Texas Instruments | Low-Emission Designs With ISOW7841 Integrated Signal and Power Isolator (Rev. C) | Application notes | Texas Instruments Low-Emission Designs With ISOW7841 Integrated Signal and Power Isolator (Rev. C) Application notes

Texas Instruments Low-Emission Designs With ISOW7841 Integrated Signal and Power Isolator (Rev. C) Application notes
Application Report
SLLA368C – May 2017 – Revised February 2019
Low-Emission Designs With ISOW7841 Integrated Signal
and Power Isolator
Anand Reghunathan, Koteshwar Rao, Anant Kamath
ABSTRACT
The ISOW7841 integrated signal and power isolation device simplifies system design and reduces board
area. The use of low-inductance micro-transformers in the ISOW7841 device necessitates the use of highfrequency switching, resulting in higher radiated emissions compared to discrete solutions. The ISOW7841
device uses on-chip circuit techniques to reduce emissions compared to competing solutions. Techniques
such as lower supply operation, the use of interlayer stitching capacitance, filters, and common-mode
chokes can further reduce radiated emissions at the system level.
1
2
3
4
5
6
7
8
9
10
Contents
Introduction ................................................................................................................... 2
Use 3.3-V Input Supply Where Possible.................................................................................. 4
Input and Output Decoupling Capacitors, Input Ballast Resistor, Ferrite Beads .................................... 4
Interlayer Side 1-to-Side 2 Capacitor ..................................................................................... 5
Direct and Capacitive Connection to Protection Earth .................................................................. 9
Using Common-Mode Chokes ............................................................................................. 9
Precautions During EMI Measurements ................................................................................ 10
Quasi-Peak versus Peak Detection ...................................................................................... 11
Conclusion .................................................................................................................. 12
References .................................................................................................................. 12
List of Figures
1
ISOW7841 Single-Chip Power and Data Isolation ...................................................................... 2
2
Common-Mode Currents Across the Isolation Barrier form a Large Return Loop .................................. 3
3
Radiated Emissions of the ISOW7841 vs a Competitive Device at 5-V Input and 80-mA Load .................. 3
4
Radiated Emissions of ISOW7841 at 5-V Input vs at 3.3-V Input ..................................................... 4
5
Supply Decoupling Capacitors and Input Ballasting Resistor .......................................................... 4
6
ISOW7841 Evaluation Module ............................................................................................. 5
7
Side 1-to-Side 2 High-Voltage Capacitor Reduces the Common-Mode Current-Loop Area
6
8
Solving for Ci for the PCB in Using
7
9
......................
.......................................................................................
3-D Diagram of PCB Layers With Interlayer Stitching Capacitance ..................................................
7
10
ISOW7841 Emissions With and Without a 30-pF Stitching Capacitance at a 5-V Input and 80-mA Load
Current ........................................................................................................................ 8
11
Triangle Corners Implemented to Enhance High-Voltage Performance
12
13
14
15
............................................. 8
Direct and Capacitive Connection to PE in an Isolated RS-485 Application Using ISOW7841 ................... 9
Common-Mode Chokes on Input and Output Power Supply and I/O Lines Reduce Emissions ................. 10
ISOW7841 Emissions Measurement Setup (CISPR 22) ............................................................. 11
Quasi Peak vs Peak Measurements With ISOW7841 ................................................................ 12
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1
Introduction
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Trademarks
All trademarks are the property of their respective owners.
1
Introduction
Texas Instruments’ ISOW7841 device is a high-performance 5000-VRMS reinforced quad-channel digital
isolator with an integrated high-efficiency, low-emissions DC/DC converter. Figure 1 shows a pinout and
simplified block diagram of the ISOW7841 device.
