Texas Instruments | Low Voltage, High Slew Rate Op-amps for Motor Drive Circuits | Application notes | Texas Instruments Low Voltage, High Slew Rate Op-amps for Motor Drive Circuits Application notes

Texas Instruments Low Voltage, High Slew Rate Op-amps for Motor Drive Circuits Application notes
Low Voltage, High Slew Rate Op-amps for Motor Drive
Circuits
Paul Goedeke, General Purpose Amps Apps
The requirements for operational amplifiers and other
ICs used in motor control systems have increased
because of the need to extract higher performance
from a motor while maintaining low system cost.
Measuring motor current is an easy and inexpensive
way to understand the torque and direction of the
motor, so current sensing forms the backbone of many
common motor control schemes for the three common
DC motor types: stepper, brushed DC and brushless
DC (BLDC).
Motor current relates proportionally to torque, allowing
a motor controller to monitor the motor load through
current sensing. It can also protect the motor from
over-torque or overload conditions in the event of a
jam or some other system failure. Typically, motors are
controlled with a pulse width modulation (PWM)
scheme generated by a controller driving different
MOSFET half-bridge driver configurations. An H-bridge
is used for brushed DC motors as shown in Figure 1,
dual H-bridge for stepper motors, or triple half-bridge
for BLDC motors. When the MOSFETs are switched
on and off with a PWM input, the inductance of the
motor acts like a low pass filter and produces an RMS
current that depends on the duty cycle of the PWM
signal.
Vsupply
Controller
PWM Input
Rshunt
Rshunt
Figure 1. H-Bridge Motor Driver with Low-Side
Current Sensing
SBOA316 – July 2018
Submit Documentation Feedback
The simplest and lowest cost motor control schemes
use a single low-side shunt resistor (shown as a dotted
resistor to ground in Figure 1) and sum the leg
currents to get an average value used for torque
control and simple fault detection like over-torque or
overload. In many cases, the demands on the current
sense amplifier are lower because over-torque
conditions can be monitored over multiple switching
cycles with lower accuracy requirements. These
schemes generally do not push the boundaries of
frequency or duty cycles either.
However, the single summing shunt method may not
provide the level of information required to use control
schemes such as field oriented control (FOC) in BLDC
motors. Accurately measuring the current through
each leg of the driving circuit is used to maximize
torque, efficiency or optimize other system goals. This
can be seen in Figure 1 where each leg has its own
shunt resistor (labeled Rshunt). FOC control schemes
have more demanding requirements for the low-side
current sense amplifiers because measurement
accuracy is critical to the control algorithm, PWM
frequencies and duty cycles are likely to be more
extreme, and measurements are made in each PWM
cycle. DC measurement accuracy is important for FOC
control schemes, requiring consideration of the offset
voltage, rail to rail behavior and input common mode
range of the op-amp.
Let’s look at how a typical low-side current sense
amplifier responds to a sudden large current in the
shunt resistor. When a PWM pulse is applied, the low
side transistor turns on and the current steps up from
nearly 0 A through Rshunt to the value of the motor
winding current. This current creates a voltage jump
and therefore a large signal input into the non-inverting
pin of the op-amp. Any large signal step Figure 1: Hbridge motor driver with low-side current sensing. into
an op-amp’s inputs causes the amplifier to go into slew
limit, where the rise time is determined by how fast the
amplifier can charge the Miller capacitance between
the input and output stages of the amplifier. Once the
signal is within a few hundred millivolts of the final
value, the small signal response of the op-amp starts
to take over and the signal settling time is determined
by the AC response of the op amp circuit. The key is
that the overall settling time is dictated by the slew rate
and the small signal settling time of the op-amp, as
seen in Figure 2. The amplifier must settle before the
Low Voltage, High Slew Rate Op-amps for Motor Drive Circuits Paul Goedeke,
Copyright © 2018, Texas Instruments Incorporated
General Purpose Amps Apps
1
Related Documentation
www.ti.com
PWM cycle ends so that the controller can make an
accurate measurement. For a given low-side current
sense amplifier circuit, the settling time is constant,
meaning that the there is an upper limit for the PWM
frequency and a lower limit for the duty cycle.
INPUT:
DIGITAL CHANGE
OR ANALOG STEP
V2
Slewing
Behavior
OUTPUT
RESPONSE
ERROR
BAND
Figure 3. Slew Rate Comparison
V1
t3
t2
SETTLING TIME
t1
Figure 2. Large Signal Response of an Op-Amp
So what can be done to get a shorter acquisition time?
One option is to choose an op-amp with a higher slew
for the current sense circuit. Figure 3 shows a simple
comparison of two op-amps with different slew rates
driving the same load. One has a slew rate of 13 V/µs
and the other has a slew rate of 2 V/µs. It is easy to
see that the higher slew rate amplifier has a much
shorter overall settling time because less time is spent
in the op-amp’s slew rate limit. The larger the input
step, the more time the op-amp will spend in slew limit.
TI now has the TLV9052, featuring a 13 V/µs slew
rate, 5 MHz bandwidth and only 330 µA Iq. The
TLV9052 represents a significant improvement over
other low voltage (<7 V supply) op-amps with similar
bandwidth, which typically have slew rates ranging
from 2 V/µs to 10 V/µs and 400 µA to 2 mA of Iq. This
combination of features in the TLV9052 makes
operational amplifier an attractive way to extract more
performance from a motor drive system while
maintaining low system cost and power consumption.
If a simpler motor control system is used with only a
single shunt, consider using the TLV9002 which has a
good trade-off of specifications and price.
1
Looking at TI’s op-amp catalog, it is easy to see that
high slew rate devices are usually high bandwidth
devices. Because motor drive systems do not usually
require passing high frequency signals, using high
bandwidth amplifiers comes with some substantial
trade-offs, including increased stability concerns and
higher cost. Arguably the biggest tradeoff is that high
bandwidth devices need higher quiescent current (Iq)
making them unsuitable for applications which are
power constrained like battery operated tools and
drones.
1.1
Related Documentation
For more information on slew rate, bandwidth
and stability please see TI Precision Labs Op
Amps videos:
• Slew Rate
• Bandwidth
• Stability
More information on designing a low-side
current sense amplifier with an op amp can be
found in the following TI Circuit Cookbooks:
• Single Supply Low-Side Current Sensing
SBOA215
• Bidirectional Low-Side Current Sensing
SBOA223
Trademarks
All trademarks are the property of their respective owners.
2
Low Voltage, High Slew Rate Op-amps for Motor Drive Circuits Paul Goedeke,
General Purpose Amps Apps
Copyright © 2018, Texas Instruments Incorporated
SBOA316 – July 2018
Submit Documentation Feedback
IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE
DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD
PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable
standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you
permission to use these resources only for development of an application that uses the TI products described in the resource. Other
reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third
party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims,
damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on
ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable
warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2018, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising