Texas Instruments | Isolated CAN solution by using ISO1050 and SN6501 (Rev. A) | Application notes | Texas Instruments Isolated CAN solution by using ISO1050 and SN6501 (Rev. A) Application notes

Texas Instruments Isolated CAN solution by using ISO1050 and SN6501 (Rev. A) Application notes
Application Report
SLLA334A – January 2013 – Revised August 2018
Isolated CAN Solution by Using ISO1050 and SN6501
Zhu Wenbin .................................................................................................. High Performance Analog
ABSTRACT
This application note presents a reference design of a compact isolated CAN module by using isolated
transceiver ISO1050 and transformer driver SN6501. Compared to current solutions in the industry, this
design has the advantages of easy implementation, high reliability, low EMI, and low cost. The ISO1042
can be used as a performance upgrade to the ISO1050.
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Contents
Introduction ...................................................................................................................
Capacitive Isolation Technology ...........................................................................................
Isolated CAN Module Design ..............................................................................................
Experiment Validation .......................................................................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
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2
2
6
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7
List of Figures
1
Magnetic Field Immunity .................................................................................................... 2
2
SN6501 Internal Block Diagram ........................................................................................... 3
3
Switching Waveforms and BBM Waveform by Pspice simulation ..................................................... 3
4
Reference Design for 5-V to 5-V Isolated Power Supply (Pspice Schematic) ....................................... 4
5
Transformer Output With Input Drops s –10% ........................................................................... 5
6
Transformer Output With Input Drops s +10% ........................................................................... 5
7
Front and Side Views of Isolated CAN Module .......................................................................... 5
8
Isolated CAN Module Test Setup .......................................................................................... 6
9
Fully Loaded bus Test (Waveform of CANH, CANL at a 1-Mbps Data Rate) ....................................... 6
List of Tables
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
Today, along with the fast popularization of CAN-bus, the application field is more diversified and
complicated than before. Un-proper node design causes poor communication, and even one node failure
can result in damage to an entire bus, especially in a harsh environment. Therefore, essential protection
circuit should be implemented into node design to improve reliability and reduce unnecessary damage.
For common design, isolation for digital signal and power is added between the controller and the
transceiver. In some applications that require higher ESD protection, TVS should be added in the bus.
The current solutions are either too complicated (for example, digital isolator, CAN transceiver, and
isolated power) or too expensive. As a viable solution, the combination of ISO1050 with SN6501, a
compact, high-performance and low-cost reference design, is provided.
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1
Capacitive Isolation Technology
2
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Capacitive Isolation Technology
ISO1050 transceiver is an isolated CAN transceiver with a maximum data rate of 1 Mbps and 4 kV of
galvanic isolation. The ISO1050 transceiver meets or exceeds all requirements of the ISO 11898
standard. This device uses capacitive isolation technology (SiO2) for signal path isolation. Compared to
inductive and optocoupler type, the main advantage of SiO2 capacitive isolation is high reliability and longlife time expectancy, which benefits from its small aging effect characteristic. With an industry lifetime
requirement at 400 V (minimum) working voltage, inductive type has only 8 years while capacitive isolator
has a significantly longer lifetime of 28 years.
100E+6
Capacitive Isolator
IEC61000-4-8
IEC61000-4-9
Magnetic Flux Density - Vs/m2
100E+3
100E+0
100E-3
100E-6
100E-9
100E-12
100E-15
100E-18
0.001
0.01
0.1
1
Frequency (MHz)
10
100
D001
Figure 1. Magnetic Field Immunity
Due to internal design construction of capacitive isolators, this type of isolator provides almost infinitely
high magnetic field immunity. Figure 1 describes the quantified magnetic immunity (the field-strength is
applied without causing false toggling). Figure 1 shows outstanding performance of the capacitive device,
which is far beyond the standard of IEC 61000-4-8 and IEC 61000-4-9.
