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RE46C129 Datasheet PDF : 8 Pages
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RE46C129
CMOS Ionization Smoke Detector ASIC with Interconnect
Product Specification
R&E International
A Subsidiary of Microchip Technology Inc.
DEVICE DESCRIPTION and APPLICATION NOTES
Internal Timing – With external components as indicated on the application drawing the period of the oscillator is
nominally 1.67 seconds in standby. Every 1.66 seconds the detection circuitry is powered up for 10.5mS and the
status of the smoke comparator is latched. In addition every 40 seconds the LED driver is turned on for 10.5mS
and the status of the low battery comparator is latched. The smoke comparator status is not checked during the
low battery test, during the low battery horn warning chirp, or when the horn is on due to an alarm condition.
If an alarm condition is detected the oscillator period increases to 41.5mS. In any alarm condition the horn is on
for 166mS and off for 83mS (typical).
Due to the low currents used in the oscillator the capacitor on pin 12 should be a low leakage type. Oscillator
accuracy will depend mainly on the tolerance of the RBIAS resistor and OSCAP capacitor.
Smoke Detection Circuit – The smoke comparator compares the ionization chamber voltage to a voltage derived
from a resistor divider across VDD. This divider voltage must be provided externally on pin 13 (VSEN). See the
application drawing (FIG 2).
The guard amplifier and outputs are always active and will be within 50mV of the DETECT input to reduce surface
leakage. The guard outputs also allow for measurement of the DETECT input without loading the ionization
chamber.
Low Battery Detection - An internal reference is compared to the voltage divided VDD supply. The battery can be
checked under load via the LED low side driver output since low battery status is latched at the end of the 10.5mS
LED pulse. Pin 3 (LBADJ) can be used to modify the low battery set point by placing a resistor to VDD or VSS.
The transmission switch on LBADJ is turned on during the LED pulse so an external resistance can be added on
pin 3 to modify the set point if desired.
LED Pulse – The LED is pulsed on for 10.5mS every 40S in standby. In alarm the LED is pulsed on for 10.5mS
every 1S.
Interconnect – Pin 2 (IO) provides the capability to common many detectors in a single system. If a single unit
goes into alarm the IO pin is driven high. This high signal causes the interconnected units to alarm. The LED
flashes every 1S for 10.5mS on the signaling unit and is inhibited on the units that are in alarm due to the active
IO signal. An internal sink device on the IO pin helps to discharge the interconnect line. This charge dump device
is active for 1 clock cycle after the unit exits a local alarm condition (1.67S).
The interconnect input has a 500mS nominal digital filter which will filter out pulsed signals on the IO pin. This
allows for interconnection to other types of alarms (carbon monoxide for example) that may have a pulsed
interconnect signal. This filter will eliminate unwanted random horn activations caused by a pulsed (<500ms
nominal) IO input signal.
Testing - By holding pin 12 (OSCAP) low the internal power strobe is active. Functional testing can be
accelerated by driving pin 12 with a 4 kHz square wave however the 10.5mS strobe period must be maintained
for proper operation of the analog circuitry. Please refer to the timing diagrams. All internal timing is reset after a
power up. By holding pins 8 and 12 low, reducing VDD and monitoring Pin 10 the low battery trip level can be
measured at power up.
© 2009 Microchip Technology Inc.
DS22172B-page 5
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