3425 Datasheet, PDF (8/12 Pages) Allegro MicroSystems – DUAL, CHOPPER-STABILIZED, ULTRA-SENSITIVE, BIPOLAR HALL-EFFECT SWITCH

3425
DUAL, CHOPPER-STABILIZED,
ULTRA-SENSITIVE,
BIPOLAR HALL-EFFECT SWITCH
APPLICATIONS INFORMATION
+B
-B
+B BOP2
-B
OUTE1
BOP1
BRP2
BRP1
OUTE2
Dwg. WH-012-1
Quadrature/Direction Detection. External logic
circuitry can be used to determine the direction of move-
ment of the magnetic field across the face of the package.
For the direction signal to be appropriately updated, a
quadrature relationship must be maintained between the
ring magnet pole width*, the sensor-to-sensor spacing,
and, to a lesser extent, the magnetic switch points. For
optimal design, the sensor should be actuated with a ring
magnet pole width* two times the sensor-to-sensor
spacing. This will produce a sinusoidal magnetic field
whose period (denoted as Τ) is then four times the sensor-
to-sensor spacing. A quadrature relationship can also be
maintained for a ring magnet that has a period that satis-
fies the relationship nΤ/4 = 1.0 mm, where n is any odd
integer. Therefore, ring magnets with pole-pair spacings
equal to 4 mm (n = 1), 1.3 mm (n = 3), etc. are permitted.
The response of the device to the magnetic field
produced by a rotating ring magnet is shown above. Note
the desired phase shift between the two integrated sensors.
Over-Current Protection. Current through each output
transistor is sensed with a low-value (<1 Ω) on-chip
aluminum resistor. The voltage drop across this resistor is
fed back to control the base drive of that output. This
feedback prevents the output transistor from exceeding its
maximum current-density rating by limiting the output
current to between 30 mA and 55 mA. In this mode, the
device will come out of saturation and dissipate an in-
creased amount of power
P_D = V_OUT x I_OM
where V_OUT = V_CC - (I_OM x R_L)
and the output transistor will be thermally stressed. This
stress, unless protected against, will cause the device
junction temperature to rise until it fails catastrophically.
Over-current limiting is intended to protect the device
from transient overloads and should not imply that output
short circuits are permitted.
* “Pole” refers to a single pole (North or South)
unless stated as “pole pair” (North and South).
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