-type backlight scheme is mainly composed of the drive circuit independently, and the specifications are relatively simple. The focus of the design is on the topology (L). Voltage/buck), single or multiple output (determine LED string mode), dimming mode, output power, etc. Direct backlight should be a system consisting of control part and drive part, such as control part to consider area The problem of dimming edge processing and intelligent backlighting, the whole system will be more complicated; only for the driving circuit, in addition to increasing the control part interface, the requirements for constant current matching and dimming ratio will be higher.
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