News and New Products

Color your world with superior LED backlighting

By Bill Schweber -- EDN, 2/3/2005

Agilent’s new HDJD-JB01 illumination and color-management system seeks to darken the prospects of the CCFLs (cold-cathode fluorescent lamps) that LCD-TV backlighting currently uses. Key to the design is a color-sensor IC, which controls a corresponding PWM current source for the RGB LED array (Picture). The system employs high-intensity Luxeon LEDs from Lumileds Lighting (www.lumileds.com), a joint venture between Agilent and Philips Electronics. The vendor claims that using an LED backlight is preferable for deep, realistic color imaging because it can provide a color gamut than spans 104% of the NTSC (National Television System Committee)-defined chromaticity space, whereas CCFLs cover only 75% of that space. According to Soo-Ghee Lee, vice president and general manager for the Optoelectronic Products Division in Agilent’s Semicon-ductor Products Group, “Now, consumers watching flat-panel TVs will be able to see all of the rainbow brighter and more clearly, for a more exciting viewing experience.”

Using LEDs for LCD backlighting sounds conceptually straightforward, but at least five practical issues have slowed their use. First, with LEDs, It is nearly impossible to maintain a fixed “color point” because the red, green, and blue LEDs degrade at different rates. Also, the wavelengths of the LEDs shift as temperature changes. It is also difficult for LEDs to hit standard setpoints in the color space, and manufacturers have to sort and bin LEDs for matching output, which adds to cost. Moreover, an overall ready-to-go system design has not yet been available.

The Agilent approach, which has five functional elements, is a closed-loop electrical and optical system. The system captures and mixes the output of the bright LEDs in a light guide, which then directs this output to a sensor IC. The sensor, in turn, provides color feedback to the controller IC, which drives the LEDs and maintains stability. Users can define the desired color setpoint, or “white point,” and brightness; those preferences go to the controller. (Note that US studio monitors and European TVs typically use a 6500K color temperature, whereas US TVs use a bluer color temperature of 7100K, and Japanese TVs us an even-bluer 9300K, as do most computer monitors worldwide.) As an additional benefit, the architecture greatly reduces motion blue because the system can pulse the LED backlight at the frame rate and go dark between frames, reminiscent of a movie-film projector’s shuttering between film-image frames.

The HDJD-S831-QT333 RGB-color-sensor IC combines a 12×12 photodiode array with a trio of transimpedance amplifiers and converts light intensity to three analog outputs, corresponding to the RGB primary colors. Its spectral sensitivity matches the LED-backlight application. The analog signals go to the HDJD-J822-SCR00 color-controller IC, which includes an A/D converter, a data-processing-logic core, a 12-bit PWM output generator, and an I²C serial interface.

The Agilent backlighting system can find use in applications other than LCD-TV displays, although that is the initial and best-fit application. Designers can also employ it in variable-color automotive-panel lighting, architectural lighting, and other embedded LCDs that require a backlight function. The system sells for less than $35 (low volumes).

Agilent Technologies Inc, www.agilent.com.