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 V
BST
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 Tab l e 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 V
DD
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 in
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 (V
IH
), 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 (V
IL
), 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”).
