Two-Wire High Precision Linear Hall-Effect Sensor IC
With Pulse Width Modulated Output Current
A1357
17
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The calibration mode is provided so that the user can compensate
for differences in the ground potential between the A1357 and
any interface circuitry used to measure the pulse width of the
A1357 current. The test mode is optional and must be enabled
by blowing programming bits. After the bit for the test mode has
been blown, the device enters 50% Duty Cycle Calibration Test
mode every time the device is powered-up. The bit enabling test
mode is key 3, bit 4.
In customer applications, the PWM interface circuitry (shown
as the system controller in figure 11) and the A1357 may be
powered via different power and ground circuits. As a result, the
ground reference for the A1357 may differ from the ground refer-
ence of the system controller. In some customer applications, this
ground difference can be as large as ±0.5 V.
Differences in the ground reference for the A1357 and the system
controller can result in variations in the threshold voltage used
to measure the duty cycle of the A1357. 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 wave-
form. This problem is shown in figure 12.
50% Duty Cycle Calibration Test Mode
PWM period
Duty Cycle
shorter than
expected
Duty Cycle at
expected duration
Duty Cycle longer
than expected
V
th
(high)
∆t
r
∆t
f
V
th
(centered)
V
th
(low)
2.5
3.5
1.5
Threshold Voltage,
V
th
(V)
Time
Figure 12. When the threshold voltage, V
th
, is correctly centered between V
th
(high) and V
th
(low) , the current duty cycle
accurately coincides with the applied magnetic field. If the threshold voltage is raised, the current duty cycle appears shorter
than expected. Conversely, if the threshold voltage is lowered, the current duty cycle is longer than expected.
GND1 GND2
System
Controller
GND
A1357
VCC
1
2
V
SUPPLY
R
SENSE
C
BYPASS
0.1 μF
Figure 11. In many applications the A1357 may be powered using a different ground reference
than the system controller. This may cause the ground reference for the A1357 (GND1) to differ
from the ground reference of the system controller (GND2) by as much as to ± 0.5 V.
