Two-Wire, Zero Speed

Differential Gear Tooth Sensor IC

ATS685LSH

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Functional Description

Sensing Technology

The ATS685 sensor IC contains a single-chip differential Hall-

effect circuit, a samarium cobalt pellet, and a flat ferrous pole

piece (a precisely-mounted magnetic field concentrator that

homogenizes the flux passing through the Hall chip). As shown in

figure 1, the circuit supports two Hall elements, which sense the

magnetic profile of the ferromagnetic gear target simultaneously,

but at different points (spaced at a 2.2 mm pitch), generating a

precise switching of the digital output signal.

The Hall IC is self-calibrating and also integrates a tempera-

ture compensated amplifier and offset cancellation circuitry. Its

voltage regulator provides supply noise rejection throughout the

operating voltage range. Changes in temperature do not greatly

affect this device due to the stable amplifier design and the offset

rejection circuitry. The Hall transducers and signal processing

electronics are integrated on the same silicon substrate, using a

proprietary BiCMOS process.

Target Profiling During Operation

Under normal operating conditions, the IC is capable of provid-

ing digital information that is representative of the mechanical

features of a rotating gear. The waveform diagram in figure 2

presents the automatic translation of the mechanical profile,

through the magnetic profile that it induces, to the digital output

signal of the ATS685. No additional optimization is needed

and minimal processing circuitry is required. This ease of use

reduces design time and incremental assembly costs for most

applications.

Diagnostics

The regulated current output is configured for two-wire applica-

tions, requiring one less wire for operation than do switches with

the traditional open-collector output. Additionally, the system

designer inherently gains diagnostics because there is always

output current flowing, which should be in either of two nar-

current level not within these ranges indicates a fault condition.

a general fault condition.

Target (Gear)

Back-biasing

Rare-earth Pellet

South Pole

North Pole

Case

(Pin 1 Side)

(Pin 4 Side)

Hall IC

Pole Piece

Element Pitch

(Concentrator)

Dual-Element

Hall Effect Device

Hall Element 1

Hall Element 2

Figure 1. Relative motion of the target is detected by the dual Hall

elements mounted on the Hall IC.

Figure 2. The magnetic profile reflects the geometry of the target, allowing

the ATS685 to present an accurate digital output response.

B

OP(#1)

B

RP(#1)

B

OP(#2)

Off

On

On

Off

Device Internal Switch State

Device Package Orientation to Target

(Pin 1 Side)

(Pin 4 Side)

IC

Element Pitch

Hall Element 1

Hall Element 2

Device Branded Face

Target Magnetic Profile

+B

+t

+t

Mechanical Position (Target moves past sensor pin 1 to pin 4)

Target

(Gear)

This tooth

sensed earlier

This tooth

sensed later

(View of Side

Away from Pins)

0

+mA





Short

Fault

Open

Figure 3. Diagnostic characteristics of supply current values.