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Sensor converts ambient light into digital signals

Posted: 16 Sep 2002 ?? ?Print Version ?Bookmark and Share

Keywords:texas advanced optoelectronic solutions? taos? tsl2550? photosensor? light sensor?

An ambient-light sensor from Texas Advanced Optoelectronic Solutions Inc. (TAOS) converts light intensity to a digital-signal output capable of direct SMBus interface.

TAOS says its TSL2550 can reduce overall system power by measuring ambient light to control the brightness of display screen backlighting in laptops, PDAs, camcorders, GPS systems, and other portable equipment. It can also be used for control of commercial and residential lighting.

TAOS' rollout follows a similar introduction by Toshiba America Electronic Components Inc.

"Portable equipment power consumption is directly proportional to display luminance," said Carlo Strippoli, VP for marketing and sales at TAOS, and such backlighting can account for 30 percent to 40 percent of platform power. "Dimming can reduce display power by as much as 70 percent, and a minimum brightness setting can extend battery life for more than an hour."

The chip combines two photodetectors, one sensitive to both visible and IR light, and the other sensitive to IR light only. Output from the two is converted to a digital format and stored in separate registers accessed through the SMBus.

By comparison, Toshiba's photo illumination sensors consist of a single-chip photodiode and amplifier circuit together with a luminosity-efficiency correction function.

Strippoli said the combined data from the photodetectors compensates for the effect of the IR component of ambient light, approximating the response of the human eye. "This information can be used to calculate light intensity in commonly used units of lux," without a costly photopic filter, he said.

The ratio of IR-to-visible light can be used to determine the type of ambient-light source, such as incandescent, fluorescent or sunlight, to allow smarter control of display contrast or color.

An ADC in the TSL2550 measures light over an effective 12-bit dynamic range. Strippoli said the integrating conversion technique effectively eliminates the effect of flicker from ac-powered lamps and increases the stability of the measurement.

A single 2.7V to 5.5V supply suits the chip for use in 3V systems. A power down mode shuts off the device when not in use.

The chip is packaged in an eight-pin SOIC and priced at $1.59 in lots of 1,000. Evaluation modules are available for $110.

EE Times

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