What type of LED should be used for HD TV backlighting?

background

The LED driver IC used to drive white light LED accounts for about 50% of the total LED drive market, and white LED is commonly used to provide backlighting for a large number of battery powered handheld portable displays. However, this type of lighting applications have matured, the next 5 years will not have a high cagr. LED lighting applications, the compound annual growth rate is far higher than the backlight lighting market handheld products of the 3 largest market are: large flat screen HD TV, car headlights lighting and general lighting.

As consumers demand high-definition TV with a larger flat-panel display and higher resolution, the demand for television has rapidly turned from plasma high-definition TVs to LED HD tv. According to DisplaySearch's data, plasma high-definition TV sales will peak in 2008, the sales revenue of $24 billion; and in 2008 LCD high-definition TV market will reach $75 billion in 2010 increased to $93 billion. Obviously, LED TV market has begun to boom.

From motion blur to color reproduction, the current LCD HD TV has a variety of drawbacks. The current generation of LCD high-definition TV can not get the real black, and the dynamic range of all colors are smaller. Ordinary high-definition TV with CCFL tube backlighting, only 450 to 650cd/m2 contrast. The main problem with these high-definition TVs is that the CCFL backlight can not be completely off or can not be localized dimming.

On the contrary, the use of high brightness LED backlight, LED array (46 inch screen, can contain up to 1600 LED) of the "group" backlight dimming or partial shutdown, so as to realize the CCFL design is almost an order of magnitude higher contrast (> 4000cd/m2). In addition, by adjusting the brightness of the backlight LED group, more intermediate tones can be reproduced to make the picture more vivid. Another advantage is that it can completely turn off the LED from local, thus reducing the motion blur problem. By completely closing the LED between frames, the blur problem caused by the fast moving object is almost eliminated. In dealing with the fast motion blur problem faced by the CCFL backlight LCD TV set, the LED very fast response speed is critical.

Designer dilemma

Cold cathode fluorescent lamps (CCFL) are often used to provide backlighting for large flat panel displays, but they are limited in chromatography and lack of color. RGB LED actually expands the range of visible light. In addition, the National Television System Committee (NTSC) defined color, CCFL can show about 80%, while the RGB can display the NTSC chromatography up to 110%, in order to display more accurately display the primitive style image. Using 3 monochromatic light sources, such as red, green, and blue (RGB) lasers, the widest possible chromatogram can be obtained.

On the other hand, the white LED backlight is very suitable for handheld and mobile display, because the white LED small size, easy to drive, the mechanical stress is less sensitive compared with CCFL, life expectancy is two times longer. However, the white light LED in chromatography and CCFL have the same shortcomings, because white LED equivalent to broadband light source. White LED is a blue light emitting diode coated with phosphor, which transforms some of the blue light into yellow light.

Compared with the monochromatic light source, the RGB LED provides a narrow band chromatography at a low cost. RGB LED not only improves the chromatography, but also improves the efficiency, because RGB LED only needs the red light, green light and blue light emitting light. In contrast, broadband light sources (such as white LED and CCFL) emit more light that does not require color, thereby reducing the purity of the chromatogram and hence the loss of efficiency. Since the different colors of the RGB LED can be driven separately, then the white point or color temperature of the RGB LED can be corrected, while the white point of the CCFL and the white LED are fixed.

LED dimming considerations

Traditionally, LED dimming is realized by using the DC signal or the filtered PWM signal to adjust the forward current flowing through the LED. LED current can be adjusted to reduce the output light density of LED. However, the forward current changes will lead to a change in the color of the LED light, because the chroma of the LED varies with the current. LCD HD TV and automotive backlight lighting and many other applications are not allowed to have any deviation LED luminous color. Due to the different changes in the surrounding environment and the human eye can perceive the light intensity changes, so in these applications need wide dimming range. PWM signal to control the intensity of the LED, you can not change the color of light to achieve LED dimming.

"True Color PWMTM dimming" through the PWM signal to adjust the brightness of LED. It is essentially a PWM frequency with full current on and off LED. The human eye is limited to 60 frames per second. By increasing the frequency of the PWM (e.g., up to 80Hz to 100Hz), the human eye senses that the pulse light source is continuously connected. In addition, the duty cycle (LED) can be controlled by modulating the duty cycle. When this method is used, the color of the LED remains unchanged because the current value of LED is either zero or constant. A lot of high-definition TV designers are required to tune the ratio of up to 3000:1, in order to adapt to a wide range of changes in ambient light.

conclusion

It is clear that, for high-definition TVs used in large LCD flat panel display backlighting, LED has become the mainstream choice. However, system designers still need to meet their specific