A3280, A3281
and A3283
Chopper-Stabilized, Precision
Hall-Effect Latches
FUNCTIONAL DESCRIPTION
Chopper-Stabilized Technique. The Hall element can be
considered as a resistor array similar to a Wheatstone bridge. A
large portion of the offset is a result of the mismatching of these
resistors. These devices use a proprietary dynamic offset cancel-
lation 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
mismatching 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 maintaing 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
processed using low-offset bipolar circuitry. This technique
produces devices that have an extremely stable quiescent Hall
output voltage, 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 low (turns
on) when a magnetic field perpendicular to the Hall element
exceeds the operate point threshold (BOP). After turn-on, the
output is capable of sinking 25 mA and the output voltage is
VOUT(SAT). Note that the device latches; that is, a south pole of
sufficient strength towards the branded surface of the device
will turn the device on; removal of the south pole will leave the
device on. When the magnetic field is reduced below the release
point (BRP), the device output goes high (turns off). The differ-
ence in the magnetic operate and release points is the hysteresis
(Bhys) of the device. This built-in hysteresis allows clean switch-
ing of the output even in the presence of external mechanical
vibration and electrical noise.
Powering up in the absence of a magnetic field (less than
BOP and higher than BRP) will allow an indeterminate output
state. The correct state is warranted after the first excursion
beyond BOP or BRP.
It is strongly recommended that an external bypass capacitor
Allegro MicroSystems, Inc.
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1.508.853.5000; www.allegromicro.com