High Precision Linear Hall-Effect Sensor IC
With an Open Drain Pulse Width Modulated Output
A1356
18
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
In customer applications, the PWM interface circuitry (body
control module; BCM in figure 9) and the A1356 may be powered
via different power and ground circuits. As a result, the ground
reference for the A1356 may differ from the ground reference of
the BCM. In some customer applications this ground difference
can be as large as ?0.5 V. Differences in the ground reference for
the A1356 and the BCM can result in variations in the thresh-
old voltage used to measure the duty cycle of the A1356. If the
PWM conversion threshold voltage varies, then the duty cycle
will vary because there is a finite rise time (t
r
) and fall time (t
f
)
in the PWM waveform. This problem is shown in figure 10. The
A1356 Output Calibration mode is used to compensate for this
error in the duty cycle. While the A1356 is in Output Calibra-
tion mode, the device output waveform is a fixed 50% duty cycle
(the programmed quiescent duty cycle value) regardless of the
applied external magnetic field. After powering-up, the A1356
outputs its quiescent duty cycle waveform for 50 ms, regardless
of the applied magnetic field (see figure 11). This allows the
BCM to compare the measured quiescent duty cycle with an ideal
50% duty cycle. This test period allows end users to compensate
for any threshold errors that result from a difference in system
ground potentials.
Output Calibration Test Mode
PWM period
Duty Cycle
shorter than
expected
Duty Cycle at
expected duration
Duty Cycle longer
than expected
V
OUT(th)high
t
r
t
f
V
OUT(th)(centered)
V
OUT(th)(low)
2.5
3.5
1.5
Time
Figure 10. When the threshold voltage, V
OUT(th)
, is correctly centered between , V
OUT(th)high
and , V
OUT(th)low
, the duty cycle
accurately coincides with the applied magnetic field. If the threshold voltage is raised, the output duty cycle appears shorter
than expected. Conversely, if the threshold voltage is lowered, the output duty cycle is longer than expected.
GND
GND1
GND2
PWMOUT
BCM
A1356
VCC
1
2
3
V
REG
R
PU
C
L
Figure 9. In many applications the A1356 may be powered using a different ground
reference than the BCM. This may cause the ground reference for the A1356 (GND 1) to
differ from the ground reference of the BCM (GND 2) by as much as to ?0.5 V.