a5349 Allegro MicroSystems, Inc., a5349 Datasheet - Page 6

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

A5349
With no external connections on the SENSITIVITY SET pin,
while the A5349 is checking for smoke:
To increase sensitivity, a resistor can be connected between
SENSITIVITY SET and VDD, with the value:
where
To decrease sensitivity, a resistor can be connected between
SENSITIVITY SET and VSS, with the value:
where
Low Supply
The low supply condition threshold is set internally by a voltage
divider connected between VDD and VSS. The threshold can
be externally adjusted by connecting a resistor between the
LOW-V SET pin and either the VDD or VSS pins.
To increase the threshold, a resistor can be connected between
LOW-V SET and VSS. Given an initial threshold, V
nally 7.5 V), and a target threshold, V
have the value:
where
To decrease the threshold, a resistor can be connected between
LOW-V SET and VDD. Given an initial threshold, V
nally 7.5 V), and a target threshold, V
have the value:
where
The supply voltage level is checked approximately every 40
seconds.
R
R
R
R
LOWVSET
K = 1 / (V
LOWVSET
K = 1 / (V
SENSITIVITYSET
SENSITIVITYSET
K = V
K = V
V
SENSITIVITYSET
(th)set
= 600E3 × K / (1 – 0.375 × K) ,
DD
= 960E3 × K / (0.6 – 1.6 × K) ,
(th)set
DD
/ V
/ (0.727 × V
/ V
/ [0.727 × V
SENSITIVITYSET
= 1.1E6 × K / (1 – K) ,
= 1.1E6 × K / (1 – K) ,
SENSITIVITYSET
= V
(th)set
(th)set
(th)init
DD
(th)init
, the resistor should
, the resistor should
/ 2 .
) – 1 .
– 1 .
] – 1) .
– 1) .
(th)init
(th)init
(nomi-
(nomi-
Timer (Hush) Mode
An internal timer is provided that can be used in various configu-
rations to allow a period of reduced smoke detector sensitivity,
referred to as Timer (or Hush) mode.
In normal operation, when a high-to-low transition occurs at the
TIMER START pin, the internal timer is reset, Timer mode is
enabled, and the circuit resets to a no-alarm condition. During
Timer mode, which is active for approximately 10.25 minutes
(368 clock cycles), the TIMER OUT pin is pulled down to VSS
every time the A5349 makes a check for smoke. A resistor con-
nected between the TIMER OUT and the SENSITIVITY SET
pins will decrease the detector’s sensitivity to smoke during
this time, and allow the user to hush alarms caused by nuisance
smoke or steam (such as from cooking). At the end of Timer
mode, the device signals its completion with two 10-ms horn
chirps, 1.67 seconds apart.
While the Timer mode is active, the LED blinks, remaining on
for approximately 8.7 s and off for approximately 1.67 s. If the
level of smoke increases such that the reduced-sensitivity level
is reached, the A5349 will signal an Alarm condition. If such an
Alarm condition does occur, the timer will still continue to com-
pletion of its cycle. If Timer mode will not be used, the TIMER
START pin should be tied to VSS.
I/O
A connection to the I/O pin allows multiple smoke detectors to
be interconnected. If any single unit detects smoke, its I/O pin is
driven high (after a nominal 3 s delay), and all connected units
will sound their associated horns. The LED remains on constantly
when an alarm is signaled from an interconnected unit, and any
local alarm condition causes the I/O pin to be ignored as an input.
When in Timer mode, the device will still signal an alarm if I/O is
driven high externally. This pin has an on-chip pulldown device
and must be left unconnected if not used.
Testing
On power-up, all internal counters are reset. Internal test cir-
cuitry allows low supply check by holding the FEEDBACK and
OSC CAP pins low during power-up, then reducing V
monitoring the HORN1 pin. HORN1 will be driven high when
V
be accelerated by driving the OSC CAP pin with a 2 kHz square
wave. The 10 ms strobe period must be maintained for proper
operation of the comparator circuitry.
DD
AC Ionization Smoke Detector
falls below the low-supply threshold. All functional tests can
with Interconnect and Timer
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
DD
and
6