ISOW7841
1
VCC
2
GND1
3
INA/OUTA
DC-DC
Primary
4
INB/OUTB
5
INC/OUTC
6
IND/OUTD
7
NC
8
GND1
DC-DC
Feedback
+
Quad
Isolators
VISO
16
GND2
15
OUTA/INA
14
OUTB/INB
13
OUTC/INC
12
OUTD/IND
11
SEL
10
GND2
9
DC-DC
Secondary
DC-DC
Feedback
+
Quad
Isolators
Copyright © 2017, Texas Instruments Incorporated
Figure 1. ISOW7841 Single-Chip Power and Data Isolation
Traditionally, discrete transformers in addition to driver integrated circuits (ICs) have been used to
generate isolated power. The ISOW7841 device uses chip-scale transformers to integrate the isolated
power converter in a small outline SOIC-16 package. These chip-scale transformers are very small in size,
use only a few turns (to reduce series resistance), and use an air core. Consequently, the DC/DC
converter driving these transformers must operate at a few tens of megahertz. Because the primary and
secondary windings of the transformers are very close to each other inside the package, a parasitic
capacitance forms between the two coils of the transformer. The fast transients in the DC/DC converter
couple through this parasitic capacitance creating a common-mode current between side 1 and side 2 of
the isolated system (see Figure 2). Because the two sides are completely isolated, the current forms a
large return loop through board-level parasitic capacitances. This large current loop can cause radiated
emissions in isolated systems. Another way to understand the emissions mechanism is that the two
isolated parts of the board form a dipole antenna transmitter.
In discrete implementations with on-board transformers, radiated emissions are lower because of the use
of high-inductance magnetic-core transformers and much lower switching frequencies.
2
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Introduction
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ISOW7841
Side 1
Side 2
Common-Mode
Current Loop
Copyright © 2017, Texas Instruments Incorporated
Figure 2. Common-Mode Currents Across the Isolation Barrier form a Large Return Loop
The ISOW7841 device uses several techniques, including symmetric design and layout, and clock
dithering to reduce electromagnetic emissions which competing solutions do not use. Figure 3 compares
the performance of the ISOW7841 device with an equivalent competitor device under identical conditions.
The ISOW7841 device passes the CISPR22B standard, and is more than 15 dB lower in emissions in
absolute terms.
60
ISOW7841 Emissions
Competition Emissions
50
CISPR22 Class B Limit Line
40
30
20
10
0
-10
30
1000
Figure 3. Radiated Emissions of the ISOW7841 vs a Competitive Device at 5-V Input and 80-mA Load
While the ISOW7841 device achieves low emissions even with a standard evaluation module (EVM),
board design and layout techniques can be used to further reduce emissions in isolated systems. This
application note discusses some of these methods. This document also discuss precautions during EMI
testing to avoid spurious effects and how to measure the true performance of the system under test.
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3
Use 3.3-V Input Supply Where Possible
2
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Use 3.3-V Input Supply Where Possible
The ISOW7841 supports both 3.3-V and 5-V input supply operation. Compared to 5-V operation, at 3.3-V
operation the slew rates of the internal DC/DC converter are lower because of the lower voltage drive
available to the power transistors. Lower slew rates reduce the common-mode current across the isolation
barrier, leading to lower radiated emissions. As shown in Figure 4, the emissions with a 3.3-V input supply
are much lower than with a 5-V supply. Where possible, TI recommends operating the ISOW7841 device
with a 3.3-V input supply.
60
ISOW7841 (5 V) Emissions
50
ISOW7841 (3.3 V) Emissions
CISPR22 Class B Limit Line
40
30
20
10
0
-10
30
1000
Figure 4. Radiated Emissions of ISOW7841 at 5-V Input vs at 3.3-V Input
3
Input and Output Decoupling Capacitors, Input Ballast Resistor, Ferrite Beads
The ISOW7841 device uses high-frequency switching to compensate for low transformer inductance, and
also duty-cycles the power-converter to provide the required output DC load while maintaining regulation.
Whenever the converter is on, a high current draw from the input supply, VCC, occurs. This current has
low-frequency content (roughly proportional to the closed-loop regulation bandwidth) and high-frequency
content at the switching frequency and harmonics of the DC/DC converter.
