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Texas Instruments How to Design Isolated CAN Systems With Correct Bus Protection Application notes
Application Report
SLLA419 – July 2018
How to Design Isolated CAN Systems With Correct Bus
Protection
Daniel Kisling, Applications Engineer .................................................................. Isolation, Interface Group
Abhi Aarey, Applications Manager...................................................................... Isolation, Interface Group
Bhavin Kumar, Test Engineer ........................................................................... Isolation, Interface Group
ABSTRACT
One common reason for transceiver damage is due to insufficient bus protection. External ESD protection
is either not designed in at all, or the protection device is chosen in a way that does not pair well with the
transceiver. It is imperative for the protection device to be chosen to interface well with the transceiver.
This paper explains the key parameters for both the protection device and the transceiver in order to
design an isolated CAN system with correct bus protection.
1
2
3
4
5
6
7
Contents
Introduction ................................................................................................................... 3
Bidirectional TVS Diode Selection ......................................................................................... 3
TVS Diode and Transceiver Curve Traces ............................................................................... 6
Other Considerations (ESD, Leakage Current, Junction Capacitance, Package Size, and Operating
Temperature) ................................................................................................................. 8
Same-Side ESD Testing .................................................................................................... 9
Conclusion .................................................................................................................. 10
References .................................................................................................................. 10
List of Figures
1
Isolated CAN transceiver With TVS Diode ............................................................................... 3
2
Typical TVS Diode IV Curve
3
Graphical Representation of Relationship Between Key TVS Diode and Transceiver Specifications for
the ISO1050 .................................................................................................................. 5
4
Graphical Representation of Relationship Between Key TVS Diode and Transceiver Specifications for
the ISO1042 .................................................................................................................. 6
5
CPDT-12V Curve Trace
6
ISO1050 Curve Trace ....................................................................................................... 6
7
ISO1050 + CPDT-12V Curve Trace (connected as shown in )........................................................ 7
8
Combined View of Curve Traces and ISO1050 Datasheet Specifications ........................................... 7
9
Combined View of Curve Traces and ISO1042 Datasheet Specifications ........................................... 8
10
Example Layout of CAN Transceiver Device and TVS Diode
...............................................................................................
....................................................................................................
4
6
........................................................
9
................................................................
VCLAMP Worst-Case Differences .............................................................................................
Common Mode Range of Isolated CAN Devices ........................................................................
VBR Worst-Case Differences ................................................................................................
TVS Diode Specifications ...................................................................................................
ESD Test Results ............................................................................................................
3
List of Tables
1
2
3
4
5
6
CANH and CANL Absolute Maximum Voltage Ratings
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5
5
9
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1
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Trademarks
All trademarks are the property of their respective owners.
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Introduction
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1
Introduction
Protection from electrostatic discharge (ESD) is often an afterthought in system design even though only
one ESD strike can permanently damage a device. ESD protection is a critical component in a system
design. If a system does not have the correct ESD protection, the high voltage of an ESD strike through
an interface connection (for example, the CAN bus) can cause a large current spike to flow directly into
the transceiver device causing damage to the transceiver. This application report provides details to
design isolated CAN systems with correct bus protection. The device selected to protect the transceiver
from electrical overstress failures is a transient-voltage suppression (TVS) diode. TVS diodes are
connected to each signal line between the interface connector (the CANH and CANL pins in this case)
and ground before the transceiver device as shown in Figure 1. If an ESD strike occurs, the TVS diode
breaks down and creates a low impedance path. The low impedance path diverts the current flow to
ground to limit the peak voltage which protects the isolated CAN transceiver. The addition of a TVS diode
can be done for a reasonable cost and with minimal additional board space.
VCC1
VCC2
U1
1
2
3
5
VCC1
VCC2
TXD
CANH
RXD
CANL
GND1
GND2
4
6
7
8
GND2
GND1
VCC1
1
VCC2
C2
0.1 µF
C1
0.1 µF
GND1
2
3
GND2
GND2
Figure 1. Isolated CAN transceiver With TVS Diode
2
Bidirectional TVS Diode Selection
A bidirectional TVS diode can protect the CAN transceiver bus pins for short overvoltage events. TVS
diodes offer the most effective protection against ESD, burst EFT, and surge transients because of short
response times of a few picoseconds and power ratings up to several kilowatts.
