Micropower Ultra-Sensitive Hall-Effect Switches
A3213 and
A3214
7
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
FUNCTIONAL DESCRIPTION
Low Average Power. Internal timing circuitry activates the
IC for 60 μs and deactivates it for the remainder of the period
(240 μs (typ) for the A3213 and 60 ms (typ) for the A3214). A
short "awake" time allows for stabilization prior to the sam pling
and data latching on the falling edge of the timing pulse. The
output during the "sleep" time is latched in the last sampled
state. The supply current is not affected by the output state.
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 de pen den cies, 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 tem per a ture 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 this device 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 dif fer ence in the mag net ic 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 pres-
ence of external mechanical vibration and electrical noise.
As used here, negative flux densities are defined as less than
zero (algebraic convention) and -50 G is less than +10 G.
0
PERIOD
60 μs "AWAKE"
"SLEEP"
IDD(EN)
IDD(DIS)
SAMPLE &
OUTPUT LATCHED
Dwg. EH-012-1
SAMPLE
& HOLD
X
+V
+V
HALL
VOLTAGE
B
+
—
Dwg. AH-011-2
