RE46C145SW16F datasheet(9/26 Pages) MICROCHIP

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 2009-2012 Microchip Technology Inc.

DS22181C-page 9

RE46C145

3.0

DEVICE DESCRIPTION

3.1

Standby Internal Timing

With the external components specified in the Typical

Application for R12 and C5, 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 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 is 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 = .6931 x R12 x C5

TF = .6931 x R9 x C5

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