ISOW7841
RF
L1
Backplane Power
VCC
CF
Loop 3
C3
C2
To Load
VISO
C1
C1
Loop 1
Loop 2
GND1
C2
C3
Loop 4
GND2
Copyright © 2017, Texas Instruments Incorporated
Figure 5. Supply Decoupling Capacitors and Input Ballasting Resistor
4
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Interlayer Side 1-to-Side 2 Capacitor
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An input capacitor bank of different capacitors (C1 = 100 nF, C2 = 1 µF, C3 = 10 µF) at the input of the IC
filters out a lot of the high-frequency content and prevents it from propagating to the backplane supply
routing. Placing these capacitors as close to the IC (see Figure 5) as possible is critical to limit the area of
loop 1. Place the smallest-value capacitor closest to the IC. A similar bank of capacitors placed on the
output side filters out switching current on the secondary side of the DC-DC converter. Minimizing the area
of loop 2 is important.
Despite the decoupling capacitors, the backplane supply routing can still draw some high-frequency and
low-frequency current (a peak current of 100 s of milliamperes lasting several microseconds) depending
on the output impedance of the supply network. If the input supply routing is long, a ballasting resistor (RF)
and an additional larger capacitor (CF, 100 µF) can prevent current into the input supply routing (loop 3)
leading to lower radiated emissions.
Figure 6 shows the image of the ISOW7841 evaluation module (EVM) and the placement of decoupling
capacitors. In this layout, stitching vias along the board edge that connect all ground planes creates a
Faraday shield. This shield prevents any radiation from noisy inner traces or planes.
Stitching Vias Along Board Edges
Stitching Vias at
Edges of a Fill
Bank of Decoupling Capacitors Close to the IC
Figure 6. ISOW7841 Evaluation Module
4
Interlayer Side 1-to-Side 2 Capacitor
As previously discussed, the common-mode current from side 1 to side 2 and a large return loop are the
main causes of emissions in isolated systems. One way of minimizing the current loop is to include a highvoltage capacitor from side 1 to side 2 and place it as close to the IC as possible as shown in Figure 7.
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5
Interlayer Side 1-to-Side 2 Capacitor
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ISOW7841
Side 1
Side 2
Common-mode
current loop is
much smaller
Side 1-to-Side 2
Capacitor
Copyright © 2017, Texas Instruments Incorporated
Figure 7. Side 1-to-Side 2 High-Voltage Capacitor Reduces the Common-Mode Current-Loop Area
The capacitors suitable for such use include high-voltage Y2 capacitors (surface-mount device [SMD] or
leaded with appropriate creepage, or clearance levels) commonly used in power supplies. Using Y2
capacitors on the printed circuit board (PCB) introduces lead inductances on both sides of the capacitor, in
addition to the parasitic inductance of the capacitor, making the capacitor ineffective at frequencies higher
than 200 MHz. One way to create a low-inductance capacitance is by overlapping internal PCB layers.
Consider a four-layer PCB with the top layer for the signal, layer 2 for ground, layer 3 for VCC, and the
bottom layer for the signal. Because of system isolation, PCB layers are separated into two groups: side 1
layers (forming signal1, GND1, and VCC1) and side 2 layers (forming signal2, GND2, and VCC2). Extending
the internal reference layers (GND and VCC) on either side of the isolation barrier (as shown in Figure 8)
creates an overlap of internal layers. This overlap area, formed by the extension of the GND1 and VCC2
layers with FR4 material between them acting as a dielectric, creates a stitching capacitance between
GND1 and VCC2. For fast transient common-mode currents, both GND and VCC points are considered to be
the same references (similar to AC analysis). Therefore, interlayer stitching capacitance forms between
the reference points of side 1 and side 2. Figure 9 shows a 3-D representation of the interlayer stitching
capacitance created in a four-layer PCB.
Use Equation 1 to calculate the equivalent capacitance formed between the sides.
Ci •
A
d
where
•
•
•
•
•
•
ε = εoεr
Ci is the interlayer stitching capacitance.
εo is the absolute permittivity of air (8.854 pF/m).
εr is the relative permittivity of the dielectric (4.2 for FR4)
A is the overlapping area.
d is the distance between the GND and VCC layers.
