A5303 senses light scattered from smoke particles

The A5303 from Allegro 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 device include 3V 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 that 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 by approximately 45 per cent 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 100us in every 10 seconds.

A three-stage-speedup sensing scheme is incorporated to minimise the time to an audible alarm and 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 1.2mm overall height) 20-pin TSSOP package.

The package is lead (Pb) free, with 100 per cent matt tin lead-frame plating.

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