True Zero Speed Miniature Differential Peak-
Detecting Gear Tooth Sensor IC
ATS645LSH
8
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Hall Technology
The gear tooth sensor IC subassembly contains a single-chip
differential Hall effect sensor IC, an optimized samarium cobalt
pellet, and a at ferrous pole piece. The Hall IC supports two
Hall elements, which sense the magnetic pro le of the ferromag-
netic target simultaneously, but at different points (spaced at a
1.5 mm pitch), generating a differential internal analog voltage
(V
PROC
) that is processed for precise switching of the digital
output signal.
The Hall IC is self-calibrating and also possesses a tempera-
ture compensated ampli er 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 ampli er 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
An operating device is capable of providing digital information
that is representative of the mechanical features on a rotating
target. The waveform diagram shown in gure 3 presents the
automatic translation of the mechanical pro le, through the mag-
netic pro le that it induces, to the digital output signal of the IC.
Output Polarity
Figure 3 shows the output polarity for the orientation of target
and package shown in gure 2. The target direction of rota-
tion shown is: perpendicular to the leads, across the face of the
device, from the pin 1 side to the pin 4 side. This results in the
IC output switching from high, I
CC(High)
, to low I
CC(Low)
, as the
leading edge of a tooth (a rising mechanical edge, as detected by
the IC) passes the package face. In this con guration, the device
output current switches to its low polarity when a tooth is the
target feature nearest to the package. If the direction of rotation
is reversed, then the output polarity inverts.
Note that output voltage polarity is dependent on the position of
the sense resistor, R
SENSE
(see gure 4).
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
of Package
Rotating Target
Branded Face
1
4
Functional Description
Figure 1. Relative motion of the target is detected by the dual Hall ele-
ments mounted on the Hall IC.
Figure 2. This left-to-right (pin 1 to pin 4) direction of target rotation
results in a low output signal when a tooth of the target gear is nearest
the face of the package (see gure 3). A right-to-left (pin 4 to pin 1) rota-
tion inverts the output signal polarity.
Figure 4: Voltages pro les for high side and low side two-wire sensing.
Figure 3. Output Pro le of a ferrous target for the polarity indicated in
gure 2.
ATS645
VCC
GND
V
CC
I
CC
1
4
V
OUT(L)
ATS645
VCC
GND
V
CC
1
4
V
OUT(H)
I
CC
R
SENSE
R
SENSE
I
OUT
V
OUT(H)
V+
V
OUT(L)
V+
I+
Representative
Differential
Magnetic Profile
Target
Mechanical Profile
IC Electrical
Output Profile, I
OUT