A5338
Package A
, 16-lead DIP
Smoke Detector
with Interconnect and Timer
The A5338CA is a low-current, CMOS circuit
providing all of the required features for an ionization-
type smoke detector. A networking capability allows
as many as 125 units to be interconnected so that if
any unit senses smoke, all units will sound an alarm.
In addition, special features are incorporated to facili-
tate alignment and test of the finished smoke detector.
This device is designed to comply with Underwriters
Laboratories Specification UL217.
The internal oscillator and timing circuitry keeps
standby power to a minimum by powering down the
device for 1.66 seconds and sensing smoke for only
10 ms. Every 24 on/off cycles, a check is made for
low battery condition. By substituting other types of
sensors, or a switch for the ionization detector, this
very-low power device can be used in numerous other
battery-operated safety/security applications.
The A5338CA is supplied in a low-cost, 16-pin
dual in-line plastic package. It is rated for continuous
operation over the temperature range of 0°C to +50°C.
Approximate Scale 1:1
Features
Interconnect up to 125 detectors
Piezoelectric horn driver
Guard outputs for detector input
Pulse testing for low battery
Power-on reset
Internal reverse battery protection
Internal timer and control for reduced sensitivity
Built-in hysteresis reduces false triggering
Typical Application
9V
50 kΩ
200 k
Ω
200 kΩ
Push to set
1
To other units
200
Ω
2
3
+SUPPLY or
–SUPPLY
RLV
16
Push to test
TIMER START
I/O
LOW-V SET
GUARD2
Smoke
Chamber
DETECT IN 15
A5338
GUARD1 14
13
12
11
COSC
0.1
μ
F
RS
+SUPPLY or
–SUPPLY
4 TIMER OUT
5
330
Ω
6
RT
8.2 M
Ω
7
8
LED
+SUPPLY
TIMING RES
FEEDBACK
SENSIVITIVTY SET
OSC CAP
HORN2
HORN1 10
–SUPPLY
9
1 MΩ
200 kΩ
Horn
1
μF
0.001
μF
RST
Dwg. EC-005A
26110.1
A5338
Smoke Detector with Interconnect and Timer
Pin-out Diagram
TIMER START
I/O
LOW-V SET
TIMER OUT
LED
+SUPPLY
TIMING RES
FEEDBACK
1
2
3
4
5
6
7
8
V
DD
OSC. &
TIMING
LOGIC
16
15
14
13
12
11
10
V
SS
9
GUARD2
DETECT IN
GUARD1
SENSITIVITY SET
OSC CAP
HORN2
HORN1
–SUPPLY
HORN
DRIVER
Dwg. PC-004
Terminal List Table
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Name
TIMER START
I/O
LOW-V SET
TIMER OUT
LED
+SUPPLY
TIMING RES
FEEDBACK
–SUPPLY
HORN1
HORN2
OSC CAP
SENSITIVITY SET
GUARD1
DETECT IN
GUARD2
Logic input for setting device mode.
Function
Logic input/output, where multiple detectors can be connected.
Terminal for a resistor, RLV, to the –SUPPLY pin (V
SS
) or to the +SUPPLY pin (V
DD
),
which may be added to modify the battery Low Voltage Threshold, V
DDL
.
Terminal for a resistor, RST, to reduce sensitivity during Timer mode.
LED output.
Device supply, V
DD
.
Terminal for timing resistor.
Feedback from horn external circuit.
Device supply, V
SS
.
Horn driver output 1.
Horn driver output 2.
Terminal for timing oscillator capacitor.
Terminal for resistor, RS, to adjust sensitvity for a particular smoke chamber.
Terminal for smoke detector guard 1.
Terminal for smoke detector input.
Terminal for smoke detector guard 2.
