Two-Wire, Zero Speed
Differential Gear Tooth Sensor IC
ATS685LSH
12
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Undervoltage Lockout
When the supply voltage falls below the minimum operating
voltage, V
CC(UV)
, I
CC
goes high and remains high regardless of
the state of the magnetic gradient from the target. This lockout
feature prevents false signals, caused by undervoltage conditions,
from propagating to the output of the device. Because V
CC
is
below the V
CC
(min) specification during lockout, the I
CC
levels
may not be within specification.
Power Supply Protection
The device contains an on-chip regulator and can operate over a
wide V
CC
range. For devices that need to operate from an unregu-
lated power supply, transient protection must be added externally.
For applications using a regulated line, EMI/RFI protection may
still be required. Contact Allegro for information on the circuitry
needed for compliance with various EMC specifications. Refer to
figure 8 for an example of a basic application circuit.
Automatic Gain Control (AGC)
This feature allows the device to operate with an optimal internal
electrical signal, regardless of the air gap (within the AG specifica-
tion). At power-on, the device determines the peak-to-peak ampli-
tude of the signal generated by the target. The gain is then auto-
matically adjusted. Figure 9 illustrates the effect of this feature.
Running Mode Gain Adjust
The ATS685 has a feature during Running mode to compensate
for dynamic air gap variation. If the system increases the mag-
netic input drastically, the device will gradually readjust the gain
downwards, allowing the chip to regain the optimum internal
electrical signal with the new, larger, magnetic signal.
Dynamic Offset Cancellation (DOC)
The offset circuitry when combined with AGC automatically
reduces the effects of chip, magnet, and installation offsets. This
circuitry is continuously active, including both Power-on mode
and Running mode, compensating for any offset drift (within
Allowable User-Induced Differential Offset). Continuous opera-
tion also allows it to compensate for offsets induced by tempera-
ture variations over time.
Running Mode Lockout
The ATS685 has a Running mode lockout feature to prevent
switching on small signals that are characteristic of vibration
signals. The internal logic of the chip evaluates small signal
amplitudes below a certain level to be vibration. In that event, the
output is blanked (locked-out) until the amplitude of the signal
returns to normal operating levels.
Watchdog
The ATS685 employs a watchdog circuit to prevent extended loss
of output switching during sudden impulses and vibration in the
system. If the system changes the magnetic input drastically such
that target feature detection is terminated, the device will fully
reset itself, allowing the chip to recalibrate properly on the new
magnetic input signal.
Mechanical Profile
AG
Small
AG
Large
AG
Small
AG
Large
Internal Differential
Analog Signal
Response, with AGC
Internal Differential
Analog Signal
Response, without AGC
Ferrous Target
V+
V+
Figure 9. Automatic Gain Control (AGC). The AGC function corrects for
variances in the air gap. Differences in the air gap cause differences in
the magnetic field at the device, but AGC prevents that from affecting
device performance, as shown in the lowest panel.
Figure 8. Typical circuit for proper device operation.
2
ATS685
1
3
4
0.01 MF (optional)
100 7
R
SENSE
C
LOAD
C
BYPASS
V
CC
