Chopper Stabilized Precision Hall Effect Switches

A1120, A1121

and A1125

9

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

A112x

VOUT

GND

0.1 μF

VCC

Output

Operation

The output of the A1120 and A1121 devices switches low (turns

on) when a magnetic field perpendicular to the Hall element

1). When the magnetic field is reduced below the release point,

A1125 devices switches high (turns off) when a magnetic field

perpendicular to the Hall element exceeds the operate point

low (turns on).

sistor is capable of sinking current up to the short circuit current

The difference in the magnetic operate and release points is the

clean switching of the output even in the presence of external

mechanical vibration and electrical noise. Powering-on the device

give an indeterminate output state. The correct state is attained

Applications

It is strongly recommended that an external bypass capacitor be

connected (in close proximity to the Hall element) between the

supply and ground of the device to reduce external noise in the

application. As is shown in panel B of figure 1, a 0.1 μF capacitor

is typical.

Extensive applications information for Hall effect devicers is

available in:

• Hall-Effect IC Applications Guide, Application Note 27701

• Guidelines for Designing Subassemblies Using Hall-Effect

Devices, Application Note 27703.1

• Soldering Methods for Allegro’s Products – SMT and Through-

Hole, Application Note 26009

All are provided in Allegro Electronic Data Book, AMS-702, and

the Allegro Web site, www.allegromicro.com.

Figure 1. Device switching behavior. In panels A and B, on the horizontal axis, the B+ direction indicates increasing south polarity magnetic field strength.

This behavior can be exhibited when using an electrical circuit such as that shown in panel C.

(A)

(B)

(C)

Functional Description

B+

V+

0

0

B+

V+

0

0