True Zero-Speed High-Accuracy

Gear Tooth Sensor IC

ATS668LSM

9

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

FUNCTIONAL DESCRIPTION

Sensing Technology

The ATS668 contains a single-chip differential Hall-effect sensor

IC and a back-biasing pellet. The Hall IC supports a pair of 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 differential internal analog volt-

output signal. The ATS668 is intended for use with ferromagnetic

targets.

The Hall IC is self-calibrating and also possesses 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

compensation circuitry. The Hall transducers and signal process-

ing electronics are integrated on the same silicon substrate, using

a proprietary BiCMOS process

Target Profiling During Operation

An operating device is capable of providing digital information

that is representative of the mechanical features of a rotating

gear. The waveform diagram in Figure 7 presents the automatic

translation of the mechanical profile, through the magnetic profile

that it induces, to the digital output signal of the IC. No addi-

tional optimization is needed and minimal processing circuitry is

required. This ease of use reduces design time and incremental

assembly costs for most applications.

Determining Output Signal Polarity

In Figure 7, the top panel, labeled Mechanical Position, repre-

sents the mechanical features of the target gear and orientation to

the device. The bottom panel, labeled IC Output Signal, displays

the square waveform corresponding to the digital output signal

that results from a rotating gear configured as shown in Figure 6

and electrically connected as in Figure 1. That direction of rota-

tion (of the gear side adjacent to the package face) is: perpen-

dicular to the leads, across the face of the device, from the pin 1

side to the pin 3 side. This results in the IC output switching from

low state to high state as the leading edge of a tooth (a rising

mechanical edge, as detected by the IC) passes the package face.

In this configuration, the device output switches to its high polar-

ity when a tooth is the target feature nearest to the package. If the

direction of rotation is reversed, so that the gear rotates from the

pin 3 side to the pin 1 side, then the output polarity inverts. That

is, the output signal goes high when a falling edge is detected,

and a valley is nearest to the package.

On

Off

Off

On

IC Internal Switch State

Package Orientation to Target

Mechanical Position (Target movement pin 1 to pin 3)

Target

(Gear)

(Package Top View)

Sensor Branded Face

Pin 1

Side

Pin 3

Side

Branded Face

Hall Element Pitch

Target Magnetic Profile

+B

This tooth

sensed earlier

This tooth

sensed later

Back-Biasing

Pellet

IC

Rotation from pin 1 to pin 3

Pin 1

Pin 3

Rotating Target

(Ferromagnetic)

Rotating Target

(Ferromagnetic)

Rotation from pin 3 to pin 1

Pin 1

Pin 3

Branded Face

of SM Package

Branded Face

of SM Package

Figure 6: Sensor and target configuration. The output

is low when a tooth of the target gear is nearest the

branded face of the package.

Figure 7: The magnetic profile reflects the features of

the target, allowing the sensor IC to present an accu-

rate digital representation of the target teeth.