Specifications

Motor Control
Monitoring and measuring current for optimal motor control
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Current monitoring
Current is a common signal to be
sensed, monitored, and fed back
to the motor-control loop. Current-
sense amplifiers make it easier to
monitor the current into and out
of the system with a high level of
precision. If current-sense amplifiers
are used, no transducer is needed,
as the electrical signal itself is being
measured. Current-sense amplifiers
detect shorts and transients, and
they monitor power and reverse-
battery conditions.
Current measurement
There is a variety of techniques to
measure current, but by far the most
popular uses a current-sense resistor.
In this technique the voltage drop
across the current-sense resistor is
first amplified by an op amp set up
in a differential gain stage, and then
measured. Traditionally, this approach
has been implemented with discrete
components. However, discrete
solutions also introduce some dis-
advantages such as the requirement
for matched resistors, poor drift, and
a larger solution area. Fortunately,
these multiple and varied disad-
vantages can be overcome by
integrating current-sense ampli-
fiers into the design. Not only do
the amplifiers measure the current,
but they also sense the direction
of current, accommodate wide
common-mode ranges, and provide
more precise measurement.
Current measurement employs either
the low-side principle in which the
sense resistor connects in series with
the ground path, or the high-side
principle in which the sense resistor
connects in series with the hot wire. In
low-side measurement, the circuit has
a low-input common-mode voltage,
and the output voltage is ground
referenced. The low-side resistor
adds undesirable extraneous resis-
tance in the ground path. In high-side
measurement, the load is grounded,
but the high-side resistor must cope
with relatively large common-mode
signals. High-side sensing also allows
detection of fault conditions such
as the motor case or winding that
shorts to ground.
High-side current-sense amplifiers
like the MAX4080/MAX4081 employ
a current-sensing resistor placed
between the positive terminal of
the power supply and the supply
input of the monitored circuit. This
arrangement avoids extraneous resis-
tance in the ground plane, greatly
simplifies the layout, and generally
improves the overall circuit perfor-
mance. Maxim’s unidirectional and
bidirectional current-sense ICs like
the MAX9918/MAX9919/MAX9920 are
available with or without internal sense
resistors. This variety of parts adds
considerable flexibility to designs
and simplifies part selection for a wide
variety of ADCs and applications.
Monitoring and measuring current for optimal motor control