26110.1
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
3
A5338
Smoke Detector with Interconnect and Timer
Characteristic
Supply Voltage Range
Supply Current
ELECTRICAL CHARACTERISTICS
1
at T
A
= 25°C, V
DD
= 9.0 V, V
SS
= 0 V, C
OSC
= 0.1
μF,
R
T
= 8.2 MΩ, unless otherwise noted
Symbol
V
DD
I
DD
I
S
V
IO
V
Ohys
V
IC(guard)
V
IC(det)
Active Guard Impedance
Z
guard
Test Pin
Test Conditions
6
Operating
V
DD
= 9.0 V, no alarm, no loads
6
V
DD
= 12 V, no alarm, no loads
15
0 to 40% RH, V
I(detect in)
= 0 to 9.0 V
14-15 Active guard
16-15 Active guard
15-13 Smoke detect comparator
13
No alarm condition to alarm
14-15
13-15
14
16
Guard amplifier
Smoke detect comparator
With reference to V
SS
With reference to V
SS
No alarm
Alarm
Min.
6.0
—
—
–1.0
–100
–100
–50
90
2.0
0.5
Typ.
9.0
5.0
—
—
—
—
—
130
—
—
Max.
12
9.0
12
1.0
100
100
50
170
V
DD
- 0.5
V
DD
- 2.0
—
—
2.00
48
12
8.0
7.8
51.5
0.5
0.9
—
—
208
12
104
48
—
2.5
80
—
—
12
48
12
60
—
—
1.99
—
—
Detector Input Current
2
Input Offset Voltage
Hysteresis
Common Mode Range
Units
V
μA
μA
pA
mV
mV
mV
mV
V
V
Ω
Ω
s
ms
ms
min
V
%
V
V
V
V
ms
ms
ms
s
V
V
μA
μA
mA
ms
s
s
μA
mA
mA
s
V
s
—
10
—
500
1.34 1.67
Oscillator Period
t
OSC
12
32
40
Oscillator Pulse Width
t
w(osc)
4
8.0
10
Timer Period
t
timer
4
After V
TIMER_START
transition high-to-low (smoke detected)
5.0
6.5
Low Voltage Threshold
V
DD(th)
6
T
A
= 0 to 50°C
7.2
—
Sensitivity Adjust Voltage
V
S
13
V
SENSITIVITY_SET
/V
DD
, SENSITIVITY SET pin open circuit
48.5
50
I
O(horn)
= 16 mA, V
DD
= 9.0 V
—
0.1
I
O(horn)
= 16 mA, V
DD
= 7.2 V
—
—
HORNx Output Voltage
V
O(horn)
10-11
I
O(horn)
= –16 mA, V
DD
= 9.0 V
8.5
8.8
I
O(horn)
= –16 mA, V
DD
= 7.2 V
6.3
—
Alarm
120
160
HORNx Output ON Time
t
on(horn)
10-11
Low battery
8.0
10
Alarm
60
80
HORNx Output OFF Time
t
off(horn)
10-11
Low Battery
32
40
1
4.5
—
V
IH(ts)
TIMER START Logic Levels
V
IL(ts)
1
—
—
TIMER START Input Current
I
I(ts)
1
V
I(ts)
= 9.0 V
20
—
TIMER OUT Output Current
I
O(to)
4
V
O(to)
= 0.5 V
500
—
LED Output ON Current
I
LED
5
V
DD
= 7.2 V, V
LED
= 1.0 V
10
—
LED Output ON Time
t
on(led)
5
8.0
10
No alarm; in Standby mode
32
40
5
LED Output OFF Time
t
off(led)
No alarm; in Timer mode after V
TIMER_START
transition high-to-low
8.0
10
2
No alarm, V
(IO)
= V
DD
- 2.0 V
25
—
2
I/O Current
I
(io)
-7.5
—
Alarm, V
(IO)
= V
DD
- 2.0 V
Charge dump, V
(io)
= 1.0 V
5.0
—
I/O Charge Dump Duration
t
f(io)
2
After local alarm
1.33 1.66
2
External alarm signal in
3.0
—
I/O Alarm Voltage
V
I(io)
I/O Delay
t
d(io)
2
Alarm signal out
—
3.0
1
Alarm (smoke detected) condition is defined as V
DETECT_IN
< V
SENSITIVITY_SET
; No-Alarm (no smoke detected) condition is defined as
V
DETECT_IN
> V
SENSITIVITY_SET
.