2.1
Clamping Voltage
The TVS diode should be selected so that the absolute maximum voltage (VABS) at the bus pins is not
violated. For example, the ISO1050 device has absolute maximum voltage range from –27 V to 40 V for
the CANH and CANL pins as specified in the ISO1050 datasheet.
Texas Instruments also offers another isolated CAN transceiver; ISO1042. The ISO1042 device has an
extended absolute maximum voltage range from –70 V to 70 V on the CANH and CANL bus pins in
addition to other benefits such as support of CAN FD datarates and higher CMTI maximizing the flexibility
for the system designer. The absolute maximum voltage range can be found in the ISO1042 datasheet.
Table 1 shows a comparison of the absolute maximum voltage rating for the CANH and CANL pins of the
ISO1050 and ISO1042 devices.
Table 1. CANH and CANL Absolute Maximum Voltage Ratings
Device
ISO1050
ISO1042
Absolute maximum voltage at CANH and CANL (negative)
–27 V
–70 V
Absolute maximum voltage at CANH and CANL (positive)
40 V
70 V
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Bidirectional TVS Diode Selection
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The maximum clamping voltage (VCLAMP) at the peak pulse current (IPP) is the most important parameter to
consider when selecting a TVS diode. The maximum clamping voltage at the peak pulse current must be
less than the absolute maximum rating of the device. This value prevents the bus pins of the transceiver
from voltages greater than the absolute maximum specified for the transceiver which will damage the
device.
(1)
An example diode to consider is the CPDT-12V. The maximum clamping voltage is 25 V at a peak pulse
current of 1 A. The specifications of this diode result in 2 V of margin for a negative voltage strike and 13
V of margin for a positive voltage for the ISO1050 device. The margin of this diode would be greater for
the ISO1042 device.
The ESDCAN05 would be a poor choice of TVS diode for the ISO1050 for the following reason. According
to the ESDCAN05 datasheet, the maximum clamping voltage is 61 V at peak pulse current of 5 A. This
surpasses the absolute maximum rating of the ISO1050. However, this could be a good candidate for the
ISO1042 with an absolute maximum rating of +/-70 V. Note that the ESDCAN05 lists multiple maximum
clamping voltages for different peak pulse current values. This is common for TVS diodes. A system
designer needs to consider what maximum peak pulse current the bus could experience and determine
the appropriate clamping voltage for that system accordingly. If one were to study an IV curve (such as
the one listed in Figure 2) for a TVS diode they would find a continuous set of clamping voltages mapping
to applied current. The maximum clamping voltage at peak pulse current is a datasheet guarantee.
IPP
VC_IPP_System
VC
VBR
VRWM
IR
IT
IR
IT
VRWM
VBR
VC
IPP_System
IPP
Figure 2. Typical TVS Diode IV Curve
Below is a table comparing the worst case difference between the maximum clamping voltages at the
highest peak pulse current listed in the TVS diode datasheet to the absolute maximum rating of the bus
pins of the transceiver.
Table 2. VCLAMP Worst-Case Differences
ISO1050
(Absolute Maximum -27 V to
+40 V)
ISO1042
(Absolute Maximum –70 V to
+70 V)
CPDT-12V Minimum Margin (Vclamp = 25 V)
2V
45 V
ESDCAN05 Minimum Margin (Vclamp = 61 V)
–34 V
9V
NUP3105LT3G Minimum Margin (Vclamp = 66 V)
–39 V
4V
TVS Diode
From this table it is clear to see the CPDT-12V may be a suitable protection diode for the ISO1050 and all
three of the TVS diodes in the table may be suitable for the ISO1042.
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Bidirectional TVS Diode Selection
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2.2
Breakdown Voltage
The TVS must also be chosen so it does not interfere with normal operation. The key parameter here is
breakdown voltage (VBR). The breakdown voltage should be outside of the recommended operating
conditions (VCM), but before the absolute maximum specifications are violated.