Another benefit of using SiO2 capacitive technology is that it is compatible with standard semiconductor
manufacturing processes. Therefore, lower production cost is achieved, thus bringing the lower cost to
customer directly.
3
Isolated CAN Module Design
For the isolated CAN design, both power supply and signal path must be electrically isolated to meet a
certain isolation level. Because ISO1050 is already internal galvanically isolated for signal path, designing
a robust isolated power supply is the key design consideration for the entire module. For a common
design, isolated power that is glue-poured as a module is always used for convenience (for example,
DCR010505, which is a 1-W isolated 5-V to 5-V, DC-DC converter). However, using such a ready-made
power module occupies more PCB room and is not suitable in some cost-sensitive cases. A PCB-level
discrete solution can also be implemented by using DC-DC converter such as TPS61085; approximately
20 external components, including transformer and diodes, are required. This kind solution is too
complicated to design the isolated power supply for a small package-isolated CAN module.
3.1
Transformer Driver
Another more compact, less expensive, and more easily designed method is to use a transformer driver.
SN6501 provides this solution. SN6501 uses push-pull topology to drive the transformer; it is designed for
low-cost, small form-factor, isolated DC-DC converters. The high primary-side can drive current up to 350
mA at 5-V power supply and 150 mA at 3.3 V with a tiny SOT23 package. Small output capacitor is
allowed; this benefits from low ripple on rectified output. Those advantages make SN6501 suitable for
isolated interface power supply.
2
Isolated CAN Solution by Using ISO1050 and SN6501
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VCC
SN6501
D2
OSC
fOSC
S
Freq.
Divider S
BBM
Logic
G2
G1
Q2
D1
Q1
GND GND
Figure 2. SN6501 Internal Block Diagram
Figure 2 shows the SN6501 internal block diagram; it includes an oscillator, a frequency divider, and a
break-before-make (BBM) logic. BBM logic outputs two complementary signals, which turn the two output
power transistors on and off alternately. Simulation by Pspice model (see Figure 3) shows the
complementary push-pull waveform with BBM (Vcc is 5 V with no load at the secondary side of
transformer).
5.0 V
Vcc = 5 V
2.5 V
0V
V(D1)
5.0 V
Vcc = 5 V
2.5 V
SEL>>
0V
V(D2)
5.0 V
Break-Before-Make
2.5 V
0V
0s
1 µs
V(D1)
V(D2)
2 µs
3 µs
Figure 3. Switching Waveforms and BBM Waveform by Pspice simulation
3.2
5-V to 5-V Isolated Power Design Consideration
Figure 4 is the reference schematic for 5-V to 5-V isolated power supply.
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Isolated CAN Module Design
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VIN
+
C4
10 µ
5
±
XFRM_NONLIN/CT-PRI/SEC
0
0
U1
GND2
V1 IS
D2
LP1
VP LP2
0
D1
VRDS
0
D1
0
LS2
GND
0
LS1 VS
VOUT
SN6501
VCC
Transient
Model
GND1
U2
1
VDiode
TX1
C3 VLDO
10 µ
V2
IP
C5
10 µ
0
3
2 IN
VDrop
OUT
TLV7xxx
SN6501_TRANS
0
Figure 4. Reference Design for 5-V to 5-V Isolated Power Supply (Pspice Schematic)
SN6501 provides low ripple on rectified output. The linear regulator may not be used when the primary
side of the transformer is supplied with a stable 5-V power (VIN). However, especially in a safety-related
application, such as distributed control systems (DCSs), the programmable logic controller (PLC) and
textile machinery controller, ±10% tolerance of VIN should be considered. So, 5-V power supply varies
from 4.5 to 5.5 V and output of rectifier (VOUT) also varies with the same ratio. While the power supply at
the secondary side (VLDO) of ISO1050 allows only 4.75 to 5.25 V, a 5-V LDO is needed to stabilize the
output of the rectifier.
