RE46C167 datasheet(13/30 Pages) MICROCHIP

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DS20002251B-page 13

RE46C165/6/7/8

3.0

DEVICE DESCRIPTION

3.1

Internal Timing

With the external components specified in the Typical

Application

section 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 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 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 con-

secutive 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 detec-

tion is sampled at a high rate. The RE46C166/8 devices

sample at a period of 330 ms. The RE46C165/7 devices

have 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 0.5 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 (car-

bon 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 = 0.693 * R12 * C5

TF = 0.693 * R9 * C5

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