RE46C144SW16F datasheet(9/26 Pages) MICROCHIP

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 2009-2012 Microchip Technology Inc.

DS22180C-page 9

RE46C144

3.0

DEVICE DESCRIPTION

3.1

Standby Internal Timing

With the external components specified in the

illustration Typical Application for ROSC and COSC,

the internal oscillator has a nominal period of 10 ms.

Normally the analog circuitry is powered down to

minimize standby current (typically 4 µA at 9V). Once

every 11 seconds, the detection circuitry (normal gain)

is powered up for 10 ms. Prior to completion of the

10 ms period, the IRED pulse is active for 100 µs. At

the conclusion of the 10 ms period, the photo amplifier

is compared to an internal reference to determine the

chamber status and latched. If a smoke condition is

present, the period to the next detection decreases and

additional checks are made. Three consecutive smoke

detections will cause the device to go into alarm, and

the horn circuit and interconnect will be active.

Once every 43 seconds, the status of the battery volt-

age is checked. This status is checked and latched at

the conclusion of the LED pulse. In addition, once

every 43 seconds the chamber is activated and, using

the high gain mode (capacitor C1), a check of the

chamber is made by amplifying background reflections.

If either the low battery or the photo chamber test fails,

the horn will chirp for 10 ms every 43 seconds.

The oscillator period is determined by the values of R9,

R12 and C5 (see Typical Application). The oscillator

period is as follows:

EQUATION 3-1:

3.2

Smoke Detection Circuitry

A comparator compares the photo amp output to an

internal reference voltage. If the required number of

consecutive smoke conditions is met, the device will go

into local alarm and the horn will be active. In local

alarm, the C2 gain is internally increased by

approximately 10% to provide alarm hysteresis.

3.3

Push-to-Test Operation

If the Test input pin is activated (VIH), after one internal

clock cycle, the smoke detection rate increases to once

every 330 ms. In this mode, the high-gain capacitor C1

is selected, and background reflections are used to

simulate a smoke condition. After the required

consecutive detections, the device will go into a local

alarm condition. When the Test input is deactivated

(VIL) and after one clock cycle, the normal gain

capacitor C1 is selected. The detection rate continues

at once every 330 ms until three consecutive no smoke

conditions are detected. At this point, the device

returns to standby timing.

3.4

LED Operation

In Standby, the LED is pulsed on for 10 ms every

43 seconds. In a local alarm condition or the push-to-

test alarm, the LED pulse frequency is increased to

once every .5 seconds. In the case of a remote alarm

the LED not active. In the Timer mode of operation, the

LED is pulsed on for 10 ms every 10 seconds.

3.5

Interconnect Operation

The bidirectional I/O pin allows for interconnection of

multiple detectors. In a local alarm condition, this pin is

driven high immediately through a constant current

source. Shorting this output to ground will not cause

excessive current. The I/O is ignored as an input during

a local alarm.

The I/O pin also has an NMOS discharge device that is

active for 1 second after the conclusion of any type of

local alarm. This device helps to quickly discharge any

capacitance associated with the interconnect line.

If a remote active high signal is detected, the device

goes into remote alarm and the horn will be active.

Internal protection circuitry allows for the signaling unit

to have a higher supply voltage than the signaled unit,

without excessive current draw.

The interconnect input has a 670 ms nominal digital

filter. This allows for interconnection to other types of

alarms (carbon monoxide, for example) that may have

a pulsed interconnect signal.

Note:

All timing references are nominal. See

electrical characteristics for limits.

T= TR +TF

Where:

TR = .693 x R12 x C5

TF =.693 x R9 xC5

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