A5348 查看數據表（PDF） - Allegro MicroSystems

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A5348

Ionization Smoke Detector with Interconnect and Timer

Allegro MicroSystems 

A5348

Ionization Smoke Detector

with Interconnect and Timer

ELECTRICAL CHARACTERISTICS1,2 (continued) at TA = 25°C, VDD = 9.0 V, VSS = 0 V, COSCCAP = 0.1 μF,

RTIMINGRES = 8.2 MΩ (unless otherwise noted)

Characteristic

Symbol

Test Conditions

Test Pin

Min.

Typ.

Max.

Units

I/O Current

IIOL

No alarm, VI/O = VDD – 2.0 V

2

IIOH Local alarm, VI/O = VDD – 2.0 V

2

25

—

–7.5

—

60

μA

—

mA

I/O Alarm Voltage

VIH(IO) External “alarm” in

2

3.0

—

—

V

I/O Delay

tr(io) Local or test alarm to I/O active

2

—

3.0

—

s

1Negative current is defined as coming out of the specified device pin (sourcing).

2Alarm (smoke) condition is defined as VDETECTIN < VSENSITIVITYSET; no alarm (no smoke) condition is defined as VDETECTIN > VSENSITIVITYSET.

Circuit Description

The A5348 is a low-current, BiCMOS circuit providing all of the

required features for an ionization-type smoke detector.

Oscillator

An internal oscillator operates with a period of 1.67 seconds

during no-smoke conditions. Every 1.67 seconds, internal power

is applied to the entire circuit for 10 ms and a check is made for

smoke. Every 24 clock cycles (approximately 40 seconds), the

LED pin is pulsed and a check is made for low battery by com-

paring VDD to an internal reference. Because very-low currents

are used in the device, the oscillator capacitor at the OSC CAP

pin should be a low-leakage type (PTFE, polystyrene, or polypro-

pylene).

Detector Circuitry

When the voltage on the DETECT IN pin is less than the volt-

age on the SENSITIVITY SET pin, the A5348 evaluates this as a

smoke condition. During a smoke condition, the resistor divider

network that sets the sensitivity (also referred to as the smoke trip

point) is altered to increase VSENSITIVITYSET by 130 mV typical

(with no external connections on the SENSITIVITY SET pin).

This provides hysteresis and reduces false triggering.

An active guard is provided on GUARD1 and GUARD2, the two

pins adjacent to the detector input, the DETECT IN pin. VGUARD1

and VGUARD2 will be within 100 mV of VDETECTIN. This will

keep surface leakage currents to a minimum and provide a

method of measuring the input voltage without loading the ion-

ization chamber. The active guard amplifier is not power strobed

and thus provides constant protection from surface leakage

currents. The detector input has internal diode protection against

electrostatic damage.

Alarm Circuitry

If smoke is detected, the oscillator period changes to 40 ms and

the horn is enabled. The horn output is typically 160 ms on,

80 ms off. During the off-time, smoke is checked and further

alarm output will be inhibited if smoke is not sensed. During a

smoke condition, the low-battery alarm is inhibited and the LED

is pulsed approximately once every second.

Sensitivity Adjust

The detector sensitivity to smoke is set internally by a voltage

divider connected between VDD and VSS. The sensitivity can,

however, be externally adjusted to the individual characteristics

of the ionization chamber by connecting a resistor between the

SENSITIVITY SET pin and either the VDD or VSS pins.

With no external connections on the SENSITIVITY SET pin,

while the A5348 is checking for smoke:

VSENSITIVITYSET = VDD / 2 .

To increase sensitivity, a resistor can be connected between

SENSITIVITY SET and VDD, with the value:

where

RSENSITIVITYSET = 1.1E6 × K / (1 – K) ,

K = VDD / VSENSITIVITYSET – 1 .

To decrease sensitivity, a resistor can be connected between

Allegro MicroSystems, Inc.

5

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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