The two main factors to select an LDO are current drive capacity and input voltage range. SN6501 only
provides current for ISO1050, thus the current drive capability depends on ISO1050 and should have
headroom for the maximum current. For 80 mA (maximum) loading of ISO1050, a 100- to 150-mA LDO,
such as TPS70950 or TLV70450, is appropriate.
The transformer output voltage range depends on transformer turns ratio. Minimum VIN and LDO
efficiency should be considered to determine the turn ratio. Larger ratio can avoid the LDO step into
nonlinear region when VIN drops (VOUT drops accordingly) and maintains a stable 5-V output. However,
this results in low efficiency due to a larger drop in voltage on the LDO, and also forces the LDO input
voltage to exceed the rating when VIN increases drastically.
Based on the power conservation principle for the transformer,
(1)
Also, according to the same volume of magnetic flux, which flows through the transformer primary and
secondary coils, we have:
•
•
•
•
•
VP = VIN - VRDS and is the voltage at the transformer primary side.
VS = VLDO + VDiode + VDrop and is the voltage at the secondary side.
VRDS is the drop voltage on the turnon resistor of the SN6501 power transistors, which varies with the
changing of the loading current.
VDiode is the diode forward voltage.
VDrop is the LDO drop voltage.
(2)
Assume ISO1050 with the maximum loading, LDO must provide 80 mA current (IS-max).
(3)
(4)
To solve this linear equation of two unknowns (n and IP) under below condition: VLDO = 5 V, VDiode = 0.2 V,
RDS = 2 Ω (at 5 V). Normally, although a voltage drop of LDO is around 150 mV at 100-mA loading, some
low cost LDO has a larger drop voltage. For a worst case, consider VDrop-max = 1 V (at 100 mA). Gets n =
1.25, IP = 100 mA.
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According to the preceding theoretical calculation, if ISO1050 is under the maximum current load, LDO
must provide about 80 mA current with a 1-V drop. Turn ratio of transformer should not be less than 1.25.
Meanwhile, the maximum primary current provided by SN6501 is 100 mA (IP) which is within the range of
350 mA at 5-V power supply. Figure 5 and Figure 6 show the Pspice simulation when input voltage drops
or increases to the design limitation. Based on the simulation and calculation, the minimum transformer
output is 5.45 V; thus, it is important to select an LDO with a drop voltage of less than 450 mV at 100 mA
maximum loading.
8
6
6
VOUT (V)
VOUT (V)
4
4
2
2
0
0
0
50
100
150
200
250
Time (Ps)
300
350
400
0
50
100
D002
VOUT = 5.45 V
200
250
Time (Ps)
300
350
400
D003
VOUT = 6.7 V
Figure 5. Transformer Output With Input Drops s –10%
3.3
150
Figure 6. Transformer Output With Input Drops s +10%
PCB Design
To meet 2500 Vrms isolation (isolation level of ISO1050DUB) requirement for the CAN module, besides
selecting a suitable transformer, PCB layout must be carefully designed.
PCB creepage distance is the most importance factor to achieve the 2500-Vrms isolation. For
ISO1050DUB, the minimum creepage (shortest terminal to terminal distance across the package surface)
is about 6.8 mm. Other components (SN6501 at the primary side and LDO at secondary side) should be
kept out for this distance. Thus, a small package LDO (SOT-23 or SC70) is better to implement the
creepage for small PCB size requirement of the module. The pin connections point of transformer selected
also should keep the creepage. Moreover, a slot with the same width of ISO1050 should be cut
underneath the transformer (isolation via air). Figure 7 shows the finial outline of the module, which is
packaged with glue.
Figure 7. Front and Side Views of Isolated CAN Module
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Experiment Validation
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Experiment Validation
To validate the reference design including a 5-V isolated power supply and an entire module
communication function, a test bench is set up (see Figure 7). Two MCU control boards are used with
essential equipments: 6/2 digit multimeter (34401A-Agilent), oscilloscope (TPS2024- Tektronix), and DC
power supply (GPS3303-GWINSTEK).
