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Texas Instruments Advantages of Integrated Current Sensing Application notes
Advantages of integrated current sensing
Ryan Kehr, Motor Drives
Monitoring and regulating current is a must for many
brushed and stepper motor applications. For brushed
motors, the current information can be used to
determine changes in load conditions or for limiting
startup and stall currents. For stepper motors, high
levels of micro-stepping require current to be regulated
for each step.
A low wattage signal path resistor compared to a
power resistor requires much smaller board space and
lowers the bill of materials (BOM). In addition to these
benefits, there is no power loss across the resistor and
one more heat source is eliminated from a design that
is most likely already under thermal budget pressure.
In Figure 1, current is plotted versus time depicting the
startup profile for a brushed DC motor. In this case,
current is limited to approximately 2 amperes before
the motor reaches a steady state condition of less than
1 ampere. Without current regulation, this same motor
peaks to over 14 amperes. This not only requires an
over-designed power supply to support this transient,
but the motor driver also needs to be rated to reliably
handle the peak currents.
Figure 2. DRV8870 with External Shunt
Figure 3. DRV8871 with Integrated Current Sensing
Figure 1. DC Motor Current vs. Time
Traditionally, this current limiting was achieved using
external shunt resistors in the ground path. The
voltage drop across these shunts was monitored and
compared to a reference voltage either provided
internally or externally depending on the device. Since
the full load current passes through these shunts, the
resistors must be power resistors and can be up to
1206 in case size which can be almost as large as the
integrated circuit itself with a 3.2 mm by 1.6 mm
dimension. Figure 2 compares the size of a shunt to a
DRV8870 DC motor driver. Figure 3 depicts the
DRV8871 with integrated current sensing where a
single 0402 resistor is used to set the current limiting
threshold. As is clearly shown in Figure 3, integrated
current sense leads to board and component savings.
SLVAE68 – March 2019
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Now that current limiting and integrated current
sensing benefits are well understood, we can also add
monitoring capability in our newest family of brushed
DC motor drivers through the IPROPI pin. The
DRV8873 is the newest product in the Brushed Motor
Driver family that provides an analog current output
proportional to the current flowing through the bridge.
By passing this proportional current through a small
resistor, a voltage proportional to current can be
monitored with an external microcontroller to
determine changes in load conditions and act
accordingly. This internal current monitoring feature
eliminates the need for an external power resistor and
conditioning circuits like an operational amplifier.
Advantages of integrated current sensing Ryan Kehr, Motor Drives
Copyright © 2019, Texas Instruments Incorporated
1
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Finally, we get to stepper motors that further benefit
from the PCB size and BOM advantages of integrated
current sensing since stepper motors have traditionally
required two external sense resistors to feedback the
current to the driver. Figure 4 compares the PCB
footprint of a traditional driver to the DRV8886AT that
benefits from internal current sensing.
Figure 4. Board Layout Reduction
One less tangible benefit that is only realized during
PCB layout is the simplicity of the layout when
choosing a device with the integrated sensing feature.
It is no longer necessary to route around the large
external shunt components and break up power,
output, and GND routing that require larger traces for
carrying device current.
In summary, integrated current sensing in TI's latest
family of motor drivers removes costly power resistors
from the BOM, reduces board size, component count,
and simplifies PCB routing.
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Table 1. Alternate Device Table
Device Name
Device Description
DRV8871
3.6A Brushed DC Motor Driver with
Integrated Current Sensing (PWM Ctrl)
DRV8871-Q1
3.6A Automotive Brushed DC Motor Driver
With Integrated Current Sensing (PWM Ctrl)
DRV8873
10A H-Bridge Motor Driver with Integrated
Current Sensing and Current Sense Output
DRV8873-Q1
Automotive H-Bridge Motor Driver
DRV8876
37-V, 3.5-A Brushed DC Motor Driver with
Integrated Current Sensing and Current
Sense Output
DRV8876-Q1
Automotive H-bridge motor drivers with
integrated current sense and regulation
DRV8884
1.0A Stepper Motor Driver With 1/16
Microstepping and Integrated Current
Sensing (STEP/DIR Ctrl)
DRV8885
1.5A Stepper Motor Driver with 1/16
Microstepping and Integrated Current
Sensing (STEP/DIR Ctrl)
DRV8886AT
2A Stepper Motor Driver With Integrated
Current Sense and Adaptive Decay Mode
References
•
•
How to use integrated current sensing in
brushed DC motors
How to use integrated current sensing in
stepper motors
Advantages of integrated current sensing Ryan Kehr, Motor Drives
Copyright © 2019, Texas Instruments Incorporated
SLVAE68 – March 2019
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