HMC6042 Datasheet, PDF (6/7 Pages) Honeywell Solid State Electronics Center – 2-Axis Magnetic Sensor Circuit

HMC6042
These resistive elements are aligned together to have a common sensitive axis (indicated by arrows on the pinouts) that
will provide positive voltage change with magnetic fields increasing in the sensitive direction. Because the output only is in
proportion to the one-dimensional axis (the principle of anisotropy) and its magnitude, additional sensor bridges placed at
orthogonal directions permit accurate measurement of arbitrary field direction. The combination of sensor bridges in two
and three orthogonal axis permit applications such as compassing and magnetometry.
The offset strap allows for several modes of operation when a direct current is driven through it. These modes are: 1)
Subtraction (bucking) of an unwanted external magnetic field, 2) null-ing of the bridge offset voltage, 3) Closed loop field
cancellation, and 4) Auto-calibration of bridge gain.
The set/reset strap can be pulsed with high currents for the following benefits: 1) Enable the sensor to perform high
sensitivity measurements, 2) Flip the polarity of the bridge output voltage, and 3) Periodically used to improve linearity,
lower cross-axis effects, and temperature effects.
Offset Strap
The offset strap is a spiral of metallization that couples in the sensor element’s sensitive axis. The offset strap measures
nominally 8 ohms, and requires 10mA for each gauss of induced field. The straps will easily handle currents to buck or
boost fields through the ±6 gauss linear measurement range, but designers should note the extreme thermal heating on
the die when doing so.
With most applications, the offset strap is not utilized and can be ignored. Designers can leave one or both strap
connections (Off- and Off+) open circuited, or ground one connection node. Do not tie both strap connections together to
avoid shorted turn magnetic circuits.
Set/Reset Strap
The set/reset strap is another spiral of metallization that couples to the sensor elements easy axis (perpendicular to the
sensitive axis on the sensor die. Each set/reset strap has a nominal resistance of 5 ohms with a nominal required peak
current of 500mA for reset or set pulses. With rare exception, the set/reset strap must be used to periodically condition the
magnetic domains of the magneto-resistive elements for best and reliable performance.
A set pulse is defined as a positive pulse current entering the S/R+ strap connection. The successful result would be the
magnetic domains aligned in a forward easy-axis direction so that the sensor bridge’s polarity is a positive slope with
positive fields on the sensitive axis result in positive voltages across the bridge output connections.
A reset pulse is defined as a negative pulse current entering the S/R+ strap connection. The successful result would be
the magnetic domains aligned in a reverse easy-axis direction so that sensor bridge’s polarity is a negative slope with
positive fields on the sensitive axis result in negative voltages across the bridge output connections.
Typically a reset pulse is sent first, followed by a set pulse a few milliseconds later. By shoving the magnetic domains in
completely opposite directions, any prior magnetic disturbances are likely to be completely erased by the duet of pulses.
For simpler circuits with less critical requirements for noise and accuracy, a single polarity pulse circuit may be employed
(all sets or all resets). With these uni-polar pulses, several pulses together become close in performance to a set/reset
pulse circuit. Figure 1 shows a quick and dirty manual pulse circuit for uni-polar application of pulses to the set/reset strap.
ASIC
Within the HMC6042, the application specific integrated circuit (ASIC) performs the set/reset strap drive and sensor
amplification functions. The ASIC has its positive power supply rails broken into VDD1 and VDD2 elements to supply the
sensors/amplifiers and set/reset driver respectively. The VDD1 rail with the sensors and amplifiers combined is designed
to permit power supply duty cycling to conserve battery energy when the circuit is not used. Both the sensors and
amplifiers are designed to stabilize within 1 millisecond after power-up to permit snapshot measurements and return to
sleep status. Either PNP or P-MOSFET devices can be used to switch VDD1 off and on. To best ensure minimal energy
consumption, place any supply decoupling capacitors outside of the switch transistor, and not across the VDD1 side of the
switch.
Set/Reset Strap Driver
To permit operation from 2.4 to 3.6 volt DC supplies, and provide the required 400mA peak current spikes on the sensor
set/reset straps; both a H-bridge driver circuit and capacitive charge pump are employed. Within the H-Bridge drive circuit
several totem-pole complementary MOSFET stages are used to buffer the low voltage logic input (S/R_IN) with the last
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