Micropower Ultra-Sensitive Hall-Effect Switches

A3214

7

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

FUNCTIONAL DESCRIPTION

Low Average Power

Internal timing circuitry activates the IC for 60 µs and deac-

tivates it for the remainder of the period (240 µs (typ) for the

A3213 and 60 ms (typ) for the A3214). A short "awake" time

allows for stabilization prior to the sampling and data latching

on the falling edge of the timing pulse. The output during the

"sleep" time is latched in the last sampled state. The supply cur-

rent is not affected by the output state.

Chopper-Stabilized Technique

The Hall element can be considered as a resistor array similar to

a Wheatstone bridge. A large portion of the offset is a result of

the mismatching of these resistors. These devices use a propri-

etary dynamic offset cancellation technique, with an internal

high-frequency clock to reduce the residual offset voltage of the

Hall element that is normally caused by device overmolding,

temperature dependencies, and thermal stress. The chopper-

stabilizing technique cancels the mismatching of the resistor

circuit by changing the direction of the current flowing through

the Hall plate using CMOS switches and Hall voltage measure-

ment taps, while maintaing the Hall-voltage signal that is induced

by the external magnetic flux. The signal is then captured by a

sample-and-hold circuit and further processed using low-offset

bipolar circuitry. This technique produces devices that have an

extremely stable quiescent Hall output voltage, are immune to

thermal stress, and have precise recoverability after temperature

cycling. This technique will also slightly degrade the device out-

put repeatability. A relatively high sampling frequency is used in

order that faster signals can be processed.

More detailed descriptions of the circuit operation can be found

in Technical Paper STP 97-10, Monolithic Magnetic Hall Sensing

Using Dynamic Quadrature Offset Cancellation and Technical

Paper STP 99-1, Chopper-Stabilized Amplifiers With A Track-

and-Hold Signal Demodulator.

Operation

The output of this device switches low (turns on) when a mag-

netic field perpendicular to the Hall element exceeds the operate

off). The difference in the magnetic operate and release points is

clean switching of the output even in the presence of external

mechanical vibration and electrical noise.

As used here, negative flux densities are defined as less than zero

(algebraic convention) and -50 G is less than +10 G.

0

PERIOD

60 µs "AWAKE"

"SLEEP"

SAMPLE &

OUTPUT LATCHED

Dwg. EH-012-1

X

+V

+V

HALL

VOLTAGE

B

+

—

Dwg. AH-011-2