LED driver integrated circuits used to drive white LEDs account for about 50% of the total LED driver market. White LEDs are often used to provide backlight for small displays of many battery powered handheld portable products. However, such lighting applications are mature and will not have a high CAGR in the next five years. In terms of LED lighting applications, the annual compound growth rate is much higher than that of handheld products. The three largest markets in the backlight market are: large flat panel display HD TV, automobile headlamp lighting and general lighting.

With consumers’ demand for HD TV with larger flat panel display and higher resolution, the demand for TV has rapidly shifted from plasma HD TV to LED HD TV. According to the data of DisplaySearch, the sales peak of plasma HD TV will appear in 2008, with a sales revenue of US $24 billion; The LCD HD TV market will reach 75 billion US dollars in 2008 and 93 billion US dollars in 2010. Obviously, the LED TV market has begun to “prosper”.

From motion blur to color reproduction, the current LCD HD TV has various disadvantages. At present, the next generation LCD HD TV can not get real black, and the dynamic range of all colors is small. Ordinary HD TV sets use CCFL tubes for backlighting, with a contrast of only 450 to 650cd / m2. The main problem with these HDTVs is that the CCFL backlight cannot be completely turned off or local dimming cannot be performed.

On the contrary, with high brightness LED backlight, the LED array (which can contain up to 1600 LEDs for 46 inch display) can dim or locally turn off the backlight “group”, so as to achieve almost an order of magnitude higher contrast than the CCFL design (> 4000cd / m2). In addition, by adjusting the brightness of the backlight LED group, more middle tones can be reproduced, so as to make the picture more vivid. Another advantage is that the LED can be completely turned off locally, thus reducing the motion blur problem. By completely turning off the LED between frames, the blurring problem caused by fast moving objects is almost completely eliminated. When solving the fast motion blur problem faced by this CCFL backlight LCD TV, the very fast response speed of LED is very important.

 

What are the advantages of high brightness LED backlight compared with other ordinary lighting

Designer’s dilemma

Cold cathode fluorescent lamps (CCFL) are often used to provide backlight for large flat panel displays, but their chromatography is limited and their colors are not bright enough. RGB LEDs actually extend the visible range. In addition, CCFL can display about 80% of the colors defined by the national television system Commission (NTSC), while RGB can display up to 110% of the NTSC chromatogram, so as to more accurately display the original style of the image on the display screen. Using three monochromatic light sources, such as red, green and blue (RGB) lasers, the widest spectrum possible can be obtained.

On the other hand, white LED backlighting is very suitable for handheld and mobile displays because of its small overall size, simple and easy to drive, less sensitive to mechanical stress and twice the expected life compared with CCFL. However, white LEDs have the same disadvantages in chromatography as CCFL, because white LEDs are equivalent to broadband light sources. White LED is formed by blue light diode covering phosphor. It converts part of blue light into yellow light, and the combined spectrum is regarded as white light.

Compared with monochromatic light source, RGB LED provides near narrowband chromatography at a lower cost. RGB LEDs not only improve the chromatography, but also improve the efficiency, because RGB LEDs only emit light energy according to the required red, green and blue light. Relatively speaking, broadband light sources (such as white LED and CCFL) emit more light without color, which reduces the purity of chromatography and therefore loses efficiency. Since RGB LEDs of different colors can be driven separately, the white light point or color temperature of RGB LEDs can be corrected, while the white light points of CCFL and white LEDs are fixed.

Dimming consideration of LED

Traditionally, LED dimming is realized by adjusting the forward current flowing through the LED with DC signal or filtered PWM signal. Reducing the LED current can adjust the LED light output density. However, the change of forward current will lead to the change of LED luminous color, because the chromaticity of LED changes with current. Many applications such as LCD HD TV and automobile backlight lighting do not allow any deviation in the luminous color of LED. Due to the different ambient light changes and the small changes in light intensity that human eyes can perceive, a wide dimming range is required in these applications. Using PWM signal to control the light intensity of LED can realize LED dimming without changing the luminous color.

True color PWMTM dimming adjusts LED brightness through PWM signal. It essentially turns on and off the LED with full current at PWM frequency. The human eye is limited to 60 frames per second. By increasing the PWM frequency (for example, to 80Hz to 100Hz), the human eye feels that the pulse light source is continuously turned on. In addition, the light intensity of the LED can be controlled by modulating the duty cycle (“length of on time”). When this method is adopted, the luminous color of the LED remains unchanged because the current value of the LED is either zero or constant. Many HD TV designers require a dimming ratio of up to 3000:1 to adapt to the wide range of ambient light.

conclusion

Obviously, LED has become the mainstream choice for the backlight of large LCD flat panel display used in HD TV. However, system designers still need LED driver integrated circuits that meet their specific design performance requirements. Therefore, the LED Driver IC must be able to provide sufficient current and voltage for many different types of LED configurations with a conversion topology that meets the input voltage range and the required output voltage and current requirements. Therefore, the LED Driver IC needs to have the following characteristics to meet the needs of designers:

(1) Wide input voltage range

(2) Wide output voltage range

(3) High efficiency conversion

(4) Strictly regulated LED current matching

(5) Operate at low noise and constant frequency

(6) Independent current and dimming control

(7) Wide dimming ratio

(8) A compact, small solution with very few external components

Source; Electronic Engineering Network

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