A5303 Datasheet, PDF (6/14 Pages) Allegro MicroSystems – Photoelectric Smoke Detector with Interconnect, Timer, and Latching Alarm Indicator

A5303
Photoelectric Smoke Detector
with Interconnect, Timer, and Latching Alarm Indicator
Pin and Circuit Description
(In Typical Application)
C1 Pin
A capacitor connected to this pin determines the gain of the
photoamplifier, A_e , during the push-to-test mode and during the
chamber monitor test. A typical capacitor value for this high-
gain (supervisory) mode is 0.047 μF, but it should be selected
based on the photochamber background reflections reaching the
detector and the required level of sensitivity. A_e = 1 + (C₁ / 12),
where C₁ is in pF. A_e should not exceed 10,000 and thus C₁
should not exceed 0.1 μF. Coupling of other signals to the C1,
C2, and DETECT inputs must be minimized.
C2 Pin
A capacitor connected to this pin determines the gain of the
photoamplifier, A_e, during standby. A typical capacitor value for
this low-gain mode is 4700 pF, but it should be selected based
on a specific photochamber and the desired level of sensitiv-
ity to smoke. A_e = 1 + (C₂ / 12), where C₂ is in pF. A_e should not
exceed 10,000 and thus C₂ should not exceed 0.1 μF. This gain
increases by a nominal 45% after a local alarm is detected (three
consecutive detections). A resistor must be installed in series
with the C2 capacitor.
DETECT Pin
This is the input to the photoamplifier and is connected to the
cathode of the photodiode. The photodiode is operated at zero
bias and should have low dark leakage current and low capaci-
tance. A shunt resistor must be installed in parallel with the
photodiode.
STROBE Pin
This output provides a strobed, regulated voltage of V_DD – 2 V.
The minus side of all internal and external photoamplifier cir-
cuitry is referenced to this pin.
VDD Pin
This pin is connected to the positive supply potential,
typically 3 V.
LVSET Pin
This pin allows the user to externally adjust the low-battery
alarm threshold. To increase the threshold, a resistor can be con-
nected between LVSET and VDD. To decrease the threshold, a
resistor can be connected between LVSET and VSS.
IRED Pin
This output provides a pulsed drive current for the external IR
emitter. To minimize noise impact, the IRED is not active when
the visible LED output is active.
ISET Pin
This pin allows the user to externally set the IRED current by
connecting a resistor between it and VSS. The IRED current
controls the amount of light generated by the IR LED in the
chamber. The IRED current, in mA, can be approximated using
the following equation:
I_IRED (mA) = 300 / R_ISET
(1)
The chosen resistor should set a maximum of 300 mA (typically
a minimum of 1Ω).
I/O Pin
A connection at this pin allows multiple smoke detectors to be
interconnected. If any single unit detects smoke, its I/O pin is
driven high, and all connected units will sound their associated
alarm indicators. As an input, this pin is sampled every 4 clock
cycles (nominally 43 ms) during standby, and two consecutive
samples and one additional clock with I/O high are required
before signaling an alarm. If the I/O line goes low at all during
the 96.8 ms, the remote alarm is not enabled, providing sig-
nificant immunity to I/O noise and other pulses on the I/O line
which are shorter than 9 clock cycles. The LED is suppressed
when an alarm is signaled from an interconnected unit, and
any local-alarm condition causes this pin to be ignored as an
input. An internal NMOS device acts as a charge dump to aid
in applications involving a large (distributed) capacitance, and
is activated at the end of a local or test alarm. This pin has an
on-chip pull-down device and must be left unconnected if not
used. In the application, there should be a series current-limiting
resistor to other smoke alarms.
SOUT0, SOUT1, SOUT2 Pins
These pins provide push-pull CMOS logic outputs to control an
external sound IC. The outputs indicate the state of the device as
follows:
Condition
Standby
Local Alarm
Remote Alarm
Push-Test Passed
Push-Test Failed
Low-Battery
Degraded Chamber
SOUT2
L
L
L
L
H
H
H
SOUT1
L
L
H
H
L
L
H
SOUT0
L
H
L
H
L
H
L
Allegro MicroSystems, Inc.
6
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com