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MC14568 Datasheet(PDF) 5 Page - Freescale Semiconductor, Inc

Part No. MC14568
Description  Low-Power CMOS Ionization Smoke Detector with Interconnect and Timer
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Maker  FREESCALE [Freescale Semiconductor, Inc]
Homepage  http://www.freescale.com
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MC14568 Datasheet(HTML) 5 Page - Freescale Semiconductor, Inc

 
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Freescale Semiconductor
5
MC14568
DEVICE OPERATION
In a NO SMOKE condition (i.e., standby mode with detect
pin 15 higher than internal reference pin 13), the oscillator
operates with a period of 1.67 s and the LED will flash once
every 24-clock periods (or every 40 s).
The smoke detect pin (pin 15) is connected to the
ionization chamber and leads to the inverting input of a
CMOS detect comparator. The voltage on this pin is
compared to an internal reference voltage on the non-
inverting input of the detect comparator set up by a resistor
divider network and a FET switch (this reference voltage can
be increased or decreased by an external resistor from pin 13
to VDD or VSS).
Periodically (every 1.67 s and for a 10 ms window) power
is supplied to the entire IC and a check is made for a SMOKE
condition. During this window the FET switch is turned on and
the detect pin voltage (pin 15) is compared to the internal
reference voltage (pin 13). If the detect pin is lower than the
reference this indicates a SMOKE condition.
A SMOKE condition speeds up the internal oscillator,
which now oscillates with an internal period of 40 ms (the
rising edge of the saw tooth periodic signal on pin 12 remains
the same while the falling edge falls faster in a SMOKE
condition).
Furthermore, a SMOKE condition causes the resistor
divider network that sets the smoke detect reference voltage
to change in such a way as to make the reference voltage
increase by approximately 130 mV (with no external
connections to pin 13 or to pin 3).
This provides hysteresis and reduces false triggering. This
hysteresis is turned off when the IC exits a SMOKE condition
and returns to standby mode.
Once in a SMOKE condition, the LED will flash every 24-
clock cycles (once every second). The horn (piezo electric
element) driver is activated during a smoke condition with a
typical ON/OFF time of 160 ms ON, 80 ms OFF.
During the off time, the IC re-checks to see if a SMOKE
condition still exists. If no smoke is detected the horn driver is
suppressed. During a SMOKE condition the low voltage
battery check is inhibited.
NO SMOKE AND LOW BATTERY CHECK
During a NO SMOKE condition (i.e., standby mode) the
LED is pulsed to load the battery every 24-clock cycles (every
40 seconds) and a fraction of the battery voltage (set
internally by a resistor divider network and pulsed “ON” by a
internal FET) is compared to an internal reference voltage set
by a reversed biased Zener diode by an internal CMOS
comparator. The fraction of the supply voltage (pin 3) to be
compared to the internal reference voltage for low battery
check can be raised or lowered by connecting an external
resistor between pin 3 and VDD or pin 3 and VSS. If low
battery is detected a warning “chirp” will be sounded by the
horn driver once every 24-clock cycles (a low battery
condition does NOT cause the oscillator to speed up as does
a SMOKE condition). Low battery check will still be performed
in reduced sensitivity mode and a low battery chirp will be
heard if VDD falls below specified value.
TIMER
An internal timer is provided that can be used in various
configurations to allow for a period of reduced smoke
detector sensitivity.


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