(1)
From Equation 1, solving for the interlayer stitching capacitance (Ci) for the PCB in Figure 9 would
produce an approximate value of 30 pF.
6
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Interlayer Side 1-to-Side 2 Capacitor
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Cross section:
Top Layer
8.0 mm
GND
0.66 mm
0.6 mm
VCC
6.28 mm
0.66 mm
Bottom
8.0 mm
Capacitance:
PCB
K
Eo (F/m)
D (mm)
L (mm)
W (mm)
C (pF)
4.2
8.85E-12
0.6
77.3
6.28
30.09
Figure 8. Solving for Ci for the PCB in Figure 9 Using Equation 1
Figure 9. 3-D Diagram of PCB Layers With Interlayer Stitching Capacitance
Implementing a stitching capacitance across the isolation barrier with appropriate spacing is required to
meet the overall isolation performance of the system. End-equipment electrical safety standards and
working-voltage conditions determine these spacing requirements. Most system standards with functional
or basic isolation do not have a minimum spacing requirement between the side 1 and side 2 layers on
the same plane or on different planes. For such systems, using the lowest spacing between the
overlapping area is best to achieve the highest capacitance value. Also consider the working voltage
expected across the barrier and the dielectric strength of the insulation material.
Standards for safety systems and systems with reinforced isolation may require a minimum spacing
between the side 1 and side 2 layers on both the same plane and on different planes. Allowing 0.4-mm
spacing for systems less than 300 VRMS and 0.6-mm spacing for 300-VRMS to 600-VRMS systems meets
most standard requirements. Aside from these spacing requirements, standards organizations stipulate
requirements on temporary overvoltages and surge voltages. For example, the International
Electrotechnical Commission (IEC) 61010-1 mandates a 5 s withstand test of 3510 VRMS and a surge or
impulse test of 6400 VPK for reinforced isolation from 300 VRMS to 600 VRMS mains voltage. The interlayer
spacing must be designed to withstand these voltages, considering the dielectric strength of the insulation.
For example, FR4 has a dielectric strength of 20 kV/mm.
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Interlayer Side 1-to-Side 2 Capacitor
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Figure 10 shows the reduction in radiated emissions from the ISOW7841 device with the use of an
interlayer stitching capacitance. With only a 30-pF capacitor between the two sides, an improvement of 10
dB to 20 dB is achieved. Higher stitching values can provide progressively higher attenuation in
emissions.
60
ISOW7841 Emissions (Without Stitching Capacitance)
50
ISOW7841 (With Stitching Capacitance)
CISPR22 Class B Limit Line
40
30
20
10
0
-10
30
1000
Figure 10. ISOW7841 Emissions With and Without a 30-pF Stitching Capacitance at a 5-V Input and 80mA Load Current
Apart from maintaining the spacing across the barrier, extra care must be given at the edges of the PCB.
At the edges, the planes can be exposed to air. Because opposite voltages come close to the edge of the
board, these opposing voltages could lead to electric field stress and then air breakdown along the edge.
Sharp edges and corners can also contribute to the air breaking down further because they increase the
intensity of the electric field in the vicinity of the sharp edge, leading to a breakdown path for high-voltage
impulse tests. To avoid such scenarios, pull in the inner layers at the edge of the PCB, and modify sharp
edges into triangle-cut shapes, which will spread the charge density. Figure 11 shows an example of such
an implementation. Rounding the corners and planes can also further enhance the solution.
Figure 11. Triangle Corners Implemented to Enhance High-Voltage Performance
8
Low-Emission Designs With ISOW7841 Integrated Signal and Power Isolator
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Direct and Capacitive Connection to Protection Earth
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While the inner layers of the PCB completely surrounded by dielectric allow relatively lower spacing (as
previously discussed), the top and bottom signal layers must maintain much higher spacing. This spacing
is based on the expected temporary overvoltage, impulse voltages, and environmental conditions such as
altitude and pollution degree.
5
Direct and Capacitive Connection to Protection Earth
The controller or cold side of most isolated systems is connected to a system chassis that is then
connected to protection earth (PE). This strong earth connection ties the controller-side ground to a stable
point and reduces the conversion of common-mode noise to electromagnetic radiation in the system.
