Chopper-Stabilized, Precision Hall-Effect Latches
for Consumer and Industrial Applications
A3290 and
A3291
5
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
FUNCTIONAL DESCRIPTION
Amp
Regulator
Low-
Pass
Filter
Sample and Hold
Figure 2: Chopper Stabilization Circuit (Dynamic
Quadrature Offset Cancellation)
Chopper-Stabilized Technique
The Hall element can be considered as a resistor array similar
to a Wheatstone bridge. A basic circuit is shown in Figure 1,
demonstrating the effect of the magnetic field flux density (B)
impinging on the Hall element. When using Hall-effect tech-
nology, a limiting factor for switchpoint accuracy is the small
signal voltage (V
HALL
) developed across the Hall element. This
voltage is disproportionally small relative to the offset that can
be produced at the output of the Hall device, caused by device
overmolding, temperature dependencies, and thermal stress.
A large portion of the offset is a result of the mismatching of
these resistors. The A3290 and A3291 use a dynamic offset
cancellation technique, with an internal high-frequency clock,
to reduce the residual offset. The chopper-stabilizing technique
cancels the mismatching of the resistor circuit by changing the
direction of the current flowing through the Hall element (refer
to Figure 2). To do so, CMOS switches and Hall voltage mea-
surement taps are used, while maintaining V
HALL
signal that is
induced by the external magnetic flux.
The signal is then captured by a sample-and-hold circuit and fur-
ther 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 to process faster signals.
More detailed descriptions of the circuit operation can be found on
the Allegro website, including: Technical Paper STP 97-10, Mono-
lithic 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 outputs of the A3290 and A3291 switch low (turn on) when
a magnetic field perpendicular to the Hall element transitions
through and exceeds the Operate Point threshold (B
OP
). This
is illustrated in Figure 3. After turn-on, the output is capable of
sinking 25 mA, and the output voltage reaches V
OUT(SAT)
.
Note that these devices latch; that is, after a south (+) polarity
magnetic field of sufficient strength impinging on the branded
face of the device turns on the device, the device remains on
until the magnetic field is reduced below the Release Point
threshold (B
RP
). At that transition, the device output goes high
(turns off). 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.
When the devices are powered on, if the ambient magnetic field
has an intensity that is between B
OP
and B
RP
, the initial output
state is indeterminate. The first time that the level of B either
rises through B
OP
, or falls through B
RP
, however, the correct
output state is obtained.
B
OP
B
RP
B
HYS
V
OUT(off)
V
OUT
V
OUT(on)(sat)
Switch to Low
Switch to High
B+
V+
Hysteresis of ∆V
OUT
Switching Due to ∆B
Figure 3: Output Voltage Responds to Sensed Mag-
netic Flux Density
Figure 1: Hall Element, Basic Circuit Operation
+V
CC
–V
HALL
+V
HALL
