Part Number: ISO1050
My application is an update to an existing design. This device is a 200W class electric motor actuator for use on a 12VDC vehicle using CAN. The legacy connector between the device and the vehicle has four pins, CAN_H, CAN_L, +12VDCin, and GND. I don't have the freedom to modify this connector.
I am concerned about this configuration. Under full power, ~20A is pulled off the vehicle. If the GND path has much more than 0.100 ohms resistance, the CAN lines will be appear to have violated the common mode specification from the perspective of the CAN transceiver in our product. We could add isolation around the CAN transceiver in the new design, however it still needs some reference for the CAN bus (?). The floating supply and the transceiver need to have a 0v reference to properly locate its own output relative to the vehicle bus.
I've seen the reference designs from TI and others, however they don't answer this question conclusively. A split termination with Vsense tied to the center tap might be a solution if this device was one of the bus terminators. Unfortunately, I can't assume that this device will always be allowed as a bus terminator. Could large value resistors be used instead of 60 ohms to avoid loading the bus, yet still supply a pseudo-reference?
Certainly, this isn't the first time this has come up. Is there an industry accepted practice to solve this challenge?
First off, thank you for the great question.
I think you are getting the internal common mode specification confused with the external common mode specification.
It is true that the bus output voltage specification has to be between 2 and 3V. So 20A at 0.100 ohms would be about a 2V drop, appearing to not meet the specification. But the standard actually does allow this. The 2-3V specification is the internal common mode. So the output of the device has to be between 2 and 3V with respect to the output ground (on the ISO1050 that would be GND2). See Bus output voltage (Recessive) in the ISO1050 datasheet in section 6.6 Electrical Characteristics: Driver on page 7 of the ISO1050 datasheet.
The CAN standard also has an external common mode specification. This allows the CAN signal common mode to go between -12V and +12V without violating spec as an input. This is where that 20A and 0.100 ohms would come into play. And here 2V is an acceptable amount. In section 6.3 Recommended Operating Conditions - Voltage at bus pins (separately or common mode) is specified at -12 to 12.
To your question about split termination and biased split termination, this can be done, but is usually done in the context of improving EMC performance.
Biased Split Termination
To your question about split termination and biased split termination, this can be done, but is usually done in the context of improving EMC performance. The 60 ohm resistors cannot be changed, but R1 and R2 in the biased split termination could be sufficiently large.
Please let me know if that answered your question or if you need me to clarify anything!
In reply to Dan Kisling:
In reply to Thom Kreider:
Thanks for the pun; it put a smile on my face.
The CAN standard changed in December of 2016 to -12V to +12V. You are correct that it was -2V to 7V previous to that.
To the question of "IS this actually a problem," I don't think it is, but we should of course do our due diligence to ensure it isn't.
If you would like to share a schematic our team would be happy to review it. You can post here if you are comfortable posting the schematic on a public forum, or we can move over to private messages to share sensitive information. I've sent you a friend request on E2E so we can use the messenger feature.
Looking forward to hearing from you!
Thanks for accepting my friend request.
You do bring up an important point: making this work with heritage equipment complying with an older specification is very important.
Perhaps I can explain how I see the problem and you can correct me if my understanding is off:
For a first pass, let’s assume we can just add these voltages together:
The team had a discussion on this thread today and we had a few thoughts:
0.2Ω would be the limit for the data path if there was a 20A pull on the data lines. We should also investigate the 20A on the data path to make sure we fully understand the problem.
Let’s take the ISO1050 as an example, though we do want to apply this to any CAN system.
Short-circuit steady-state output current is limited to a max of 105mA according to the Electrical Characteristics: Driver of the ISO1050 datasheet (see page 7 of the datasheet). If I recall correctly, the CAN specification limits this to 115mA for any CAN transceiver.
We have to consider what is driving side 2 of the isolator such that it would be able to sustain such current. Typically (though I cannot say every time) side 2 isolated power supply is derived from side 1. For example, let’s use the Typical Application circuit in section 9.2 on page 22 of the ISO1050 datasheet.
In this case the secondary power supply comes from the SN6501 paired with a transformer and LDO. The absolute maximum rating for peak output switch current of the SN6501 is 500mA – far from our original problem statement of 20A.
So at this point it seems appropriate for us to question the 20A pull on the CAN data path that would push us outside of our common mode limit.
Here are some clarifying questions we had:
With this information we should be able to explore what limitations the system will have and how to account for them.
Please let me know if I can clarify any parts of my post!
I see the root of our communication issue. The issue is 20A through the +13.8v bus and power return, not down the CAN lines. I took your picture and doctored it up a little. The isolation boundary isn't interesting to this discussion - the power supply to the isolated side is removed because I'm not worried about the iso boundary.
Notice that the very large load in the star shape (made it ludicrously large to get the idea across). That load is sharing the GND return line with CAN. There's 0.2 ohms stuck in that shared line on the way back to the battery and SPG.
This will drive CM onto the CAN lines. It will NOT draw 20A down those lines, just assert a bunch of apparent voltage onto the two lines from the perspective of the other CAN bus nodes. There's nothing special or unusual about the CAN bus. The nodes will see a bunch of CM and there's no way to solve that other than to bring in a dedicated CAN reference to float the PHY against OR some other mystery solution that I'm trying to get figured out.
I think we are on the same page now.
One thing I would like to bring up is that VCM is internally driven on the ISO1050, and virtually all CAN transceivers do this as well..
While VCM may be exposed externally on some chips and not on others, it is still present internally. Driving the bus to VCM externally (for example, through a biased split termination as we discussed earlier) could help with things such as EMC, but are not really expected to have a large impact on common mode. Furthermore, this technique can only be used at terminating nodes.
So this brings us to the critical questions: how do we solve this common mode problem when we get outside of the bounds we solved for earlier – and how much is that going to cost?
The good news is there are parts that far exceed the -2V to +7V of the old CAN standard. As I mentioned earlier the ISO1050 can operate from -12V to +12V. As another example, the TCAN1042 can exceed +/-20V Common Mode Voltage (See VIT+ and VIT- on page 9 of the TCAN1042 datasheet). Here is a list of CAN transceivers on ti.com that have a high voltage common mode voltage.
Admittedly CAN transceivers from other semiconductor companies can also have a large common mode voltage range. I’ll leave those to a google search as an exercise for the reader.
I am not at liberty to discuss specific pricing on the forum, but I think we can still talk to this. As you put it “If the answer is $0.50 worth of parts to get another 12v of headroom that will be one thing. If the answer is $50 to get another 1v, that will be another.” The TCAN device I listed earlier are in the ballpark of $1 and will give you large common mode headroom.
As always, let me know if I need to clarify anything or if I've answered your question.
It is pleasure discussing this with you.
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