Tuesday 13 July 2010

Smoke detector application

Smoke-detector integrated circuit features ultra-low current consumption and networking capability

The new A5303 from Allegro MicroSystems Europe is a low-current BiCMOS photoelectric smoke detector integrated circuit designed to be used with an infrared optical chamber to sense light scattered from smoke particles.

Key features of the new device include 3 V operation with ultra-low standby current that results in an average 10-year battery life, plus a networking capability that allows units to be interconnected so that if any unit senses smoke all units will sound an alarm. Also included are a latching alarm indicator and a timer function that minimises standby power consumption by controlling the duration of the reduced sensitivity period.

The A5303 incorporates a variable-gain photo-amplifier which can be directly interfaced to an infrared emitter-detector pair. The amplifier gain levels are determined by two external capacitors, and are internally selected depending on the operating mode. Low gain is selected during standby and timer modes. During a local alarm, this low gain is increased (internally) by approximately 45% to provide hysteresis. High gain is used during pushbutton test and to periodically monitor the chamber sensitivity during standby.

The internal oscillator and timing circuitry minimise standby power by sensing for smoke for only 100 µs in every ten seconds. A special three-stage-speedup sensing scheme is incorporated to minimise the time to an audible alarm and also to reduce false triggering. Chamber sensitivity is periodically monitored, with two consecutive cycles of degraded sensitivity required for a warning signal to occur.

The A5303 is supplied in a low-profile (less than 1.2 mm overall height) 20-pin TSSOP package (0.65 mm nominal lead pitch). The package is lead (Pb) free with 100% matt tin lead-frame plating.

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