Therefore, wherever possible, the control side of isolated systems should be strongly connected to
protection earth.
The isolated ground of interfaces, such as analog and digital I/O modules and isolated RS485/CAN, can
be AC connected to PE with the use of high-voltage safety capacitors. This capacitive connection is used
to provide a return path to PE for ESD, surge, and EFT strikes on the bus and I/O cables. This connection
also serves the purpose of reducing the common-mode noise on the isolated ground which reduces
emissions through the interface and I/O cables.
Figure 12 shows an example of an isolated RS-485 system using the ISOW7841 device. The
microcontroller ground is directly connected and the isolated ground is capacitively connected to PE,
minimizing electromagnetic radiation from the input supply cables as well as from the RS-458 bus.
ISOW7841
VIN
VCC
VISO
A
RS-485
Transceiver
µC
B
GND1
GND2
PE (Protective Earth)
Copyright © 2017, Texas Instruments Incorporated
Figure 12. Direct and Capacitive Connection to PE in an Isolated RS-485 Application Using ISOW7841
6
Using Common-Mode Chokes
Long cables or wires connected to the system containing the ISOW7841 device can potentially pick up
high-frequency switching noise from the convertor and act as transmitter antennas; longer cables result in
higher radiation levels. The solution is to keep the common-mode current loop as small as possible and to
keep the cables as short as possible. In case the system cannot avoid the presence of long cables and
cannot provide a good connection between the ground and power earth or chassis, common-mode chokes
(see Figure 13) can be used for attenuating the common-mode noise. For the ISOW7841 designs, a
choke helps reduce the common-mode current as well as the return-path loop area, indirectly reducing
radiated emissions. Common-mode chokes can be inserted on the input and output power supplies or on
any long cables connected to the system.
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Precautions During EMI Measurements
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ISOW7841
VCC
VISO
Common-Mode
Choke
To Load
0
Common-Mode
Choke
0
VIN
CF
GNDIN
C3
C2
C1
C4
C5
C6
0
0
GND1
GND2
Copyright © 2017, Texas Instruments Incorporated
(1)
Any I/O cables should have common-mode choke with respect to GND.
Figure 13. Common-Mode Chokes on Input and Output Power Supply and I/O Lines Reduce Emissions
7
Precautions During EMI Measurements
When performing emissions measurements, make sure that only the emissions from the equipment under
test (EUT) or device under test (DUT) are being measured and not emissions from other parts of the
setup. While the following practices do not lower radiated emissions, they are critical to determining the
true performance of the ISOW7841 device in the system under test.
The primary reason for radiation is the formation of antennas on the board. Long cables used to power up
the system or probes used to measure any parameter can act like antennas and cause a higher emissions
reading. The solution is simple: the setup used for emissions should closely mimic the final system
conditions in which it will be operating. Properly shield all supporting equipment other than the system
being tested with a Faraday shield.A box lined with a conductive metal, such as copper or aluminum, can
serve as a Faraday shield. Keep cables connected to and originating from the system as short as
possible, and keep the module to which the cables are connected within the Faraday shield. Any direct or
capacitive connections of the board or metal shields to PE that are eventually planned for use in the final
system must be present during EMI testing as well.
Figure 14 shows the setup used to measure the ISOW7841 emissions. The board containing the
ISOW7841 device is exposed for measuring the radiated emissions. The cables connected to the board to
power up the IC are as short as possible and in a tight twisted-pair configuration. The rest of the
equipment—a 12-V battery and low-dropout regulator (LDO) in this case—are inside the black box and
lined with aluminum foil to shield the components inside from the measurements. The cables connecting
the LDO to the board are as short as possible and in a tight twisted-pair configuration. The cables extend
out through a small hole at the top of the box. This hole is as small as possible to avoid compromising the
effect of the Faraday shield.