For example, the recommended operating conditions for the ISO1050 bus pins are +/-12 V as can be
seen in section 6 of the ISO1050 datasheet. .
The ISO1042 offers an impressive +/-30 V common mode range. The table below details the common
mode for each device.
Table 3. Common Mode Range of Isolated CAN Devices
Device
ISO1050
ISO1042
VCM CANH / CANL
+/-12 V
+/-30 V
(2)
The breakdown voltage should be chosen such that it is greater than the recommended operating
conditions of the transceiver. This will ensure it does not interfere with the operational range of the
transceiver.
The breakdown voltage of the TVS diodes is compared to the recommended operating conditions of the
isolated CAN transceivers in the following table.
Table 4. VBR Worst-Case Differences
TVS Diode
ISO1050 (VCM = +/-12 V)
ISO1042 (VCM = +/-30 V)
CPDT-12V (VBR = 13 V)
1
-17
ESDCAN05 (VBR = 39 V)
27
9
NUP3105LT3G (VBR = 35.6 V)
23.6
5.6
Once again the CPDT-12V is found to be a suitable match for the ISO1050. The CPDT-12V does come
within the recommended operating conditions of the ISO1042. This is not an issue from an overvoltage
perspective; this will just limit the common mode of the system. With this TVS diode in place, the new
common mode will now be +/- 13 V because, beyond the voltage, the CPDT-12V will conduct.
The ESDCAN05 and NUP3105LT3G will not interfere with the common mode range of the ISO1050.
However, they have already been disqualified as a suitable match for the ISO1050 because the clamping
voltage is greater than the absolute maximum voltage ratings for the ISO1050. ESDCAN05 and
NUP3105LT3G do have margin to the ISO1042 recommended operating conditions specification, so these
TVS diodes are a good match as they will allow the system to fully utilize the extended common mode of
the ISO1042.
Putting these two requirements together, |VCLAMP|< |VABS MAX| and |VBR| > |VCM|, we get the following picture
of how these parameters relate to each other.
Absolute Maximum
Outside the
Absolute
Maximum
Ratings of Device
Absolute Maximum
TVS
Clamping
Voltage
±27 V
Recommended Operating
Conditions
TVS
Breakdown
Voltage
±12 V
0V
TVS
Clamping
Voltage
TVS
Breakdown
Voltage
12 V
27 V
Outside the
Absolute
Maximum
Ratings of Device
40 V
Figure 3. Graphical Representation of Relationship Between Key TVS Diode and Transceiver
Specifications for the ISO1050
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TVS Diode and Transceiver Curve Traces
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Absolute Maximum
TVS
Clamping
Outside the
Voltage
Absolute
Maximum
Ratings of Device
±70 V
Absolute Maximum
TVS
Clamping
Outside the
Voltage
Absolute
Recommended Operating Conditions
TVS
Breakdown
Voltage
TVS
Breakdown
Voltage
30 V
0V
±30 V
Maximum
Ratings of Device
70 V
Figure 4. Graphical Representation of Relationship Between Key TVS Diode and Transceiver
Specifications for the ISO1042
3
TVS Diode and Transceiver Curve Traces
To further illustrate this point, the TVS diodes and isolated CAN transceivers were put on a curve tracer.
Note that these curve traces were done for only one device and at room temperature. These curve traces
are provided purely for illustrative purposes. Protection circuitry must be chosen based on datasheet
specifications which consider temperature, process variation, and so forth.
0.01
0.008
0.006
Current (A)
0.004
0.002
0
-0.002
-0.004
-0.006
-0.008
-0.01
-60
-40
-20
0
Voltage (V)
20
40
60
D001
Figure 5. CPDT-12V Curve Trace
The CPDT-12V curve trace in Figure 5 shows that the TVS diode will draw minimal currents from +/-17
V – which makes sense as the datasheet guarantees the device will not draw more than 1 mA of current
up to 13 V and lists 17 V as the typical breakdown. As more energy is injected into the device, TVS diode
holds the bus at 17 V and steers the energy to ground.
