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Texas Instruments How to isolate signal and power for I2C (Rev. A) Application notes
How to Isolate Signal and Power for I2C Interfaces
Anthony Viviano, Product Marketing Engineer, Isolation, Interface Group
Introduction:
Inter-integrated circuit (I2C) bus communication is a
two wire half duplex method for communication over
short distances and has been widely adopted for a
variety of applications because of its simplicity. In
systems where the I2C bus is used to communicate
between two domains of different potentials or where
high voltages are present, galvanic isolation can be
used to protect circuitry and human operators as well
as break ground loops that can create noise that will
interfere with signal communication.
Since digital isolators are inherently unidirectional,
implementing the bidirectional communication of the
I2C bus presents numerous challenges for system
designers. This document will address the different
methods available to isolate the I2C bus and how to
provide isolated power for these solutions.
Signal isolation:
Signal isolation for the I2C bus can be achieved with
two methods. The first method is to use a digital
isolator with external circuitry to separate the
bidirectional data path into two unidirectional channels.
After the bidirectional data is separated into
unidirectional signals, the digital isolator will modulate
the input signal for each channel and pass the signal
across the isolation barrier before demodulating the
signal at the output. The application note Designing an
Isolated I2C Bus interface by Using Digital Isolators
explains in more detail the design considerations and
methodology for separating the bidirectional I2C
signals into unidirectional signals to interface with the
digital isolator.
3.3 V
5V
0.1 …F
0.1 …F
16
1
VCC1
VCC2
2k
ISO7731
Figure 1 shows the implementation of this method in
an application with bidirectional data and unidirectional
clock using a three channel digital isolator such as the
ISO7731. For multi-master systems requiring
bidirectional data and clock signals, the same methods
explained in the application note can be used with a
four channel digital isolator such as the ISO7742.
The second method for isolating the I2C bus is to use
an integrated solution such as the ISO154x family of
devices. These integrated solutions use internal
circuitry combined with the digital isolator to achieve
the same isolated I2C buffer functionality. The
ISO1540 is designed for multi-master systems with
bidirectional data and clock signals and the ISO1541 is
designed for systems with bidirectional data and
unidirectional clock. If an application with bidirectional
data and unidirectional clock requires clock stretching
function, it is recommended to use ISO1540 to achieve
clock stretching.
VCC1
VCC2
A
RPU1
B
SDA1
RPU2
ISO1541
Cnode
SDA2
Cbus
C
D
GND1
GND2
VREF
Figure 2. ISO154x Method for Separating The
Bidirectional SDA Signal Into Unidirectional
Isolation Channels
4.7 k
4.7 k
3
14
SCL1
SCL2
1 nF
(optional)
2.49 k
7
3.12 k
10
EN1
EN2
1.1 k
13
4
1.45 k
10 pF
(optional)
D2
MMBT3904
84.5
12
5
SDA1
SDA2
612
BAS40
BAS40
GND1
2, 8
GND2
9, 15
GND1
GND2
Figure 1. Discrete Implementation for Isolated I2C
with a Digital Isolator
SLLA417A – August 2018 – Revised October 2018
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Figure 2 shows a functional diagram of how the
bidirectional serial data line (SDA) signal from the I2C
bus is internally separated into two unidirectional
signals that are isolated using the channels of the
digital isolator. The isolated I2C devices are designed
to interface with a low capacitance I2C node on side 1
and a fully loaded I2C bus with up to 400 pF on side 2.
The arrangement and connection of internal
unidirectional channels creates a closed signal loop
that is prone to latch-up. This latch-up condition is
prevented by implementing an output buffer (B) whose
output low-level is raised by a diode drop to
How to Isolate Signal and Power for I2C Interfaces Anthony Viviano, Product Marketing
Copyright © 2018, Texas Instruments Incorporated
Engineer, Isolation, Interface Group
1
www.ti.com
approximately 0.75 V, and the input buffer (C) that
consists of a comparator with defined hysteresis. The
comparator's upper and lower input thresholds then
distinguish between the low potential from SDA1 and
the potential from output buffer B.
