5347
SMOKE DETECTOR
with INTERCONNECT
and TIMER
ELECTRICAL CHARACTERISTICS continued
Test
Pin
4
5
5
5
No Alarm, In Standby
No Alarm, Timer Mode After Pin 1
High-to-Low
I/O Current
2
No Alarm, V
I/O
= V
DD
- 2.0 V
Alarm, V
I/O
= V
DD
- 2.0 V
I/O Alarm Voltage
I/O Delay
Supply Current
2
2
6
External “Alarm” In
“Alarm” Out
V
DD
= 9.0 V, No Alarm, No Loads
V
DD
= 12 V, No Alarm, No Loads
NOTE 1: Negative current is defined as coming out of (sourcing) the specified device pin.
NOTE 2: Alarm (Smoke) Condition is defined as V
15
< V
13
; No Alarm (No Smoke) Condition as V
15
> V
13
.
Limits
Test Conditions
V
OUT
= 0.5 V
V
DD
= 7.2 V, V
OUT
= 1.0 V
Min.
500
10
8.0
32
8.0
25
-7.5
3.0
—
—
—
Typ.
—
—
10
40
10
—
—
—
3.0
5.0
—
Max.
—
—
12
48
12
60
—
—
—
9.0
12
Units
µA
mA
ms
s
s
µA
mA
V
s
µA
µA
Characteristic
Timer Out Output Current
LED Output ON Current
LED Output ON Time
LED Output OFF Time
CIRCUIT DESCRIPTION
The A5347CA 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 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
DD
to an internal reference.
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.
When smoke is detected, the resistor divider
network that sets the sensitivity (smoke trip point) is altered to increase
the sensitivity set voltage (pin 13) by typically 130 mV with no external
connections to pins 3 or 13. This provides hysteresis and reduces
false triggering. 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 amplifier is not
power strobed and thus provides constant protection from surface
leakage currents. The detector input has internal diode protection
against static damage.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
