8

A1201-DS, Rev. 2

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Continuous-Time Bipolar Switch Family

A1201, A1202, A1203, and A1204

Functional Description

Bipolar Device Switching

The devices of the A120X family provide highly sensitive

switching for applications using magnetic fields of alternating

polarities, such as ring magnets. There are three switching modes

for bipolar devices, referred to as latch, unipolar switch, and

negative switch. Mode is determined by the switchpoint charac-

teristics of the individual device. The characteristic hysteresis,

strength and polarity of the switchpoints of the device. (Note

that, in the following descriptions, a negative magnetic value

indicates a north polarity field, and a positive magnetic value

indicates a south polarity field. For a given value of magnetic

strength, but opposite polarity. B = 0 indicates the absence of a

magnetic field.)

Bipolar devices typically behave as latches. In this mode,

magnetic fields of opposite polarity and equivalent strengths

are needed to switch the output. When the magnetic fields are

removed (B

→ 0) the device remains in the same state until a

magnetic field of the opposite polarity and of sufficient strength

causes it to switch. The hysteresis of latch mode behavior is

shown in panel A of figure 1.

In contrast to latching, when a device exhibits unipolar switch-

ing, it only responds to a south magnetic field. The field must

back to the high state, as shown in panel B of figure 1. Devices

exhibiting negative switch behavior operate in a similar but

opposite manner. A north polarity field of sufficient strength,

Figure 1. Bipolar Device Output Switching Modes. These behaviors can be exhibited when using a circuit such as that shown in panel D. Panel A

such as the 120x family, can operate in any of the three modes.

V+

0

V+

0

V+

0

B+

B–

B+

B– 0

0

B+

B–

0

(A)

(B)

(C)

VCC

Sensor Output

GND

VOUT

A120x

(D)