Each solution for isolating the signal in an I2C system
will have trade-offs. The discrete solution using a
digital isolator provides more freedom in part selection
with the ISO7731 or ISO7742 both available in multiple
package options with different isolation ratings to fit the
specific use case; however, this solution also has the
drawback of needing external circuitry that occupies
more board space . The integrated solution with
ISO1540 or ISO1541 will occupy less board space and
require less design efforts than the discrete solution,
but is only available in limited package and isolation
rating options.
transformer integrated into the chip provides a typical
efficiency of around 50% and can provide up to 650
mW of isolated power. With the integrated power
solution, since the transformers are smaller in size,
switching frequencies are higher, leading to higher
emissions as compared to the discrete solution. These
emissions can be reduced by stitching capacitors as
shown in the application note, Low-Emission Designs
With ISOW7841 Integrated Signal and Power Isolator.
Figure 4 shows the implementation of ISOW7842 in a
system with bidirectional data and a unidirectional
clock. The same methodology used to separate the
SDA signal can be applied to the clock signal if
bidirectional clock is needed.
3.3 V
5V
0.1 …F
1
VCC
0.1 …F
16
VISO
2k
ISOW7842
3
14
SCL1
SCL2
Power Isolation:
1 nF
(optional)
Regardless of the method chosen to isolate the I2C
signal, an isolated power supply will be required to
power the secondary side of the digital isolator or
isolated I2C buffer. The first solution for providing
isolated power is to use a circuit similar to Figure 3,
which uses the SN6501 transformer driver to drive a
transformer in a push pull configuration. The benefit of
this solution is that it provides greater than 80%
efficiency and the transformer and LDO can be
selected to optimize for specific design considerations.
The SN6501 provides up to 1.5 W of power and can
be replaced with the SN6505 for up to 5 W if isolated
power is needed for additional devices.
4
D2
GND
SN6501
5
VCC
4
8
1
3
2
7
6
3
1
1
5
2
D1
GND
3
2
1
3.3 V
VDD
VCC2
VCC1
8
GND
NC
5
4
3.3 V
RPU2
RPU1
2
MCU_SDA
OUT
IN
TPS76350
EN
7
SDA1
SDA2
Cnode
Cbus
MCU
ISO1541
MCU_SCL
3
SCL1
SCL2
GND1
GND2
DGND
4
Digital
Ground
Galvanic
Isolation Barrier
6
5
ISO
Ground
Figure 3. Isolated I2C Solution for Signal and
Power with ISO1541
The second solution for providing isolated power in the
I2C system is to replace the digital isolator in the
discrete approach to signal isolation solution with
ISOW7842. The ISOW7842 device is a digital isolator
with integrated signal and power isolation in a 16 pin
SOIC package. The advantage of this device is that it
greatly reduces board space by integrating the
transformer, transformer driver, and LDO. The small
solution size comes with a trade off in efficiency as the
2
10
3.12 k
SEL
1.1 k
13
4
1.45 k
10 pF
(optional)
D2
MMBT3904
84.5
12
5
SDA1
SDA2
612
BAS40
GND1
BAS40
GND1
2, 8
GND2
9, 15
GND2
Figure 4. Isolated I2C Solution for Signal and
Power Using ISOW7842
Conclusion:
There are many methods to isolate signal and power
for an I2C system and the correct choice will depend
on the specific application requirements. Isolated I2C
buffers such as the ISO154x family make design easy
by integrating all of the external circuitry needed to
isolate the SDA and SCL signals while preventing
latch up and complying with the I2C standard. In some
cases, it may be beneficial to have the flexibility of
numerous packages and isolation ratings available. A
discrete solution using the ISO77xx series of digital
isolators offers this flexibility and can still achieve the
same isolated I2C functionality as the integrated
solution, when designed correctly.
For isolated power the key trade-off is efficiency
versus board space. The SN6501 solution provides a
compact, low noise, and high efficiency solution for
generating isolated power. For applications where
further board space reduction is desired, the
ISOW7842 solution simplifies design and reduces
board space compared to the first solution. A system
designer must weigh the trade-offs of each solution for
isolating signal and power in an isolated I2C system to
determine the best fit for their particular application.
How to Isolate Signal and Power for I2C Interfaces Anthony Viviano, Product Marketing
Engineer, Isolation, Interface Group
2.49 k
SLLA417A – August 2018 – Revised October 2018
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