Hall-effect sensor switching
for low-power applications
Allegro MicroSystems has introduced a new Hall-effect
switch, the A3210, which meets the demand for an
accurate, small and relatively low-cost devices that
offers the added benefit of consuming minimal power.
The key design requirements for modern consumer electronic equipment
can be summarised as high reliability, small size, low cost, and low power
consumption. One result of these requirements is that product development
engineers have had to look at alternatives to traditional electromechanical
switching devices.
Because mechanical switches (such as reed switches) tend to have low
reliability without offering cost or packaging advantages, the consumer
electronics industry has been forced to re-evaluate their long-term
viability. One result is an increased interest in Hall-effect switches to
provide a low-cost, reliable solution. Because Hall-effect sensors have no
electromechanical contacts, one of the main causes of unreliability in
traditional switches is removed. In fact, Hall-effect devices typically
outlive the product in which they are used, making them reliable substitutes
for mechanical switches. The new Hall-effect device described in this
article meets the demand for an accurate, small and relatively low-cost
devices that offers the added benefit of consuming minimal power.
Accuracy and sensitivity
The new A3210 device is fabricated using an advanced BiCMOS process,
which allows component matching with low-input offset errors and provides
small geometry chopping and logic circuitry. The ultrasensitive Hall-effect
switch allows for accurate, low magnetic switch-points over a wide range of air
gaps. A patented chopper stabilisation technique reduces offset drift caused
by temperature and stress, allowing for sensitive and stable switch points.
The typical offset drift is ±5 Gs. A switch-point specification of less than
50 Gs means that a small magnet is sufficient for proper operation at air
gaps between two and four times greater than the typical operating air gap
of other Hall-effect switches.
The new device also operates on low power, with a typical current
consumption of only 12 µA at 3 V, compared with 5 mA at 12 V for earlier
Hall-effect switches. The device is a single-chip, chopper-stabilised,
unipolar switch with an on-chip oscillator that maintains a 0.1% duty cycle
for 'on' time and latched open-drain output. The signal is sampled every 60
ms, with an 'awake' time of 60 µs for sampling. The output of the
Hall-effect sensor during sample time is latched and held until the next
sample occurs. This technique allows for an average supply current of 12 µA,
making the device a perfect fit for products that require low-power
consumption.
Pole-independent sensing
The design of the new device offers another advantage by using a novel
pole-independent sensing technique. This makes it an easy 'drop-in'
replacement for reed switches. The pole-independent chip orientation allows
for operation with either a north or south pole magnet, making the device
easier to manufacture. The new technology gives the same output polarity for
either pole. A north pole is typically thought of as a negative field, and a
south pole is typically a positive field.
Applications
Hall effect sensors provide an electrical voltage when excited by a
perpendicular magnetic field. Most Hall-effect devices have their own signal
conditioning circuitry. Both analogue and digital output Hall devices are
available; their specifications are focused on measuring various magnetic
configurations.
One such configuration is in 'flip top' mobile telephones. A typical
open-or-closed detection scheme is shown in Fig.1. The magnet orientation
(i.e. the pole facing the sensor face) is typical for Hall-effect sensors.
Because of the new device's pole-independent sensing technique, however,
the device will work with a reed switch magnet with a 90° orientation from a
typical Hall-effect magnet.
The device does require a regulated power supply to protect against
transients on the supply line. The external circuitry required, however, is
minimal.
The switch is available in two packages: a SOHED package for surface-mount
applications, and a single in-line package for through-hole mounting.
Picture. This mobile telephone's 'open or closed' detection scheme uses the
A3210 sensor. A switch point of below 50 Gs allows a small magnet to deliver
proper operation at air gaps 2-4 times greater than the typical operating
air gap of other Hall-effect switches
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Article - by Dominikus Maisl, Allegro MicroSystems Europe
June 2000
