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Electronic Design

Light Sensor Sees Like We Do

Light sensors are important in power management, as they control the dimming of display backlights. Yet typical semiconductor-based light sensors are too sensitive to infrared energy, causing them to trigger dimming circuits inaccurately. Directionality also degrades their accuracy. The LX1970 visible light sensor corrects these problems.

Developed by Microsemi Corp., the sensor combines a PIN-diode array photosensor with a high-gain amplifier to achieve a spectral response that approximates that of the eye (see the figure). To achieve omnidirectionality, the sensor chip is encapsulated with a clear molding material. The sensor ensures very linear response and good temperature stability. Gain varies by ±10% from −50°C to 100°C. Also, the LX1970 interfaces easily with LED and CCFL drivers.

Although PIN-diode photosensors have been around for years, they generally lacked adequate IR rejection and suffered from low (picoamp) output levels. Microsemi achieved a high degree of IR rejection with a semiconductor process that creates two diodes that are sensitive to different wavelengths. The on-chip amplifier boosts the sensor's output into the microamp range. Integrating the amp and photosensor is challenging because of the need to shield the amplifier from ambient light while exposing the sensor.

The LX1970 costs $1.14 each in 10,000-piece quantities.

Microsemi Corp.

TAGS: Components
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