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ISO1540: ISO1540 in series

Axel Schoenfeld
Prodigy 50 points
Part Number: ISO1540
Other Parts Discussed in Thread: TCA9517

Dear Sirs,

I have a question about the ISO1540
Can I connect two ISO1540 in series?

I would like to increase the capacitive switching capacity to 400pF for each of the separate IC2 buses

Please refer to the attached circuit diagram.

Two ISO1540 (IC201 and IC202) are connected in series.
(the power signals DAM_5V and VCC correspond to a supply voltage of +5V DC)
The IC203 is used for a reset of the I2C bus.

Many thanks for the help

Axel

  • 0
    TI__Mastermind 49495 points

    Hi Axel,

    Thank you for reaching out to us with your question.

    I am not sure if I understood your question correctly. Did you mean to say that you wanted to achieve load capacitance support of upto 400pF on both sides of the I2C bus? Any particular reason why you want to do that? Do you expect such high loads on the I2C bus? Can you please share which devices do you expect to have such high capacitive load?

    ISO1540 is an I2C isolator that is intended to isolate an I2C node where the node can be a master or a slave. This means that ISO1540 can only have one node on one side while can have more than one node on the other side. Thus, Side1 of ISO1540 is designed to be interfaced to only one one and hence only supports upto 40pF load. While Side2 can be interfaced to more than one node and hence supports cap load of upto 400pF.

    You are probably already familiar with the information I have shared above. One other thing to note here is that, ISO1540 Side1 of one unit can't be connected to Side1 of another ISO1540. This limitation is due to the bidirectional channel implementation in device.
    Hence, the implementation that you have done in your schematic will not be able to support reliable communication. Let me know if you have any questions, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 50 points

    Thanks for the help.

    Yes, unfortunately I found out that I can't put two ISO1540 in series and thanks to them I now understand why.

    We have an old circuit board that we have to use, on which the I2C bus was not laid out properly.
    This increases the capacitive load immensely and there are several nodes. Unfortunately I can't separate the master on the old board.

    Could I alternatively use the TCA9517 for the IC202 (see schematic)?
    Pin 5 of course would not have to be connected.

  • 0 in reply to Axel Schoenfeld
    TI__Mastermind 49495 points

    Hi Axel,

    Thank you for your inputs.

    I now understand why you are looking for higher load cap support. You mentioned that there are multiple nodes on the PCB (including the master) that connect to one side of ISO1540, do you also have multiple nodes on the other side of ISO1540?

    Unfortunately, TCA9517 is going to have somewhat similar challenge. SideB of TCA9517 has LOW input voltage threshold (VILC) and LOW output voltage (VOL) very close to each other. This makes it challenging to connect more than one node to SideB of TCA9517 and SideB also cannot be connected to Side1 of ISO5140.

    This limitation is usually common for most I2C isolators and buffers. Most of these devices use two unidirectional channels to achieve a single bidirectional channel. When two unidirectional channels are connected back-to-back directly, the channels will latch-up in LOW and will never come back. To avoid such a latch-up, a break in the loop is introduced and that is achieved by keeping the two thresholds I referred earlier unusually close to each other. Hence, the limitation.

    The solution to your issue would be to isolate only a single node. If that is not possible, then a basic MCU between two ISO1540 devices maybe needed to achieve I2C bus isolation. Let me know if you have any questions, thank you.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 50 points

    Like I said, I can't get the knots resolved.

    If I do without galvanic isolation, is there an IC2 bus driver that would be suitable for me?



  • 0 in reply to Axel Schoenfeld
    TI__Mastermind 49495 points

    Hi Axel,

    I will notify this post to the non-isolated I2C team so that you get best device recommendations. Thanks for your patience and inputs so far.

    Regards,
    Koteshwar Rao

  • 0 in reply to Axel Schoenfeld
    TI__Mastermind 49370 points

    Hey Axel,

    TCA9517 can interface with the ISO1540 but A side of TCA9517 would need to be connected to side 1 of the ISO1540. It doesn't look like the device is a pin to pin replacement though seeing as pin 5 is GND for ISO1540 and EN for TCA9517. This would mean you would need to respin the board.

    Please note though, that B side of TCA9517 should not connect to any other I2C buffers with a static voltage offset or rise time accelerator.

    Thanks,

    -Bobby