Ionization Smoke Detector with Interconnect
A5350
5
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The A5350 is a low-current, BiCMOS circuit providing all of
the required features for an ionization-type smoke detector. The
alarm polarity on DETECT IN is inverted as compared with typi-
cal ion-type devices. The DETECT IN voltage must rise above
the SENSITIVITY SET voltage in order to cause an alarm.
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 for 10 ms and a check is made for
smoke. Every 24 clock cycles (approximately 40 seconds), the
LED pin is pulsed and a check is made for low battery by com-
paring V
DD
to an internal reference. Because very-low currents
are used in the device, the oscillator capacitor at the OSC CAP
pin should be a low-leakage type (PTFE, polystyrene, or polypro-
pylene).
Detector Circuitry
When the voltage on the DETECT IN pin is greater than the volt-
age on the SENSITIVITY SET pin, the A5350 evaluates this as a
smoke condition.
An active guard is provided on GUARD1 and GUARD2, the two
pins adjacent to the detector input, the DETECT IN pin. V
GUARD1
and V
GUARD2
will be within 100 mV of V
DETECTIN
. This will
keep surface leakage currents to a minimum and provide a
method of measuring the input voltage without loading the ion-
ization chamber. The active guard amplifier is not power strobed
and thus provides constant protection from surface leakage
currents. The detector input has internal diode protection against
electrostatic 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 checked and further
alarm output will be inhibited if smoke is not sensed. During a
smoke condition, the low-battery alarm is inhibited and the LED
is pulsed approximately once every second.
Sensitivity Adjust
The detector sensitivity must be externally set to the individual
characteristics of the ionization chamber by connecting the
Circuit Description
Characteristic Symbol Test Conditions Test Pin Min. Typ. Max. Units
I/O Current
I
IOL
No alarm, V
I/O
= V
DD
– 2.0 V 2 25 — 60 μA
I
IOH
Local alarm, V
I/O
= V
DD
– 2.0 V 2 –7.5 — — mA
I/O Alarm Voltage V
IH(IO)
External “alarm” in 2 3.0 — — V
I/O Delay t
r(io)
Local or test alarm to I/O active 2 — 3.0 — s
1
Negative current is defi ned as coming out of the specifi ed device pin (sourcing).
2
Alarm (smoke) condition is defi ned as V
DETECTIN
> V
SENSITIVITYSET
; no alarm (no smoke) condition is defi ned as V
DETECTIN
< V
SENSITIVITYSET
.
ELECTRICAL CHARACTERISTICS
1,2
(continued) at T
A
= 25°C, V
DD
= 9.0 V, V
SS
= 0 V, C
OSCCAP
= 0.1 μF,
R
TIMINGRES
= 8.2 MΩ (unless otherwise noted)