2
Negative current is defined as coming out of (sourcing) the specified device pin.
26110.1
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
4
A5338
Smoke Detector with Interconnect and Timer
Functional Description
The A5338 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 s
during no-smoke conditions. Every 1.67 s, internal power is
applied to the entire circuit and a check is made for smoke. Every
24 oscillator cycles (40 s total), the LED is pulsed and a check is
made for low battery by comparing V
DD
to an internal reference.
Because very low currents are used in the device, the oscillator
capacitor, COSC, at the OSC CAP pin 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 voltage on the SENSITIVITY SET pin by
130 mV typical, when there are no external connections to the
SENSITIVITY SET pin or LOW-V SET pin. This provides hys-
teresis and reduces false triggering. An active guard is provided
on both GUARDx pins, adjacent to the DETECT IN detec-
tor input pin. The voltage at the GUARDx pins will be within
100 mV of DETECT IN. 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 protec-
tion 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 160
ms ON, and 80 ms OFF, typical. During the OFF time, smoke is
again checked and the circuit will inhibit further horn output if
smoke is not sensed. During smoke conditions, the low-battery-
warning is inhibited. The LED is driven at a 1 Hz rate, and will
only turn ON while the horn outputs are in the OFF state.
Sensitivity Adjust.
The detector sensitivity to smoke is set
internally by a voltage divider connected between V
DD
and V
SS
.
The sensitivity can be externally adjusted to the individual char-
acteristics of the ionization chamber by connecting a resistor, RS,
between the SENSITIVITY SET pin and +SUPPLY, or between
SENSITIVITY SET pin and –SUPPLY.
Low Battery.
The low-battery threshold is set internally by a
voltage divider connected between V
DD
and V
SS
. The threshold
can be increased by connecting a resistor, RLV, between the
LOW-V SETpin and +SUPPLY. The threshold can be decreased
by connecting RLV between LOW-V SETpin and –SUPPLY. The
battery voltage level is checked every 40 s during the 10 mA, 10
ms LED pulse. If an LED is not used, it should be replaced with
an equivalent resistor (typically 500 to 1000
Ω),
such that the bat-
tery loading remains at 10 mA.
Timer.
An internal timer is provided that can be used in various
configurations to allow for a period of reduced smoke detector
sensitivity,
Hush
mode. When a high-to-low transition occurs at
the TIMER START pin, the internal timer is reset, Timer mode is
enabled, and the circuit is reset to a no-alarm condition. Dur-
ing Hush mode, the LED will flash at a 10 s rate. If the level
of smoke is increased such that the reduced sensitivity level is
reached, the device will go into the alarm condition. The timer,
however, will continue to completion of the nominal 6.5 minute
period (232 oscillator cycles). If Timer mode is not used, the
TIMER START pin should be tied low. While in Hush mode, the
battery is checked. If a low-battery condition is detected, the low-
battery warning chirp will be activated.
I/O.
The I/O pin provides a terminal 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 s delay. The LED is suppressed when an
alarm is signaled from another interconnected unit. An inter-
nal NMOS device acts as a charge dump to aid in applications
involving a large (distributed) capacitance on the I/O pin. The
charge dump is activated only on the removal of a local alarm.
Testing.
On power-up, all internal counters are reset. Internal
test circuitry allows for low-battery check by holding the
FEEDBACK and OSC CAP pins low during power-up, then
reducing V
DD
and monitoring HORN1. All functional tests can be
accelerated by driving OSC CAP with a 2 kHz square wave. The
10 ms strobe period must be maintained for proper operation of
the comparator circuitry.
26110.1
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
5
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