10
Low-Emission Designs With ISOW7841 Integrated Signal and Power Isolator
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Quasi-Peak versus Peak Detection
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Antenna
10 Meters
DUT
Power Supply
(12 V Battery + LDO)
Enclosed within box
Short Twisted-Pair
Cables
Faraday Cage
Lined With Aluminum
Figure 14. ISOW7841 Emissions Measurement Setup (CISPR 22)
If the supply connections are long wires coming from a power supply placed far away from the DUT, then
common-mode chokes are recommended near the DUT so that the emissions are not unnecessarily
enhanced by the long wires. By using common-mode chokes, emissions of the actual setup are measured
and the effect of the long cables is nullified.
Any extra components required on the board, such as load resistors, must be soldered directly to the
board rather than connecting them to the board using long wires.
8
Quasi-Peak versus Peak Detection
According to the CISPR 22 standard, the radiated emissions specification is specified as a quasi-peak
limit, although the peak-detector measurement can typically be used to get a quick result.
The ISOW7841 device uses clock dithering to change the switching frequencies across a small band
instead of concentrating all of the power at one single peak. Techniques such as this lower the peak scan
levels and also show significantly better results when subjected to quasi-peak scans. At lower load
operations, the power of radiated emissions is less because the DC/DC converter is on for a much shorter
time compared to the full load condition. This lower power is reflected as a significant improvement in the
quasi-peak results.
Texas Instruments recommends taking a peak-detector measurement first to find out the frequencies for
the worst-case measurements. Then take a quasi-peak measurement at those frequencies to estimate the
true margin from the CISPR 22B quasi-peak limit line.
Figure 15 shows the CISPR 22B measurement results with the ISOW7841 device. Up to 8.8-dB
improvements occur with quasi-peak (QP) measurement compared to a peak-detector measurement.
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11
Conclusion
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Frequency (MHz) Polarization Peak (dBuV/m) QP (dBuV/m)
ISOW7841
5 V to 3.3 V at 5 mA
ISOW7841
5 V to 3.3 V at 80 mA
Improvement (dB)
240
H
18
11.8
240
V
20
13
6.2
7
465
V
32
24.5
7.5
465
H
29
21
8
676
H
39
30.2
8.8
676
V
41
32.5
8.6
240
V
20
13.3
6.7
240
H
19
13
6
465
H
30
23.6
6.4
465
V
32
26.3
5.7
676
H
38
31.6
6.4
676
V
40
34.5
5.5
Figure 15. Quasi Peak vs Peak Measurements With ISOW7841
9
Conclusion
The ISOW7841 integrated data and power isolator simplifies system designs and reduces board area.
Through chip-design techniques, the ISOW7841 device reduces electromagnetic emissions by more than
15 dB when compared to competition solutions. However, techniques such as lower supply operation and
the use of interlayer stitching capacitance, filters, and common-mode chokes can further reduce radiated
emissions at the system level. Care should be taken during emissions measurement to measure the true
performance of the system under test. Quasi-peak measurements result in lower readings than peak
measurements, and should be used to assess the exact margin to the CISPR22B standard.
Read our blogs
• Optimize the emissions performance in your isolated system
• Small doesn’t mean you compromise performance
• Cool down your industrial system design with integrated data and power isolation
10
References
•
•
•
•
•
•
12
Read the How to isolate signal and power for an RS-485 system tech note for an example of isolated
power and interface
Read about advantages of using an integrated data and power isolation in the Fully integrated signal
and power isolation – applications and benefits white paper.
Texas Instruments, ISOW7841 High-Efficiency, Low-Emissions, Reinforced Digital Isolator With
Integrated Power product folder
Texas Instruments, 8-ch Isolated High Voltage Analog Input Module with ISOW7841 Reference Design
Texas Instruments, Isolated RS-232 With Integrated Signal and Power Reference Design
Texas Instruments, Size and Cost Optimized Binary Module Reference Design Using Digital Isolator
with Integrated Power Reference Design
Low-Emission Designs With ISOW7841 Integrated Signal and Power Isolator
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (May 2017) to A Revision ........................................................................................................... Page
•
Changed the IEC specification number in the Interlayer Side 1-to-Side 2 Capacitor section ................................... 7
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Revision History
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