Micropower, Ultrasensitive
Hall-Effect Switches
A3211 and
A3212
8
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Operation
The output of the A3212 switches low (turns on) when a magnetic
field perpendicular to the Hall element exceeds the operate point
B
OPS
(or is less than B
OPN
). After turn-on, the output is capable
of sinking up to 1 mA and the output voltage is V
OUT(ON)
. When
the magnetic field is reduced below the release point B
RPS
(or
increased above B
RPN
), the device output switches high (turns
off). The difference in the magnetic operate and release points is
the hysteresis (B
hys
) of the device. This built-in hysteresis allows
clean switching of the output even in the presence of external
mechanical vibration and electrical noise. The A3211 functions in
the same manner, except the output voltage is reversed from the
A3212, as shown in the figures to the right.
As used here, negative flux densities are defined as less than zero
(algebraic convention), i.e., -50 G is less than +10 G.
Applications
Allegro's pole-independent processing technique allows for opera-
tion with either a north pole or south pole magnet orientation,
enhancing the manufacturability of the device. The state-of-the-
art technology provides the same output polarity for either pole
face.
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. This
is especially true due to the relatively high impedance of battery
supplies.
The simplest form of magnet that will operate these devices is
a bar magnet with either pole near the branded surface of the
device. Many other methods of operation are possible. Extensive
applications information for Hall-effect devices is available in:
• Hall-Effect IC Applications Guide, Application Note 27701;
• Hall-Effect Devices: Guidelines for Designing Subassemblies
Using Hall-Effect Devices, Application Note 27703.1;
• Soldering Methods for Allegro’s Products — SMD and Through-
Hole, Application Note 26009.
All are provided at
www.allegromicro.com
0
+B
0
-B
OUTPUT ON
OUTPUT ON
5 V
MAX
B
OPS
B
RPS
B
OPN
B
RPN
OUTPUT VOLTAGE
MAGNETIC FLUX
A3212
OUTPUT OFF
0
+B
0
-B
5 V
MAX
OUTPUT VOLTAGE
MAGNETIC FLUX
OUTPUT OFF
OUTPUT OFF
B
RPS
OPS
B
OPN
RPN
OUTPUT ON
B
B
OUTPUT
Dwg. EH-013-2
0.1 µF
SUPPLY
(3 V BATTERY)
50 k Ω
X
V
DD
1
2
3
