Two-Wire, Zero Speed
Differential Gear Tooth Sensor IC
ATS685LSH
10
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Power-On
The ATS685 is guaranteed to power-on in the high current state,
I
CC(High)
. When power (V
CC
> V
CC
(min) ) is applied to the
device, a short period of time is required to power the vari-
ous portions of the circuit. During this period, the ATS685 will
power-on in the high current state, I
CC(High)
.
Initial Edge Detection
The device self-calibrates using the initial teeth sensed, and then
enters running mode. This results in reduced accuracy for a brief
period, CAL
I
. However, this period allows the device to optimize
for running mode operation. As shown in figure 6, the first three
high peak signals corresponding to rising output edges are used to
calibrate AGC (Automatic Gain Control). There is a slight vari-
ance in the duration of initialization, depending on what target
feature is opposite the sensor IC when power-on occurs. Also, a
high speed of target rotation at power-on may increase the quan-
tity required in the CAL
I
period.
Figure 6. Power-On Initial Edge Detection. This figure demonstrates four typical power-on scenarios. All of these examples assume that the target is
moving relative to the sensor IC in the direction indicated (from pin 1 to pin 4) and the voltage output is configured for low-side sensing, V
OUT(Low)
. The
length of time required to overcome Start Mode Hysteresis, as well as the combined effect of whether it is overcome in a positive or negative direction
plus whether the next edge is in that same or opposite polarity, affect the point in time when AGC calibration begins. Three high peaks are always
required for AGC calibration when f
OP
d 200 Hz, and more may be required at greater speeds.
Target
(Gear)
I
CC
I
CC
I
CC
I
CC
I
CC
I
CC
I
CC
I
CC
Power-on
opposite
tooth
Power-on
at falling
mechanical
edge
Power-on
at rising
mechanical
edge
Power-on
opposite
valley
AGC Calibration
Running Mode
AGC Calibration
Running Mode
AGC Calibration
Running Mode
AGC Calibration
Running Mode
Device
Position
1
1
4
4
2
2
3
3
Start Mode
Hysteresis
Overcome
Start Mode
Hysteresis
Overcome
Start Mode
Hysteresis
Overcome
Start Mode
Hysteresis
Overcome