a5355ca Allegro MicroSystems, Inc., a5355ca Datasheet - Page 4

• Manufacturer Part Number: a5355ca
• Description: Smoke Detector With Interconnect
• Manufacturer: Allegro MicroSystems, Inc.

all of the required features for an ionization-type smoke
detector.
Oscillator. An internal oscillator operates with a period of
1.67 seconds during no-smoke conditions. Every 1.67
seconds, internal power is applied to the entire circuit and
a check is made for smoke. Every 24 clock cycles (40
seconds), the LED is pulsed and a check is made for low
battery by comparing V
Since very-low currents are used in the device, the
oscillator capacitor at pin 12 should be a low-leakage type
(PTFE, polystyrene, or polypropylene).
Detector Circuitry. An active guard is provided on both
pins adjacent to the detector input (pin 15). The voltage
at pins 14 and 16 will be within 100 mV of the input. This
will keep surface leakage currents to a minimum and
provide a method of measuring the input voltage without
loading the ionization chamber. The active guard ampli-
fier is not power strobed and thus provides constant
protection from surface leakage currents. The detector
input has internal diode protection against static damage.
Alarm Circuitry. If smoke is detected, the oscillator
period changes to 40 ms and the horn is enabled. The
horn output is typically 160 ms ON, 80 ms OFF. During
the OFF time, smoke is again checked and will inhibit
further alarm output if smoke is not sensed. During
smoke conditions the low battery alarm is inhibited and
the LED is driven at a 1 Hz rate.
5355
SMOKE DETECTOR
with INTERCONNECT
The A5355CA is a low-current CMOS circuit providing
DD
to an internal reference.
CIRCUIT DESCRIPTION
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
Sensitivity Adjust. The detector sensitivity must be
externally adjusted to the individual characteristics of the
ionization chamber by connecting resistors between pin
13 and V
and V
Low Battery. The low battery threshold is set internally
by a voltage divider connected between V
The threshold can be increased by connecting a resistor
between pin 3 and V
by connecting a resistor between pin 3 and V
battery voltage level is checked every 40 seconds during
the 10 mA, 10 ms LED pulse. If an LED is not used, it
should be replaced with an equivalent resistor (typically
500
10 mA.
I/O. A connection is provided at pin 2 to allow multiple
smoke detectors to be commoned. If any single unit
detects smoke (I/O is driven high), all connected units will
sound their associated horns after a nominal 3 second
delay. The LED is suppressed when an alarm is signaled
from an interconnected unit.
Testing. On power up, all internal counters are reset.
Internal test circuitry allows for low battery check by
holding pins 8 and 12 low during power up, then reducing
V
can be accelerated by driving pin 12 with a 2 kHz square
wave. The 10 ms strobe period must be maintained for
proper operation of the comparator circuitry.
DD
and monitoring HORN
SS
to 1000
.
DD
, and between pin 13
) such that the battery loading remains at
DD
. The threshold can be decreased
1
(pin 10). All functional tests
DD
and V
SS
. The
SS
.