A1220, A1221,
A1222, and A1223
Chopper Stabilized Precision Hall Effect Latches
Functional Description
Operation
Applications
The output of these devices switches low (turns on) when a mag-
netic field perpendicular to the Hall element exceeds the operate
point threshold, BOP (see panel A of figure 1). After turn-on, the
output voltage is VOUT(SAT). The output transistor is capable of
sinking current up to the short circuit current limit, IOM, which is
a minimum of 30 mA. When the magnetic field is reduced below
the release point, BRP, the device output goes high (turns off).
The difference in the magnetic operate and release points is the
hysteresis, BHYS, of the device. This built-in hysteresis allows
clean switching of the output even in the presence of external
mechanical vibration and electrical noise.
It is strongly recommended that an external bypass capacitor be
connected (in close proximity to the Hall element) between the
supply and ground of the device to reduce both external noise
and noise generated by the chopper stabilization technique. As is
shown in panel B of figure 1, a 0.1 μF capacitor is typical.
Extensive applications information for Hall effect devices is
available in:
• Hall-Effect IC Applications Guide, Application Note 27701
• Guidelines for Designing Subassemblies Using Hall-Effect
Devices, Application Note 27703.1
Removal of the magnetic field will leave the device output
latched on if the last crossed switchpoint is BOP, or latched off if
the last crossed switch point is BRP.
• Soldering Methods for Allegro’s Products – SMT and Through-
Hole, Application Note 26009
Powering-on the device in the hysteresis range (less than BOP and
higher than BRP) will give an indeterminate output state. The cor-
All are provided in Allegro Electronic Data Book, AMS-702, and
rect state is attained after the first excursion beyond BOP or BRP
.
the Allegro Web site, www.allegromicro.com.
VS
V+
VCC
VCC
RL
A122x
CBYP
VOUT
0.1 μF
Output
GND
VOUT(SAT)
B+
0
B–
0
BHYS
(A)
(B)
Figure 1. Switching behavior of latches. In panel A, on the horizontal axis, the B+ direction indicates increasing south polarity magnetic field strength,
and the B– direction indicates decreasing south polarity field strength (including the case of increasing north polarity). This behavior can be exhibited
when using a circuit such as that shown in panel B.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
10
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com