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MC14468 Datasheet(PDF) 5 Page - Freescale Semiconductor, Inc
FREESCALE [Freescale Semiconductor, Inc]
MC14468 Datasheet(HTML) 5 Page - Freescale Semiconductor, Inc
/ 9 page
The internal oscillator of the MC14468 operates with a
period of 1.67 seconds during no-smoke conditions. Each
1.67 seconds, internal power is applied to the entire IC and a
check is made for smoke, except during LED pulse, Low
Battery Alarm Chirp, or Horn Modulation (in smoke). Every 24
clock cycles a check is made for low battery by comparing
to an internal zener voltage. Since very small currents
are used in the oscillator, the oscillator capacitor should be of
a low leakage type.
If smoke is detected, the oscillator period becomes 40 ms
and the piezoelectric horn oscillator circuit is enabled. The
horn output is modulated 160 ms on, 80 ms off. During the off
time, smoke is again checked and will inhibit further horn
output if no smoke is sensed. During local smoke conditions
the low battery alarm is inhibited, but the LED pulses at a
1.0 Hz rate. In remote smoke, the LED is inhibited as well.
An active guard is provided on both pins adjacent to the
detect input. The voltage at these pins will be within 100 mV
of the input signal. This will keep surface leakage currents to
a minimum and provide a method of measuring the input
voltage without loading the ionization chamber. The active
guard op amp is not power strobed and thus gives constant
protection from surface leakage currents. Pin 15 (the Detect
input) has internal diode protection against static damage.
The I/O (Pin 2), in combination with VSS, is used to
interconnect up to 40 remote units for common signaling. A
Local Smoke condition activates a current limited output
driver, thereby signaling Remote Smoke to interconnected
units. A small current sink improves noise immunity during
non-smoke conditions. Remote units at lower voltages do not
draw excessive current from a sending unit at a higher
voltage. The I/O is disabled for three oscillator cycles after
power up, to eliminate false alarming of remote units when
the battery is changed.
Sensitivity/Low Battery Thresholds
Both the sensitivity threshold and the low battery voltage
levels are set internally by a common voltage divider (please
see Figure 2) connected between VDD and VSS. These
voltages can be altered by external resistors connected from
pins 3 or 13 to either VDD or VSS. There will be a slight
interaction here due to the common voltage divider network.
The sensitivity threshold can also be set by adjusting the
smoke chamber ionization source.
Since the internal op amps and comparators are power
strobed, adjustments for sensitivity or low battery level could
be difficult and/or time-consuming. By forcing Pin 12 to VSS,
the power strobing is bypassed and the output, Pin 1,
constantly shows smoke/no smoke. Pin 1 = VDD for smoke.
In this mode and during the 10 ms power strobe, chip current
rises to approximately 50
The 9-volt battery level is checked every 40 seconds
during the LED pulse. The battery is loaded via a 10 mA
pulse for 10 ms. If the LED is not used, it should be replaced
with an equivalent resistor such that the battery loading
remains at 10 mA.
When smoke is detected, the resistor/divider network that
sets sensitivity is altered to increase sensitivity. This yields
approximately 100 mV of hysteresis and reduces false
Figure 6. Typical Application as Ionization Smoke Detector
*Note: Component values may change
depending on type of piezoelectric horn used.
Related Electronics Part Number
Freescale Semiconductor, Inc
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