0.01
0.008
0.006
Current (A)
0.004
0.002
0
-0.002
-0.004
-0.006
-0.008
-0.01
-60
-40
-20
0
20
Voltage (V)
40
60
80
D002
Figure 6. ISO1050 Curve Trace
The ISO1050 curve trace shows there are some internal ESD structures that try to clamp the voltage
outside of the absolute maximum rating of the part. These may be sufficient for some ESD strikes, but an
external ESD structure will better protect the device and ensure that the absolute maximum ratings are not
violated.
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TVS Diode and Transceiver Curve Traces
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0.01
0.008
0.006
Current (A)
0.004
0.002
0
-0.002
-0.004
-0.006
-0.008
-0.01
-60
-40
-20
0
Voltage (V)
20
40
60
D003
Figure 7. ISO1050 + CPDT-12V Curve Trace (connected as shown in Figure 1)
When the TVS diode is connected as shown in Figure 1, a curve trace CANH or CANL will result in the
curve shown in Figure 7. In the middle of the curve, TVS diode has not broken down and curve takes the
shape of the ISO1050. At +/-17 V, the curve takes the shape of the CPDT-12V as the diode steers the
energy to ground.
All three curves are combined in Figure 8 with some markup.
0.01
0.008
0.006
Current (A)
0.004
0.002
0
-0.002
-0.004
-0.006
CPDT-12V
CPDT-12V+ISO1050
ISO1050
-0.008
-0.01
-60
-50
-40
-30
-20
-10
0
10
Voltage (V)
Absolute Maximum
Outside the Absolute Maximum
Ratings of Device
±27 V
TVS
Clamping
Voltage
TVS
Breakdown
Voltage
±12 V
Recommended
Operating Conditions
0V
20
30
40
50
Absolute Maximum
TVS
Clamping
Voltage
TVS
Breakdown
Voltage
12 V
27 V
60
70
Outside the Absolute
Maximum Ratings of Device
40 V
Figure 8. Combined View of Curve Traces and ISO1050 Datasheet Specifications
The green region in the center marks the recommended operating conditions. In a proper design, the TVS
diode is not interfering with this region and so the common mode of the transceiver is the common mode
of the system. The TVS breakdown occurs before reaching the red region which designates voltages
beyond that of the absolute maximum voltage specifications of the ISO1050 isolated CAN transceiver.
The combined plots and markup are for ESDCAN05, NUP3105LT3G, and ISO1042 is shown below in
Figure 9.
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Other Considerations (ESD, Leakage Current, Junction Capacitance, Package Size, and Operating Temperature) www.ti.com
0.01
0.008
0.006
Current (A)
0.004
0.002
0
-0.002
-0.004
ESDCAN05
ESDCAN05 + ISO1042
ISO1042
NUP3105LT3G
NUP3105LT3G + ISO1042
-0.006
-0.008
-0.01
-100
-80
-60
-40
-20
Absolute Maximum
Outside the
Absolute Maximum
Ratings of Device
TVS Clamping
Voltage
±70 V
0
Voltage (V)
20
40
TVS
Breakdown
Voltage
TVS
Breakdown
Voltage
0V
80
TVS Clamping
Voltage
Recommended Operating Conditions
±30 V
60
100
Absolute Maximum
30 V
Outside the
Absolute Maximum
Ratings of Device
70 V
Figure 9. Combined View of Curve Traces and ISO1042 Datasheet Specifications
4
Other Considerations (ESD, Leakage Current, Junction Capacitance, Package Size,
and Operating Temperature)
ESD Capability is the amount of voltage the TVS diode can handle without damage. Note that the ESD
rating of the diode is not the same as the ESD rating of the system. The rating gives no indication of
whether the downstream circuitry will be protected. If the system is to survive an 8 kV ESD strike, the TVS
diode should have a rating of at least 8 kV. Check out www.ti.com/ESD-ClampingVoltage for more
information.
Leakage Current is the amount of current the diode conducts at voltages less than the breakdown voltage.
An ideal TVS diode would have zero leakage current which means it would have zero effect on the
recommended operating range of the bus pins. An actual TVS diode will have leakage currents in the
microamp range which will have a negligible effect. Leakage current is typically specified for a smaller
range than breakdown voltage of the TVS diode. Make sure the voltage listed is outside of the operating
range of the transceiver bus pins to avoid any issues.