DC Power Supply
00.0
MCU
Isolated
CAN
Module
ON
OFF
Isolated
CAN
Module
MCU
Oscilloscope
Multi-meter
-
- -
Figure 8. Isolated CAN Module Test Setup
A nonisolated CAN device SN65HVD1050 is used as receiver in the experiment to test an isolated CAN
module. One MCU sends a prefixed data sequence (handshake) via isolated CAN module. The other
MCU receives the data through SN65HVD1050 and checks the error rate at the same time; if an error is
found, a green LED on the receiver MCU board turns off. The oscilloscope is also used to observe the
CAN bus waveform: 250 Kbps, 500 Kbps, and 1 Mbps data rates are tested and no data errors are found.
The second experiment uses two isolated CAN modules as transmitter and receiver, respectively. The
same data rates as in the first experiment are tested successfully.
Isolated CAN module
Receiver
47 Ÿ
Isolated CAN module
Transmitter
Figure 9. Fully Loaded bus Test (Waveform of CANH, CANL at a 1-Mbps Data Rate)
6
Isolated CAN Solution by Using ISO1050 and SN6501
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Conclusion
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Bus loading is an important feature for a CAN device. This feature is related to the driver current capacity
of a CAN transmitter. Larger output current ability means more nodes can be added on the bus, assuming
other conditions remain the same. For isolated CAN, the isolated power supply must provide enough
current for the required output swing when the bus is fully loaded. In the CAN standard of ISO 11898-2,
the differential output must be greater than 1.5 V with a 60- Ω load, and must be greater than 1.2 V with a
fully loaded bus (bus node of a CAN device also depends on receiver differential input impedance besides
the driver capacity).
long as the power supply is stable and does not drop when larger current is drawn by ISO1050 with heavy
loading, the device can support at least 167 nodes. For this isolated power designed by SN6501, 100 mA
on the secondary side is ensured. To test bus loading capacity of the module, a 167-transceiver network is
simulated by a resistor. The differential input resistance of ISO1050 is a minimum of 30 kΩ with a
maximum of 80 kΩ. Assume 167 nodes are in parallel on a bus with all 30-kΩ resistance: this is equivalent
to a 180-Ω differential load. Take two 120-Ω termination resistors into account: this yields a total 45 Ω. In
the test, a 47-Ω resistor is shunted on the receiver side. The data rate test is successful at 250 Kbps, 500
Kbps, and 1Mbps. Figure 9 shows the waveform of CANH, CANL at 1 Mbps data rate. The differential
output voltage is 2.4 V, which is in the range of 1.4 to 3 V (the value ensured by the data sheet) and also
has more margin for 1.2 V (ISO11898-2 requirement).
5
Conclusion
An isolated CAN module reference is discussed in this application report. A designer can easily complete
a compact, high-reliability, low-cost 5-V to 5-V isolated power by using SN6501 and a few external
components. In place of the ISO1050, ISO1042 can also be used as a performance upgrade. Detailed
design consideration of how to select LDO and calculate appropriate transformer turn ratio is presented.
Meanwhile, Pspice simulation is used to validate the theoretical calculation. Finally, the maximum bus
loading (167 nodes) and highest data rate (1 Mbps) is achieved in a set of experiments to demonstrate the
robustness of the isolated CAN module.
6
References
For related documentation, refer to the following:
• Texas Instruments, SN6501 Transformer Driver for Isolated Power Supplies data sheet
• Texas Instruments, ISO1050 Isolated CAN Transceiver data sheet
• Texas Instruments, ISO1042 Isolated CAN Transceiver With 70-V Bus Fault Protection and Flexible
Data Rate data sheet
• Texas Instruments, ISO72x Single Channel High-Speed Digital Isolators data sheet
• Texas Instruments, High-Voltage Lifetime of the ISO72x Family of Digital Isolators application report
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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 (January 2013) to A Revision .................................................................................................... Page
•
8
Added the ISO1042 device recommendation
Revision History
.........................................................................................
1
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