Two-Wire, Zero Speed
Differential Gear Tooth Sensor IC
ATS685LSH
7
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 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
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 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-
row ranges, shown in figure 3 as I
CC(HIGH)
and I
CC(LOW)
. Any
current level not within these ranges indicates a fault condition.
If I
CC
> I
CC(HIGH)
(max), then a short condition exists, and if I
CC
< I
CC(LOW)
(min), then an open condition exists. Any value of I
CC
between the allowed ranges for I
CC(HIGH)
and I
CC(LOW)
indicates
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 OnOnOff
Device Internal Switch State
Device Package Orientation to Target
Device Internal Differential Analog Signal, V
PROC
Device Output Signal, I
CC
(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)
I
CC(HIGH)
(max)
I
CC(HIGH)
(min)
I
CC(LOW)
(max)
I
CC(LOW)
(min)
Range for Valid I
CC(HIGH)
Range for Valid I
CC(LOW)
0
+mA
Short
Fault
Open
Figure 3. Diagnostic characteristics of supply current values.
