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ISO1540: Can I have multiple isolator chips on the same bus

Gary Sinde1
Prodigy 20 points

Part Number: ISO1540
Other Parts Discussed in Thread: TXS0104E,

I have a master controller I2C on a CPU. This goes through a Level translator (TXS0104E) so the CPU can stay in the 3.3V and the I2C can be at 5.0V. 

This I2C bus then goes to a back plane with 8 card connectors (allowing up to 8 slave "cards")

Each card has an ISO1540 which then goes to an I2C slave chip (could be I/O chip, could be temp chip, could be DAC chip, etc.) but up to 8 cards of any type can be used.

When I plug in any one card, they work fine. All cards work. When I plug in more than 1 card, I cannot communicate to any of them.

My Question: Are multiple ISO1540 chips allowed to sit on the same I2C bus in parallel in front of each slave chip?

  • 0
    TI__Guru 53535 points

    Hi Gary,

    Great question here, thanks for posting on E2E!

    Side-2 of ISO1540 is able to function in parallel with other ISO1540s side-2s, but not side-1. 

    ISO1540 uses a static voltage offset (SVO) on side-1 of the device to avoid latchup states. Because of this, this side of the device is unable to be connected to other devices that also use a static voltage offset (this includes other isolators or I2C buffers that use this method). Side-1 of ISO1540 also has a lower capacitive load limit of 40pF which is better suited for low capacitive local nodes and less-so for backplanes or long traces with multiple connections. 

    Side-2 of ISO1540 has neither of these restrictions and can be connected to any other I2C compliant device(s) and can tolerate capacitive loads up to the I2C specified 400pF. This includes compatibility with other devices that use static voltage offsets.

    It sounds like the cards you are using with ISO1540 may have side-1 facing the connection to the backplane. When only one SVO is present, there is no issue. However, once another device with an SVO is introduced, the system no longer works. The added capacitance of a second board may also exceed the maximum load of this side of the isolator. 

    Is it possible to check the orientation of the ISO1540 devices on the cards? If side-2 is able to be used for the backplane and parallel connections, I don't foresee what I described above to be an issue. Let me know which is the case and I can help further diagnose or may be able to suggest alternate solutions. 

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    Prodigy 20 points

    Hi Eric,

    Thank you for the quick response. 

    Yes, the ISO1540's on the "cards" have their side "1" facing the backplane and I think I understand the problem with the SVO.

    Some of our "cards" have only 1 I2C chip so flipping it would work. But some have several I2C chips on them so even flipping the ISO1540 sounds like it would not work in those cases. 

    Gary

  • 0 in reply to Gary Sinde1
    TI__Guru 53535 points

    Hi Gary,

    Glad to hear we could identify the source of this part of the issue.

    It's true that the 40pF load limit on ISO1540 does not lend itself well to multiple device connections. If the traces are short enough and device capacitive loads small enough (they should specify this on their datasheet), you may be able to stay below or near this limit. However, if this is not the case, you may need to add I2C buffers between devices (taking care not to attach another SVO side to ISO1540's side-1) to reduce the presented capacitance to ISO1540. 

    Let me know if these solutions sound possible for your system or if there are any other constraints we need to consider.

    Regards,
    Eric