2010 Microchip Technology Inc. DS22245A-page 11
RE46C162/163
3.0 DEVICE DESCRIPTION
3.1 Internal Timing
With external components as indicated on the
application drawing, the period of the oscillator is
nominally 1.67 seconds in standby. Every
1.67 seconds, the detection circuitry is powered up for
10.5 ms and the status of the smoke comparator is
latched. In addition, every 40 seconds the LED driver is
turned on for 10.5 ms and the status of the low battery
comparator is latched. The smoke comparator status is
not checked during the low battery test, during the low
battery horn warning chirp, or when the horn is on due
to an alarm condition.
If an alarm condition is detected, the oscillator period
increases to 41.7 ms.
Due to the low current used in the oscillator, the
capacitor on the OSCAP pin should be a low leakage
type.
3.2 Smoke Detection Circuit
The smoke comparator compares the ionization
chamber voltage to a voltage derived from a resistor
divider across V
DD
. This divider voltage is available
externally on the VSEN pin. When smoke is detected,
this voltage is internally increased by 130 mV nominal
to provide hysteresis and make the detector less
sensitive to false triggering.
The VSEN pin can be used to modify the internal set
point for the smoke comparator by use of external
resistors to V
DD
or V
SS
. Nominal values for the internal
resistor divider are indicated on the block diagram.
These internal resistor values can vary by up to ±20%,
but the resistor matching should be <2% on any one
device. The transmission switch on VSEN prevents any
interaction from the external adjustment resistors.
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 Low Battery Detection
An internal reference is compared to the voltage-
divided V
DD
supply. The battery can be checked under
load via the LED low side driver output since low
battery status is latched at the end of the 10.5 ms LED
pulse.
3.4 LED Pulse
The LED is pulsed on for 10.5 ms every 40s in standby.
In alarm, the LED is pulsed on for 10.5 ms every
1 second.
3.5 Interconnect
The I/O pin provides a capability common to many
detectors in a single system. If a single unit goes into
alarm, the I/O pin is driven high. This high signal
causes the interconnected units to alarm. The LED
flashes every 1s for 10.5 ms on the signaling unit and
is inhibited on the units that are in alarm due to the I/O
signal. An internal sink device on the I/O pin helps to
discharge the interconnect line. This charge dump
device is active for 1 clock cycle after the unit exits the
alarm condition (1.67s).
The interconnect input has a 500 ms nominal digital
filter. This allows for interconnection to other types of
alarms (carbon monoxide for example) that may have
a pulsed interconnect signal.
3.6 Testing
At power-up all internal registers are reset. The low
battery set point can be tested at power-up by holding
FEED and OSCAP low at power-up. HB will change
state as V
DD
passes through the low battery set point.
By holding the OSCAP pin low, the internal power
strobe is active. Functional testing can be accelerated
by driving OSCAP with a 4 kHz square wave; however,
the 10.5 ms strobe period must be maintained for
proper operation of the analog circuitry. Refer to
Figure 3-1 timing diagram.
3.7 Timer Mode
The transition of the TSTART pin from a high to low
level initiates a timer period (10 minutes maximum for
RE46C162, and 1 minute maximum for RE46C163).
During this timer period, the open drain NMOS on the
TSTROBE pin is strobed simultaneously with the inter-
nal clock. A resistor connected to this pin and the
VSEN pin is used to modify the detector sensitivity for
the timer period.
During the timer period, the LED flashes for 10.5 ms
every 10 seconds.
If the smoke level exceeds the reduced sensitivity set
point during the timer period, the unit will go into a local
alarm condition, the horn will sound and the timer mode
is cancelled. If an external only alarm occurs during the
timer mode, the timer mode is cancelled.
If the test button is pushed in a standby, reduced
sensitivity mode, the unit is tested normally. Upon
release of the test button, the timer mode counter is
reset and restarted.
