3212
MICROPOWER,
ULTRA-SENSITIVE
HALL-EFFECT SWITCHES
FUNCTIONAL DESCRIPTION
Low Average Power.
Internal timing circuitry activates the
sensor for 45
µs
and deactivates it for the remainder of the
period (45 ms). A short "awake" time allows for stabilization
prior to the sensor sampling 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.
PERIOD
"AWAKE"
+V
SAMPLE
& HOLD
X
I
DD(EN)
"SLEEP"
SAMPLE &
OUTPUT LATCHED
Dwg. EH-012-1
I
DD(DIS)
0
Dwg. WH-017-2
B
+V
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
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 dependen-
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 temperature
cycling. A relatively high sampling frequency is used for faster
signal processing capability can be processed.
More detailed descriptions of the circuit operation can be
found in: Technical Paper STP 97-10,
Monolithic Magnetic
Hall Sensor Using Dynamic Quadrature Offset Cancellation
and Technical Paper STP 99-1,
Chopper-Stabilized Amplifiers
With A Track-and-Hold Signal Demodulator.
—
HALL
VOLTAGE
+
Dwg. AH-011-2
Operation.
The output of this device switches low (turns on)
when a magnetic field perpendicular to the Hall sensor 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.
As used here, negative flux densities are defined as less
than zero (algebraic convention) and -50 G is less than +10 G.
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5
SANKEN [ SANKEN ELECTRIC ]
