RE46C180S16TF【PDF 11/58 页】IC ION SMOKE DETECT ASIC 16SOIC

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Sheet目录56

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?2011 Microchip Technology Inc.

DS22275A-page 11

RE46C180

3.0

DEVICE DESCRIPTIONS

3.1

Standby Internal Timing

The internal oscillator is manufactured to ?% toler-

ance. The oscillator period, TPOSC, is 625 祍. The inter-

nal clock period, TPCLK, of 10 ms is derived from the

internal oscillator period.

In Standby, once every 10s, the smoke detection cir-

cuitry is powered on for 5 ms. At the conclusion of the

5 ms, the status of the smoke comparator is latched. If

a Smoke condition is present, the period to the next

detection decreases and additional checks are made.

In Standby, once every 80s, the Low Battery detection

circuitry is powered on for 10 ms. At the conclusion of

the 10 ms, the status of the Low Battery comparator is

latched. RLED is enabled for 10 ms every 320s to

provide a battery load in the loaded battery test.

In Standby, once every 320s, the chamber test circuitry

is powered on for 5 ms. At the conclusion of the 5 ms,

the status of the chamber test is latched. See

Section 3.3 Supervisory Tests for details.

3.2

Smoke Detection Circuitry

The collection electrode voltage (CEV) of the ionization

chamber is compared to the stored reference voltage at

the conclusion of the 5 ms smoke sample period. After

the first Smoke condition is detected, the smoke detec-

tion rate increases to once every 1s. Three consecutive

smoke detections will cause the device to go into Local

Alarm, and the horn circuit and IO will be active. RLED

will turn on for 10 ms at 1 Hz rate.

In Local Alarm, the smoke reference voltage (smoke

sensitivity) is internally increased to provide alarm

hysteresis.

There are three separate smoke sensitivity settings (all

user programmable):

" Standby sensitivity

" Local alarm (hysteresis) sensitivity

" HUSH sensitivity

During PTT, the standby smoke sensitivity is used in

smoke detection; but the CHAMBER voltage is user

programmable.

The guard amplifier and outputs are always active, and

will be within 50 mV of the DETECT input to reduce

surface leakage. The guard outputs also allow for

measurement of the DETECT input without loading the

ionization chamber.

3.3

Supervisory Tests

Once every 80s, the status of the battery voltage is

checked by comparing a fraction of the V

voltage to

an internal reference. In each period of 320s, the bat-

tery is checked four times. Of these four battery

checks, three are unloaded and one is loaded with

RLED enabled, which provides a battery load. Low bat-

tery status is latched at the end of the 10 ms RLED

pulse.

If the Low Battery test fails, the horn will chirp for 10 ms

every 40s, and will continue to chirp until the next

loaded Low Battery check is passed. The unloaded

Low Battery checks are skipped in Low Battery

condition.

As a user programmable option, a Low Battery Hush

mode can be selected. If a Low Battery condition

exists, upon release of PTT, the unit will enter the Low

Battery Hush mode, and the 10 ms horn chirp will be

silenced for 8 hours. At the conclusion of the 8 hours

the audible indication will resume, if the Low Battery

condition still exists

In addition, every 320s, a background chamber test is

performed by internally lowering the CHAMBER volt-

age to a pre-determined level (user programmable) for

3.7s. This will emulate a Smoke condition. At the end of

this 3.7s period, the smoke detection circuitry is pow-

ered on for 5 ms, and the Smoke condition is detected.

If two consecutive chamber tests failed to detect a sim-

ulated Smoke condition, the chamber fail latch is set

and the failure warning is generated. The horn will chirp

three times every 40s. Each chirp is 10 ms long and

three chirps are spaced at a 0.5s interval. The chamber

fail warning chirp is separated from the Low Battery

warning chirp by about 20s.

The horn will continue this pattern until the chamber fail

latch is reset. The chamber fail latch resets when any

one of the followings is active:

" Two consecutive chamber tests pass

" Local smoke alarm

" PTT smoke alarm

After the chamber test is completed, the CHAMBER

voltage goes back to its normal standby level.

Chamber test is performed approximately 140s after

the loaded Low Battery test.

In a Local Alarm, PTT Alarm or Remote Alarm condi-

tion, the chamber test is not performed, and the Low

Battery chirping is prohibited.

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