Electronic Design

Sensor Detects Light And Proximity For Unique Mobile Applications

Suppose you were designing a digital camera, and you wanted to extend its battery life by turning off its big power-hungry display screen whenever its users have their eye up to the optical viewfinder. Or suppose you were designing a notebook and you wanted a feature that would turn the keyboard backlight on only when the user’s hands were near the keyboard. Or suppose you were designing a touchscreen phone and you wanted to deactivate the on-screen buttons when users hold the phone up against their head.

Now suppose you could do that with a chip that adjusts screen brightness for ambient lighting as well. That’s what you get with Capella Microsystems’ CM3612 proximity and ambient-light sensor chip, which integrates patented optical filters, photodiodes, an analog-to-digital converter (ADC), and an I2C interface (Fig. 1).

Applied as part of the semiconductor fabrication process, the company’s patent-pending Filtron technology tops the proximity sensor diode with an 830- to 880-nm infrared filter and the ambient-light sensor diode with a 450- to 650-nm optical band-pass filter. By limiting the wavelengths impinging on the ambient light sensor, Capella Microsystems mimics the human eye’s brightness-sensing mechanism.

When it’s used to deactivate touchscreen phones, the proximity sensor function drives an integrated infrared emitter diode. The chip then uses its infrared detector to look for light reflected from the user’s head.

The 12-bit ADC both provides a representation of light intensity via the serial interface and feeds a digital low-pass filter that removes florescent light flicker. The CM3612 operates on voltages from 2.6 to 3.7 V and draws 130 µA (typical). The lower-cost CM3601 eliminates the I2C interface and uses a digital-to-analog converter (DAC) to output the ambient light value as an analog signal.

The CM3612 and CM3601 come in 2.35- by 1.8- by 1-mm optical land-grid array (OPLGA) packaging and are sampling now (Fig. 2). In quan ti ties of 10,000, the CM3612 costs $1.50, and the CM3601 costs $1.20.


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