Analysis of the characteristics that LED driver integrated circuits need to have

LED driver ICs used to drive white LEDs account for about 50% of the total LED driver market. White LEDs are often used to provide backlighting for small displays in many battery-powered handheld portable products. However, this type of lighting application has matured and will not have a high CAGR in the next five years. As far as LED lighting applications are concerned, the three largest markets with a much higher CAGR than the handheld product backlighting market are: large flat-panel display HDTVs, automotive headlights, and general lighting.

As consumers demand larger flat-panel screens and higher resolutions for HDTVs, demand has shifted rapidly from plasma to LED HDTVs. According to DisplaySearch, plasma HDTV sales will peak in 2008 at $24 billion, while the LCD HDTV market will reach $75 billion in 2008 and grow to $93 billion by 2010. Clearly, the LED TV market has begun to "boom."

Current LCD HDTVs have a variety of shortcomings, from motion blur to color reproduction. Current generation LCD HDTVs cannot achieve true blacks and have a low dynamic range for all colors. Regular HDTVs use CCFL tubes for backlighting and only have a contrast ratio of 450 to 650cd/m2. The main problem with these HDTVs is that the CCFL backlighting cannot be completely turned off or local dimming is not possible.

In contrast, with high-brightness LED backlighting, an array of LEDs (up to 1,600 LEDs for a 46-inch display) can dim or locally shut down “groups” of the backlight, achieving contrast ratios that are almost an order of magnitude higher than CCFL designs (> 4,000 cd/m2). Additionally, by adjusting the brightness of groups of backlight LEDs, more mid-tones can be reproduced, making the picture more vivid. Another benefit is the ability to completely shut down the LEDs locally, thereby mitigating motion blur. By completely shutting off the LEDs between frames, blur caused by fast-moving objects is almost completely eliminated. The very fast response speed of the LEDs is critical in solving the fast-motion blur problem faced by such CCFL-backlit LCD TVs.

The Designer’s Dilemma

Cold cathode fluorescent lamps (CCFLs) are often used to backlight large flat panel displays, but they have a limited color spectrum and lack vivid colors. RGB LEDs actually extend the visible light range. In addition, CCFLs can display about 80% of the colors defined by the National Television System Committee (NTSC), while RGB can display up to 110% of the NTSC color spectrum, more accurately showing the original appearance of the image on the display. Using three monochromatic light sources, such as red, green and blue (RGB) lasers, can achieve the widest possible color spectrum.

On the other hand, white LED backlighting is very suitable for handheld and mobile displays because white LEDs are small, simple to drive, less sensitive to mechanical stress, and have a life expectancy twice as long as CCFLs. However, white LEDs have the same disadvantages as CCFLs in terms of color spectrum because white LEDs are equivalent to broadband light sources. White LEDs are formed by blue diodes covered with phosphors, which convert part of the blue light into yellow light. The combined spectrum is considered white light.

Compared to monochromatic light sources, RGB LEDs provide a color spectrum close to a narrow band at a lower cost. RGB LEDs not only improve the color spectrum, but also improve efficiency because RGB LEDs only emit light energy in the required red, green, and blue colors. In contrast, broadband light sources (such as white light LEDs and CCFLs) emit more light of unwanted colors, reducing the purity of the color spectrum and thus losing efficiency. Since different colors of RGB LEDs can be driven separately, the white light point or color temperature of RGB LEDs can be calibrated, while the white light point of CCFLs and white light LEDs is fixed.

LED Dimming Considerations

LED dimming is traditionally achieved by adjusting the forward current through the LED using a DC signal or a filtered PWM signal. Reducing the LED current adjusts the LED light output density. However, changes in forward current can cause changes in the color of the LED light because the chromaticity of the LED changes with current. Many applications such as LCD high-definition televisions and automotive backlighting cannot tolerate any deviation in the color of the LED light. A wide dimming range is required in these applications due to the varying ambient light conditions and the human eye's ability to perceive small changes in light intensity. Using a PWM signal to control the light intensity of the LED allows LED dimming without changing the color of the light.

"True Color PWMTM Dimming" adjusts the brightness of the LED using a PWM signal. It essentially switches the LED on and off at full current at the PWM frequency. The human eye is limited to 60 frames per second. By increasing the PWM frequency (e.g. to 80Hz to 100Hz), the human eye perceives the pulsed light source as being continuously on. In addition, by modulating the duty cycle (the length of the "on time"), the intensity of the LED can be controlled. With this method, the color of the LED remains constant because the current through the LED is either zero or constant. Many HDTV designers require dimming ratios as high as 3000:1 to accommodate wide variations in ambient light.

in conclusion

It is clear that LEDs have become the mainstream choice for backlighting large LCD flat panel displays used in high-definition televisions. However, system designers still need LED driver ICs that meet their specific design performance requirements. Therefore, LED driver ICs must be able to provide sufficient current and voltage for many different types of LED configurations with conversion topologies that meet the input voltage range and required output voltage and current requirements. Therefore, LED driver ICs need to have the following features to meet the designer's needs:

(1) Wide input voltage range

(2) Wide output voltage range

(3) High-efficiency conversion

(4) Strictly regulated LED current matching

(5) Low noise, constant frequency operation

(6) Independent current and dimming control

(7) Wide dimming ratio

(8) A compact solution with minimal external components