Chopper-Stabilized, Precision Hall-Effect Switch
A3340
7
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Dwg. EH-012
REG
X
SAMPLE
& HOLD
+V
CC
B
–V
HALL
+V
HALL
+B
0
FLUX DENSITY
0
B
CC
V
B
+V
OPRP
OUTPUT VOLTAGE
V
OUT(SAT)
SUPPLY
OUTPUT
Dwg. EH-013
0.1 μF
SUPPLY
X
PTCT
VCC
GND
VOUT
Chopper-Stabilized Technique. The Hall element can be con-
sidered as a resistor array similar to a Wheatstone bridge. A large
portion of the offset is a result of the mismatching of these resis-
tors. These devices use a proprietary dynamic offset cancellation
technique, with an internal high-frequency clock to reduce the
residual offset voltage of the Hall element that is normally caused
by device overmolding, temperature dependencies, and thermal
stress. The chopper-stabilizing technique cancels the mismatch-
ing of the resistor circuit by changing the direction of the current
flowing through the Hall plate using CMOS switches and Hall
voltage measurement taps, while maintaining the Hall-voltage
signal that is induced by the external magnetic flux. The signal
is then captured by a sample-and hold circuit and further pro-
cessed using low-offset bipolar circuitry. This technique produces
devices that have an extremely stable quiescent Hall output volt-
age, are immune to thermal stress, and have precise recoverability
after temperature cycling. This technique will also slightly
degrade the device output repeatability. A relatively high sampling
frequency is used in order that faster signals can be processed.
More detailed descriptions of the circuit operation can be found
in: Technical Paper STP 97-10, Monolithic Magnetic Hall Sensing
Using Dynamic Quadrature Offset Cancellation and Technical
Paper STP 99-1, Chopper-Stabilized Amplifiers With A Track-and-
Hold Signal Demodulator. Operation. The output of these devices
switches high (turns off) when a magnetic field (south pole) per-
pendicular to the Hall element exceeds the operate point thresh-
old (B
OP
). When the magnetic field is reduced below the release
point (B
RP
), the device output goes low (turns on). After turn-on,
the output is capable of sinking 25 mA and the output voltage is
V
OUT(SAT)
. The difference in the magnetic operate and release
points is the hysteresis (B
hys
) of the device. This built-in hyster-
esis allows clean switching of the output even in the presence of
external mechanical vibration and electrical noise. Applications.
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.
The simplest form of magnet that will operate these devices is a
ring magnet. Other methods of operation, such as linear magnets,
are possible. Extensive applications information on magnets and
Hall-effect devices is also available in Application Note 27701, or
at www.allegromicro.com.
FUNCTIONAL DESCRIPTION
