MC14468 查看數據表（PDF） - Motorola => Freescale
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MC14468 Datasheet PDF : 8 Pages
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Freescale Semiconductor, Inc.
DEVICE OPERATION
MC14468
TIMING
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 Bat-
tery Alarm Chirp, or Horn Modulation (in smoke). Every 24
clock cycles a check is made for low battery by comparing
VDD to an internal zener voltage. Since very small currents
are used in the oscillator, the oscillator capacitor should be of
a low leakage type.
DETECT CIRCUITRY
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 out-
put 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 volt-
age without loading the ionization chamber. The active guard
op amp is not power strobed and thus gives constant protec-
tion from surface leakage currents. Pin 15 (the Detect input)
has internal diode protection against static damage.
INTERCONNECT
The I/O (Pin 2), in combination with VSS, is used to inter-
connect 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 volt-
age. The I/O is disabled for three oscillator cycles after power
up, to eliminate false alarming of remote units when the bat-
tery 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 1) connected between VDD and VSS. These volt-
ages can be altered by external resistors connected from
pins 3 or 13 to either VDD or VSS. There will be a slight inter-
action here due to the common voltage divider network. The
sensitivity threshold can also be set by adjusting the smoke
chamber ionization source.
TEST MODE
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 µA.
LED PULSE
The 9–volt battery level is checked every 40 seconds dur-
ing 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.
HYSTERESIS
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
triggering.
TO
OTHER
UNITS
0.1 µF
330 Ω
+ 8.2 MΩ
9V
1
16
MC14468
2
15
3
14
4
13
5
12
6
11
7
10
8
9
1M
1M
TEST
0.1 µF
*NOTE: Component values may change
depending on type of piezoelectric horn used.
1.5 MΩ*
0.001 µF
220 kΩ*
Figure 5. Typical Application as Ionization Smoke Detector
Motorola Sensor Device Data For More Information On This Product,
5
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