Junction capacitance should be considered as this could violate systems specifications if it is too large,
and will also limit the maximum data rate of the bus. As a rule of thumb, junction capacitance should be
less than 50pF.
Package size and layout will affect the total solution size. Many TVS diodes come in a SOT-23 package
that is not only a small form factor, but is relatively easy to add to a layout. Figure 10 shows an example
layout of the ISO1042 device in the DWV package. The D1 component is the TVS diode. Pin 1 and 2
connect to CANH and CANL. Pin 3 is connected to GND.
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Same-Side ESD Testing
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Minimize
distance to VCC
VCC1
VCC1
TXD
RXD
MCU
Isolation
Capacitor
C
VCC2
C
VCC2
C1
CANH
D1
CANL
C2
GND2
GND1
GND1
PLANE
CAN
BUS
GND2
PLANE
Figure 10. Example Layout of CAN Transceiver Device and TVS Diode
The operating temperature of the selected TVS diode should be greater than or equal to the specified
operating temperature of the system.
Table 5 shows the specifications for the example TVS diodes selected in this application report.
Table 5. TVS Diode Specifications
5
TVS Diode
VBR
VCLAMP
Package
ESD Contact
Leakage
Current
Junction
Capacitance
Operating
Temperature
CPDT-12V
13 V
25 V
SOT-23
8 kV
2 µA
12 pF
125°C
ESDCAN05
35.6 V
66 V
SOT-23
30 kV
0.1 µA
30 pF
150°C
NUP3105LT3G
39 V
61 V
SOT-23
30 kV
0.1 µA
3.5 pF
175°C
Same-Side ESD Testing
One form of testing for industrial or automotive CAN would be subject to is same side ESD testing. A
system including the components shown in Figure 1 are subjected to high voltage strikes from ESD tester.
In this test, an ESD gun would be attached to the CANH or CANL pin and tested for a specific voltage with
respect to GND2. The device is then checked for damage.
Table 6 shows the bench test results for a limited number of tests for each isolated CAN transceiver and
TVS diode configuration. These tests were done to illustrate the potential benefits of the TVS diode on the
system. A very small sample size of tests was completed for each configuration. Process variation and
temperature were not considered. As such, these pass levels should not be considered as guaranteed for
all devices but rather should be read as relative performance differences for systems with and without a
TVS diode.
Table 6. ESD Test Results
Devices Being Tested
Typical Pass Level
ISO1050
4 kV
ISO1050 + CPDT-12C
8 kV
ISO1042
8 kV
ISO1042 + ESDCAN05
15 kV
ISO1042 + NUP3105LT3G
15 kV
These conclusions can be made from the results:
• A transceiver can withstand ESD strikes with a higher voltage when paired with an appropriate TVS
diode. This is because the TVS diode guides the ESD energy away from the transceiver pins into the
local ground. The TVS diode also clamps the ESD energy at a safe value within the absolute maximum
voltage rating of the transceiver. Therefore the transceiver does not receive the full energy impact of
the ESD event.
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Conclusion
•
•
•
6
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The ISO1042 device can withstand higher voltage same-side ESD strikes than the ISO1050 device
when a TVS device is not installed.
For a system specification of 8 kV same-side ESD, a TVS diode may not be required when using the
ISO1042.
The ISO1042 device can withstand a 15-kV ESD strike with an appropriate bus protection device.
Conclusion
Using a TVS diode helps protect a device from overvoltage events on a CAN bus. Choosing the right TVS
diode based on the clamping voltage and breakdown voltage with respect to the transceiver specifications
will ensure that the TVS diode can protect the device when necessary but does not interfere with the
operational range. If a correct TVS diode is selected, the system can withstand higher ESD strikes on the
bus pins than that of a system with no TVS diode.
7
References
See these documents for additional reference:
• Texas Instruments, ISO1042 Isolated CAN Transceiver With 70-V Bus Fault Protection and Flexible
Data Rate data sheet
• Texas Instruments, ISO1050 Isolated CAN Transceiver data sheet
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