RE46C190 datasheet(13/42 Pages) MICROCHIP

13 / 42 page

 2010 Microchip Technology Inc.

DS22271A-page 13

RE46C190

3.0

DEVICE DESCRIPTION

3.1

Standby Internal Timing

The internal oscillator is trimmed to ±6% tolerance.

Once every 10 seconds, the boost converter is

powered up, the IRcap is charged from VBST and then

the detection circuitry is active for 10 ms. Prior to

completion of the 10 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

will cause the device to go into Alarm and activate the

horn and interconnect circuits. In Alarm, the horn is

driven at the high boost voltage level, which is

regulated based on an internal voltage reference, and

therefore results in consistent audibility over battery

life. RLED will turn on for 10 ms at a 2 Hz rate. In Local

Alarm, 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 Push-to-Test

There are four separate sets of integration limits (all

user programmable):

• Normal Detection

• Hysteresis

• HUSH

• Chamber Test and Push-to-Test 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. In addition, once every 43 seconds the

chamber is activated and a High Gain mode and

chamber test limits are internally selected. A check of

the chamber is made by amplifying background

reflections. The Low Boost mode is used for the

chamber test.

If either the low battery test or the chamber test fails,

the horn will pulse on for 10 ms every 43 seconds, and

will continue to pulse until the failing condition passes.

If two consecutive chamber tests fail, the horn will pulse

on three times for 10 ms, separated by 330 ms every

43 seconds. Each of the two supervisory test audible

indicators is separated by approximately 20 seconds.

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 will turn on. If the TEST

input is held for more than 0.5 second, the unit will

enter

the

Push-to-test

operation

described

Section 3.4 “Push-to-Test Operation (PTT)”

. After

the TEST input is driven low, the unit enters in Low

Battery Hush mode, and the 10 ms horn pulse is

silenced for 8 hours. The activation of the test button

will also initiate the 9 minute Reduced Sensitivity mode

described in Section 3.6 “Reduced Sensitivity

Mode”

. At the end of the 8 hours, the audible indication

will resume if the low battery condition still exists.

3.4

Push-to-Test Operation (PTT)

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

detection rate increases to once every 250 ms after

one internal clock cycle. In Push-to-Test, 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 will go 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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