01-12-2012, 04:29 PM
Hall Effect Latch / Bipolar Switch
1Hall Effect Latch.pdf (Size: 359.79 KB / Downloads: 18)
Description
The A1250 Hall-effect sensor IC is a temperature stable,
stress-resistant bipolar switch. This device is the most sensitive
Hall-effect device in the Allegro® bipolar switch family and is
intended for ring-magnet sensing. Superior high-temperature
performance is made possible through an Allegro patented
dynamic offset cancellation that utilizes chopper-stabilization.
This method reduces the offset voltage normally caused by
device overmolding, temperature dependencies, and thermal
stress.
The A1250 includes the following on a single silicon chip:
a voltage regulator, Hall-voltage generator, small-signal
amplifier, chopper stabilization, Schmitt trigger, and a short
circuit protected open-drain output. Advanced BiCMOS
wafer fabrication processing takes advantage of low-voltage
requirements, component matching, very low input-offset
errors, and small component geometries.
The A1250 Hall-effect bipolar switch turns on in a south
polarity magnetic field of sufficient strength and switches
off in a north polarity magnetic field of sufficient strength.
Because the output state is not defined if the magnetic field
is diminished or removed, to ensure that the device switches,
Allegro recommends using magnets of both polarities and of
sufficient strength in the application.
Functional Description
The output of this device switches low (turns on) when a magnetic
field perpendicular to the Hall sensor IC exceeds the operate point
threshold, BOP . After turn-on, the output voltage is VOUT(SAT) .
The output transistor is capable of sinking current up to the short
circuit current limit IOM , which is a minimum of 30 mA. When
the magnetic field is reduced below the release point, BRP , the
device output goes high (turns off). The difference in the magnetic
operate and release points is the hysteresis, BHYS , of the device.
This built-in hysteresis allows clean switching of the output even
in the presence of external mechanical vibration and electrical
noise.
Given the magnetic parameter specifications (refer to Magnetic
Characteristics table), bipolar switches will operate in one of three
modes, depending on switchpoints. For typical values of BOP and
BRP , the device will operate as a latch, as shown in figure 1a.
Note that, when the magnetic flux density exceeds a switchpoint,
the output will retain its state when the magnetic field is removed.
Chopper Stabilization Technique
When using Hall-effect technology, a limiting factor for
switchpoint accuracy is the small signal voltage developed across
the Hall element. This voltage is disproportionally small relative
to the offset that can be produced at the output of the Hall sensor
IC. This makes it difficult to process the signal while maintaining
an accurate, reliable output over the specified operating
temperature and voltage ranges. Chopper stabilization is a unique
approach used to minimize Hall offset on the chip. Allegro
employs a patented technique to remove key sources of the output
drift induced by thermal and mechanical stresses. This offset
reduction technique is based on a signal modulation-demodulation
process. The undesired offset signal is separated from the
magnetic field-induced signal in the frequency domain, through
modulation. The subsequent demodulation acts as a modulation
process for the offset, causing the magnetic field-induced signal
to recover its original spectrum at base band, while the DC offset
becomes a high-frequency signal.