RE46C120

CMOS Ionization Smoke Detector ASIC

R&E International

Product Specification

A Subsidiary of Microchip Technology Inc.

© 2009 Microchip Technology Inc.

DS22157A-page 4

DEVICE DESCRIPTION and APPLICATION NOTES

Note: All timing references are nominal values. Refer to the electrical specifications for limits

Internal Timing – With external components as indicated on the application drawing the period of the oscillator is

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.

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 is available externally on pin 13 (VSEN). When smoke is

detected this voltage is internally increased by 130mV nominal to provide hysteresis and make the detector less

sensitive to false triggering.

Pin 13 (VSEN) can be used to modify the internal set point for the smoke comparator by using external resistors

to VDD or VSS. Nominal values for the internal resistor divider are indicated on the block diagram. These internal

resistor values can vary by up to ±20% but the resistor matching will typically be <2% on any one device. A

transmission switch on pin 13 isolates this pin during the low battery test so that the low battery set point will not

be affected if external resistors are used to modify the smoke sensitivity set point.

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 Zener 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

10mS LED pulse. Pin 3 (LBADJ) can be used to modify the low battery set point by placing a resistor to VDD or

VSS. Note that the internal resistor string is common to both pin 3 and pin 13 so there will be some interaction

between the two. Modification of the low battery set point may affect the smoke sensitivity setting.

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.

Testing – At power up all internal registers are reset. 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 should be maintained for proper operation of the analog circuitry. Please refer to the timing

diagrams.

Horn Tone – Pin 4 selects the NFPA72 horn tone (high or floating) or the 2/3 duty cycle continuous tone (low). If

this pin is externally connected high, use a current limiting resistor from pin 4 to VDD.

Reverse Battery Protection – The RE46c120 internally limits the current from VSS to VDD in the event of

accidental polarity reversal. If an input is connected to VDD it should be done through a resistance of at least

1.5K to limit the reverse current through this path.