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Texas Instruments How to Improve Speed and Reliability of Isolated Digital Inputs in Motor Drives Application notes
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How To Improve Speed and Reliability of Isolated Digital
Inputs in Motor Drives
Anant Kamath, Systems Engineer, Isolation Products, Interface Group
Digital Input Receivers in Motor Drives
Digital Input receivers are used in AC and servo motor
control to interface various 24-V signals to the control
module of the drive. These signals include inputs from
field sensors and switches, position and speed
feedback encoded as 24-V signals, clock or PWM
inputs for speed control, and emergency stop signals,
such as Safe Torque Off (STO). Isolation is used to
manage ground potential differences.
5V
Optocoupler
R2
24 V
Sensor/
Switch
designer to independently adjust these parameters.
The ISO121x can reduce power dissipation by a factor
of 5x compared to traditional approaches, as shown in
the How To Simplify Isolated 24-V PLC Digital Input
Module Designs TI Tech Note.
RSENSE
(sensor / switch /
Position Feedback /
Safety Device)
Schmitt
Trigger
Buffer
Host
Controller
ISO1211
RTHR
24-V Input
SENSE
VCC1
IN
OUT
FGND
Host
Controller
GND1
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Figure 2. Digital Input Module with ISO1211
R1
A CMOS based integrated solution such as the
ISO121x offers several advantages over traditional
optocoupler solutions for motor drive digital inputs.
Figure 1. Traditional implementation of Digital
Input Receivers in Motor Drives
Common Implementations
Figure 1 shows the most common implementation of
Digital Input receivers in motor drive applications in
use today. The voltage thresholds and the receiver
input current are set by the resistors R1 and R2, in
combination with the optocoupler’s input
characteristics. A Schmitt trigger buffer is usually
needed after the optocoupler to provide hysteresis for
noise immunity.
New Digital Input Solutions for Motor Drive
Applications
Standard optocouplers have a response time of tens of
micro-seconds, and are not capable of high speed
operation. ISO121x devices offer much higher speed
and response time, and are ideal for high speed digital
inputs such as for position and speed feedback and
control signals in motor drives.
Max. Response Time /
Propagation Delay
Maximum Clock
Frequency
Optocoupler
20 µs
20 kHz
ISO121x
140 ns
2 MHz
Device
Controlled Thresholds and Noise Immunity
Texas Instruments' ISO1211 and ISO1212 devices are
high-speed isolated digital input receivers with
integrated current limit, voltage comparator with
hysteresis and reverse polarity protection. The
ISO1211 can be used for channel-to-channel isolation
and the ISO1212 is ideal for multichannel designs.
Figure 2 shows the implementation of one channel of a
Digital Input receiver with ISO1211. The resistor RSENSE
controls the current limit, and the resistor RTHR, the
voltage transition thresholds. This allows the systems
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Higher Speed and Lower Response Times
Motor drives are noisy systems, and it is imperative to
design interfaces including digital inputs for noise
immunity. Optocoupler input response and current
transfer ratio varies greatly from device to device, and
with temperature. Over a period of time, the LEDs
used in the optocoupler also age, resulting in a further
shift in input and transfer characteristics. Keeping
these factors in mind, digital inputs designed with
optocouplers have a wide variation in input voltage
transition thresholds. To keep the current draw low,
often the system designer is forced to design the
digital input receiver to draw the minimum required
current at a higher input voltage (i.e. the maximum
threshold is pushed higher, beyond the desired
How To Improve Speed and Reliability of Isolated Digital Inputs in Motor
Drives Anant Kamath, Systems Engineer, Isolation Products, Interface Group
Copyright © 2017, Texas Instruments Incorporated
1
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specification). In comparison, as shown in Figure 3, an
ISO121x based solution has controlled thresholds and
hysteresis, and sufficient margin to the low and high
levels, to account for noise, variation in the 24-V
supply and voltage drop in the input wiring.
5V
Optocoupler
+/- 24V
Host
Controller
24 V
24 V
Schmitt Trigger
Buffer
Max = 18 V
Max = 15 V
Min = 12 V
Min HYST = 1 V
No HYST
ISO1212
RTHR
Min = 4 V
0V
0V
CIN
RSENSE
SENSE1
IN1
FGND1
Optocoupler Solution
VCC1
OUT1
ISO121x
OUT2
Figure 3. ISO121x Based Receivers Have Higher
Noise Margin to the High and Low Levels
+/- 24V
RSENSE
SENSE2
Host
Controller
IN2
FGND2
GND1
Current for positive polarity
Safety Monitor
Devce
STO
Switch 1
ISO1212
STO1
Current for negative polarity
CIN
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RSENSE
SENSE1
Figure 5. Implementing Bipolar, AC or Sink-Source
Configuration using ISO1212
IN1
FGND1
OUT1
Power Stage
Enable
OUT2
STO
Switch 2
24 V
Motor Drive
RTHR
RTHR
STO2
CIN
RSENSE
Bipolar, AC or Sink-Source Operation
SENSE2
IN2
FGND2
GND1
STO_COM
Copyright © 2017, Texas Instruments Incorporated
Figure 4. Safe Torque Off (STO) Signals
Implemented with ISO1212
Increased Reliability
The ISO121x devices, like other digital isolation
products from Texas Instruments are manufactured
using a well-controlled CMOS semiconductor
manufacturing process. These devices have a very
low Failure in Time (FIT) rate of <0.3 per 1 billion
hours of operation. In comparison, optocouplers have
a FIT rate of 10 to 30 per 1 billion hours of operation,
orders of magnitude worse. The ISO121x devices also
do not age with time, unlike optocouplers.
Device
FIT rate
Aging
Optocoupler
20
Significant
ISO121x
0.3
Negligible
Critical signals such as Safe Torque Off (STO) that are
active-low, and hence operate at a continous highlevel in normal operation can cause accelerated aging
in optocouplers. Safe Torque Off (STO) when
implemented with ISO121x as shown in Figure 4
benefits from a lower FIT rate, and negligible impact
from aging.
Digital inputs for motor drives are sometimes
implemented in bipolar or sink-source configuration. To
implement bipolar operation, usually a full-bridge
rectifier using four diodes is used ahead of an
optocoupler. The ISO1212 device can be used to
create an input module that can be used for bipolar,
AC or sink-source operation, as shown in Figure 5,
resulting in a compact solution, while maintaining the
benefits of current limit, and precise voltage
thresholds.
Conclusion
The ISO1212 and ISO1211 devices bring a modern
solution for Digital Input receivers in motor drive
applications offering higher speed, well controlled
voltage thresholds, better noise immunity and
increased reliability and longevity compared to
traditional solutions with optocouplers.
Table 1. Alternative Device Recommendations
Device
Optimized Parameters
Performance Trade-Off
ISO1211
Single-channel isolated
digital input receiver
For channel to channel
isolation
ISO1212
Dual-channel isolated
digital input receiver
For group isolated inputs.
Lower cost per channel
8-channel digital input
serializer
Non-isolated, Lower Speed,
Needs field side supply
SN65HVS880
Table 2. Adjacent Tech Notes
SLLA370
2
How To Improve Speed and Reliability of Isolated Digital Inputs in Motor
Drives Anant Kamath, Systems Engineer, Isolation Products, Interface Group
Copyright © 2017, Texas Instruments Incorporated
How To Simplify Isolated 24-V PLC Digital
Input Module Designs
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