RE46C191 datasheet(11/44 Pages) MICROCHIP

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DS20005225H-page 11

RE46C191

3.0

DETAILED DESCRIPTION

3.1

Standby Internal Timing

The internal oscillator is trimmed to ±6% tolerance. Once

every ten seconds, the boost converter is powered up,

the IRCAP is charged from VBST and the detection

circuitry is active for 3 ms. Prior to completion of the

3 ms period, the IRED pulse is active for a

user-programmable duration of 100-400 µs. During this

IRED pulse, the photo diode current is integrated and

then digitized. The result is compared to a limit value

stored in EEPROM during calibration to determine the

photo chamber status. If a Smoke condition is present,

the period to the next detection decreases and

additional checks are made.

3.2

Smoke Detection Circuitry

The digitized photo amplifier integrator output is

compared to the stored limit value at the conclusion of

the IRED pulse period. The IRED drive is all internal,

and

both

the

period

and

current

are

user

programmable. Three consecutive smoke detections

cause the device to go into Alarm mode, and activate

the horn and interconnect circuits. In Alarm mode, the

horn is driven at the high boost voltage level, which is

regulated based on an internal voltage reference, and

results in consistent audibility over battery life. RLED

turns on for 10 ms at a 2 Hz rate. In Local Alarm mode,

the integration limit is internally decreased to provide

alarm hysteresis. The integrator has three separate

gain settings:

• Normal and Hysteresis

• Reduced Sensitivity (Hush)

• High Gain for Chamber Test and PTT

There are four separate sets of alarm limits, which are

all user programmable:

• Normal Detection

• Hysteresis

•Hush

• Chamber Test and PTT Modes

In addition, there are user-selectable integrator gain

settings to optimize detection levels (see Table 4-1).

3.3

Supervisory Tests

Once every 86 seconds, the status of the battery

voltage is checked by enabling the boost converter for

10 ms and comparing a fraction of the VDD voltage to

an internal reference. In each period of 344 seconds,

the battery voltage is checked four times. Three checks

are unloaded and one check is performed with the

RLED enabled, which provides a battery load. The

High Boost mode is active only for the loaded low

battery test. If the low battery test fails, the horn pulses

on for 10 ms every 43 seconds and continues to pulse

until the failing condition passes.

As an option, a Low Battery Silence mode can be

invoked. If a Low Battery condition exists and the TEST

input is driven high, the RLED turns on. If the TEST

input is held for more than 0.5 seconds, the unit enters

the

PTT

operation

described

Section 3.4

“Push-to-Test Operation (PTT)”. After the TEST

input is driven low, the unit enters the Low Battery Hush

mode and the 10 ms horn pulse is silenced for eight

hours. The activation of the test button also initiates the

nine-minute Reduced Sensitivity mode described in

Section 3.6 “Reduced Sensitivity Mode”. After eight

hours, the audible indication resumes if the Low Battery

condition still exists.

Once every 43 seconds, the chamber test is activated

and the High Gain mode with chamber test limits is

internally selected. The chamber is checked by

amplifying background reflections. The Low Boost

mode is used for the chamber test.

If two consecutive chamber tests fail, the horn pulses

on for 10 ms three times, with the pulses separated by

330 ms every 43 seconds. The horn continues to pulse

until the failing condition passes.

Each of the two supervisory test audible indicators is

separated by approximately 20 seconds.

3.4

Push-to-Test Operation (PTT)

When the TEST input pin is activated (VIH), the smoke

detection rate increases to once every 250 ms after

one internal clock cycle. In PTT, the photo amplifier

High Gain mode is selected and background reflections

are used to simulate a Smoke condition. After the

required three consecutive detections, the device goes

into a Local Alarm condition. When the TEST input is

driven low (VIL), the photo amplifier normal gain is

selected after one clock cycle. The detection rate

continues at once every 250 ms until three consecutive

No Smoke conditions are detected. At this point, the

device returns to standby timing. In addition, after the

TEST input goes low, the device enters the Hush mode

(see Section 3.6 “Reduced Sensitivity Mode”).

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