Texas Instruments | High-Side Motor Current Monitoring for Over-Current Protection (Rev. A) | Application notes | Texas Instruments High-Side Motor Current Monitoring for Over-Current Protection (Rev. A) Application notes

Texas Instruments High-Side Motor Current Monitoring for Over-Current Protection (Rev. A) Application notes
____________________________________________________
High-Side Motor Current Monitoring for Over-Current
Protection
Scott Hill, Current Sensing Products
High power, precision motor systems commonly
require detailed feedback such as speed, torque and
position to be sent back to the motor control circuitry in
order to precisely and efficiently control the motor’s
operation. Other motor control applications, such as
fixed motion tasks, do not require the same level of
system complexity in order to carry out their jobs.
Ensuring that the motor has not stalled or encountered
an unintended object in the motor's path or that a short
in the motor's winding exists can frequently be all of
the information provided back to the motor control
circuitry. More complex motor control systems
implementing dynamic control and active monitoring
can also benefit from adding simple out-of-range
detection function because of the faster indication of
out-of-range events.
By placing a current sense amplifier in series with the
DC power supply driving the high side of the motor
drive circuitry as shown in Figure 1, the overall current
to the motor can be measured easily detecting out-ofrange conditions. To detect small leakages the lowside return current can also be measured. A difference
between the high-side and low-side current levels
indicates a leakage path exists within the motor or
motor control circuitry.
RSHUNT
RSHUNT
Figure 1. Low & High-Side Current Sensing
The DC voltage level varies depending on the voltage
rating of the motor leading to multiple current
measurement solutions to accommodate the
corresponding voltage levels. For low voltage motors
(~5V) the selection of a circuitry to monitor this current
is much simpler with multiple amplifier types (current
sense, operational, difference, instrumentation) are
able to perform the current measurement function to
support this common-mode input voltage range.
SBOA163A – July 2016 – Revised December 2016
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For larger voltage motors (24V and 48V for example),
the available options decrease down to dedicated
current sense and differential amplifiers. As the
voltage requirements continue to increase,
measurement errors begin to impact the ability to
effectively identify out-of-range conditions. One
specification that describes an amplifier's effectiveness
at operating at high input voltage levels is the
common-mode rejection (CMR) term. This
specification directly describes how well an amplifier's
input circuitry can reject the influence of large input
voltages.
Ideally, an amplifier is able to completely reject and
cancel out any voltage common to both input pins and
amplify only the differential voltage seen between
them. However, as the common-mode voltage is
increased, leakage currents in the amplifier's input
stage result in additional input offset voltage. Larger
input range levels being monitored will create
proportionally larger measurement errors.
For example, an amplifier (difference amplifier or
current sense amplifier) that has a CMR (CommonMode Rejection) specification of 80dB will have a
significant offset voltage introduced in the
measurement based on the input voltage level. An
80dB CMR specification corresponds to an additional
100µV of offset voltage induced into the measurement
for every volt applied to the input.
Many devices are specified under defined conditions
(VCM = 12V and VS = 5V for example) which
establishes the base-line for the default specifications
(CMR and PSRR specifically). In this example
operating at 60V common-mode voltage creates a
change in VCM of 48V (60V-12V). A 48V change with a
80db CMR results in an additional 4.8mV of offset
voltage in addition to the specified input offset voltage
found in the device’s datasheet.
Applications employing calibration schemes are less
concerned by this additional induced offset voltage.
However, for applications where system calibration
cannot account for this shift in offset, selection of an
amplifier with better common-mode voltage rejection is
required.
High-Side Motor Current Monitoring for Over-Current Protection Scott Hill, Current
Copyright © 2016, Texas Instruments Incorporated
Sensing Products
1
www.ti.com
A key requirement for over-current detection circuitry is
the ability to detect and respond quickly to out-ofrange conditions. A signal bandwidth 100kHz and
2V/µs allows for the INA240 to accurately measure
and amplify the input current signal, send the output to
the high-speed comparator for an alert to be issued
based on a shorted condition in the span of a few
microseconds. This fast response ensures other
critical system components are not damaged by the
unintended excess current flowing in the system.
SUPPLY
VCM =
-4V to +80V
RPULL-UP
+
RSHUNT
-
GPIO
CONTROlLER
The INA240 is a dedicated current sense amplifier with
a common-mode input voltage range of -4V to +80V
and a worst case CMR (Common-Mode Rejection)
specification of 120dB over the entire input and
temperature range of the device. 120dB of CMR
corresponds to an additional 1µV of input offset
voltage induced for every 1V change in common-mode
voltage. The temperature influence on the amplifier's
ability to rejection common-mode voltages is not well
documented in many product datasheets so it should
be evaluated in addition to the room temperature
specification. The INA240 maintains a guaranteed
120dB CMR specification over the entire -40ºC to
+125ºC temperature range. The typical CMR
performance for the INA240 over the entire
temperature range is 135dB (less than 0.2µV for every
1V change) as shown in Figure 2.
+
REF
Figure 3. High-Side Over-Current Detection
Alternate Device Recommendations
Figure 2. Common-Mode Rejection vs.
Temperature
A system controller has the ability to utilize the current
sense amplifier's measurement to evaluate the
operation of the system. Comparing the current
information to pre-defined operating threshold allows
for detection of out-of-range events. A comparator
following the high-side current sense amplifier can
easily detect and provide alerts quickly to the system
allowing for corrective actions to be taken.
Figure 3 illustrates the signal chain path for monitoring
and detecting out-of-range excursions when
measuring currents on a high voltage rail driving the
motor drive circuitry. The output signal proportional to
the measured input current is directed to the ADC in
addition to be sent to the comparator to detect
overcurrent events. The comparator alert will assert if
the input current level exceeds the predefined
threshold connected as the comparators reference
voltage.
2
For applications measuring high voltage capability
needing a faster signal bandwidth or smaller package,
use the LMP8640. For applications requiring higher
voltage capability, the INA149 is a high performance
difference amplifier capable of interfacing with
common-mode voltages up to +/-275V off of a +/-15V
supply and has a guaranteed CMR of 90dB (or 31.6µV
for every 1V input change). The INA301 is a precision
current sense amplifier with an on-board comparator
that is ideal for detecting over-current events on
common-mode voltages up to 36V.
Table 1. Alternate Device Recommendations
Device
Optimized Parameter
Performance Trade-Off
LMP8640HV
Package: SOT23-6,
Signal Bandwidth
Accuracy
INA149
VCM Range: +/-275V
CMR, Gain
INA301
On-Board Comparator;
35µV VOS
VCM: 0V to 36V
Table 2. Related TI TechNotes
SBOA160
High Precision, Low-Drift In-Line Motor
Current Measurements
SBOA161
Low-Drift, Low-Side Current Measurements
for Three-Phase Systems
SBOA162
Measuring Current To Detect Out-of-Range
Conditions
SBOA165
Precision Current Measurement On High
Voltage Power Rail
High-Side Motor Current Monitoring for Over-Current Protection Scott Hill, Current
Sensing Products
SBOA163A – July 2016 – Revised December 2016
Submit Documentation Feedback
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