RE46C122

CMOS Ionization Smoke Detector ASIC with

R&E International

Interconnect and Timer Mode

A Subsidiary of Microchip Technology Inc.

Product Specification

© 2009 Microchip Technology Inc.

DS22173A-page 5

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.

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 use of 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 should be <2% on any one device. Transmission

switches on VSEN and LBADJ prevent any interaction from the external adjustment resistors.

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.

Transmission switches on VSEN and LBADJ prevent any interaction from external adjustment resistance.

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 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 the alarm condition (1.67S).

The interconnect input has a 500mS nominal digital filter. This allows for interconnection to other types of alarms

(carbon monoxide for example) that may have a pulsed interconnect signal.

Testing – At power up all internal registers are reset. The low battery set point can be tested at power up by

holding FEED and OSCAP low at power up. HB will change state as VDD passes through the low battery set

point. 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.

Timer Mode – The transition of pin 1 (TSTART) from a high to low level initiates a ten minute timer. During this 10

minute period the open drain NMOS on pin 4 (TSTROBE) is strobed on with the internal clock. A resistor

connected to this pin could be used to modify the detector sensitivity for the timer period.