a5349 Allegro MicroSystems, Inc., a5349 Datasheet - Page 5

• Manufacturer Part Number: a5349
• Description: Ac Ionization Smoke Detector With Interconnect And Timer
• Manufacturer: Allegro MicroSystems, Inc.

A5349
ELECTRICAL CHARACTERISTICS
R
The A5349 is a low-current, CMOS circuit providing 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 for 10 ms and a check is made for
smoke. Every 24 clock cycles (approximately 40 seconds), a
check is made for low supply by comparing V
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 polypropylene).
Detector Circuitry
When the voltage on the DETECT IN pin is less than the volt-
age on the SENSITIVITY SET pin, the A5349 evaluates this as a
smoke condition. During a smoke condition, the resistor divider
network that sets the sensitivity (also referred to as the smoke trip
point) is altered to increase V
(with no external connections on the SENSITIVITY SET pin).
This provides hysteresis and reduces false triggering.
I/O Current
I/O Alarm Voltage
I/O Delay
1
2
Negative current is defi ned as coming out of the specifi ed device pin (sourcing).
Alarm (smoke) condition is defi ned as V
TIMINGRES
Characteristic
= 8.2 MΩ (unless otherwise noted)
SENSITIVITYSET
Symbol
V
I
t
I
IH(IO)
IOH
r(io)
IOL
DETECTIN
No alarm, V
Local alarm, V
External “alarm” in
Local or test alarm to I/O active
1,2
(continued) at T
< V
by 130 mV typical
DD
Test Conditions
SENSITIVITYSET
to an internal
I/O
I/O
= V
Circuit Description
= V
DD
DD
– 2.0 V
; no alarm (no smoke) condition is defi ned as V
A
– 2.0 V
= 25°C, V
An active guard is provided on GUARD1 and GUARD2, the two
pins adjacent to the detector input, the DETECT IN pin. V
and V
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-supply alarm is inhibited and the LED
is turned off until the condition is removed.
Sensitivity Adjust
The detector sensitivity to smoke is set internally by a voltage
divider connected between VDD and VSS. The sensitivity can,
however, be externally adjusted to the individual characteristics
of the ionization chamber by connecting a resistor between the
SENSITIVITY SET pin and either the VDD or VSS pins.
AC Ionization Smoke Detector
Test Pin
DD
GUARD2
2
2
2
2
with Interconnect and Timer
= 9.0 V, V
will be within 100 mV of V
Min.
–7.5
SS
3.0
25
—
= 0 V, C
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
OSCCAP
Typ.
DETECTIN
3.0
—
—
—
= 0.1 μF,
> V
DETECTIN
SENSITIVITYSET
Max.
60
—
—
—
. This will
.
Units
mA
GUARD1
μA
V
s
5