MPXM2102AS datasheet(650/670 Pages) MOTOROLA

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MC145017

4–58

Motorola Sensor Device Data

www.motorola.com/semiconductors

DEVICE OPERATION

TIMING

The internal oscillator of the MC145017 operates with a

period of 1.65 seconds during no–smoke conditions. Each

1.65 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

41.67 ms and the piezoelectric horn oscillator circuit is

enabled. The horn output is modulated 500 ms on, 500 ms

off. During the off time, smoke is again checked and will in-

hibit further horn output if no smoke is sensed. During smoke

conditions the low battery alarm is inhibited, but the LED

pulses at a 1.0 Hz rate.

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.

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 outputs, Pins 1 and

4, constantly show smoke/no smoke and good battery/low

battery, respectively. Pin 1 = VDD for smoke and Pin 4 = VDD

for low battery. 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 11.6 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.

Figure 5. Typical Application as Ionization Smoke Detector

MC145017

116

330

Ω

8.2 M

Ω

9 V

0.1

µF

1.5 M

Ω*

0.001

µF

220 k

Ω*

0.1

µF

1 M

TEST

*NOTE: Component values may change depending on type of piezoelectric horn used.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 650 Page - Motorola, Inc

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