RE46C165SW datasheet(13/30 Pages) MICROCHIP

13 / 30 page

 2010 Microchip Technology Inc.

DS22251A-page 13

RE46C165/6/7/8

3.0

DEVICE DESCRIPTION

3.1

Internal Timing

With the external components specified in the 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 10 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

voltage 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 the Typical Application figure). The

oscillator period is as follows:

EQUATION 3-1:

3.2

Smoke Detection Circuit

A comparator compares the photo amplifier 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 (VIH4), the smoke

detection is sampled at a high rate. The RE46C166/8

device samples at a period of 330 ms. The

RE46C165/7 device has a first sample delay of up to

330 ms. After one sample, the smoke detection rate

increases to once every 250 ms. In this mode the high

gain capacitor C1 is selected, and background

reflections are used to simulate a smoke condition.

After the required three consecutive detections, the

device will go into a local alarm condition. When the

TEST input is deactivated (VIL4) and after one clock

cycle, the normal gain capacitor C2 is selected. The

detection rate continues once every 330 ms for the

RE46C166/8, and every 250 ms while the horn is not

sounding

for

the

RE46C165/7.

When

three

consecutive no smoke conditions are detected, the

device returns to standby timing.

Push-to-test will not work while the alarm memory is

set. The alarm memory notification will be activated

instead.

3.4

LED Pulse

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 once

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

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

LED is pulsed on for 10 ms every 10 seconds.

3.5

Interconnect

The bidirectional I/O pin allows the 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 has a 280k nominal pull-down resistor, so

the pin may be left unconnected.

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 digital filter that ensures

filtering out pulses of up to 300 ms. Filter pulses will be

ignored and not affect internal timing of the part. This

allows for interconnection to other types of alarms

(carbon monoxide for example) that may have a pulsed

interconnect signal.

The remote alarm delay (370 ms to 1.27s) specifies the

time from the interconnect going active to sounding the

piezo horn alarm.

T= TR + TF

Where:

TR = .693 * R12 * C5

TF = .693 * R9 * C5

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