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ISOW7841: Using 2 ISOW7841 parts in parallel

Part Number: ISOW7841
Other Parts Discussed in Thread: ISO7841, ISO7741

Hi,

Currently in our design we are using 2 ISOW7841 parts in parallel as we have 7 SPI lines that we need to isolate. When we do this, we have both of our parts supplied by 3V3, but only 1 part feeding 3V3 to isolation. This setup causes our part that is not supplying 3V3 to get hot and not work (we know this from if we take off 1 part the system works.)

Are there any issues when doing this? Would another part be better suited for this?

Thanks,

James

  • Hi James,

    How are the two ISOW7841 parts connected? If one is supplying power, is the other ISOW7841 Viso pin floating or connected somewhere? Multiple ISOW7841 can be used without issues, so we should be able to determine and fix the root cause :)

    Since two devices generating isolated power are not needed, the second ISOW7841 can be replaced with an ISO7741 or ISO7841. These devices could be powered by the same 3.3V on the non-isolated side and the Viso supply of the ISOW7841 on the isolated side.

    Please let me know your response.


    Thank you for your time,
    Manuel Chavez
  • Hi Manuel,

    The 2 parts are only connected via the Grounds on both sides. Part 1 is supplying 3V3 and part 2 Viso is left floating. A weird thing we are seeing is where if I remove part 2, and branch over the connections with a wire, the device still doesn't work unless I am touching the ground plane. Does this help?

    thanks,

    James
  • Hi James,

    Manuel is going to be away from work for a couple days, I will try to assist you with this issue in his absence.
    Thank you for your inputs so far, to better understand the issue and help debug can you please help me with the below information?

    1. Schematic and PCB layout showing both the ISOW devices.
    2. Do you know what is the current rating of the 3.3V supply that is powering both ISOW devices?
    3. Did you try to replace the non-working ISOW device with a new one to see if the existing ISOW device is somehow damaged?
    4. When the device gets hot, do you notice an increase in the input current? If you already know the increase in current, please do let me know the value.
    5. How do you check if the ISOW device is working or not? Do you try to establish communication through ISOW device data channels and then notice no response? Or do you monitor VISO voltage?
    6. Even though the VISO of non-working device is left floating, I am assuming you still have the 10uF + 0.1uF capacitors connected at VISO.

    Please do help me with the above information so that I can try debug the issue.
    If you are not comfortable sharing schematic and PCB layout on this public forum then please let me know if I can reach out to you on your email? Thanks.


    Regards,
    Koteshwar Rao
  • Hi Koteshwar,

    Thanks for helping:

    1. Schematic and PCB layout showing both the ISOW devices. - I can supply these to you over email, I think that would be better
    2. Do you know what is the current rating of the 3.3V supply that is powering both ISOW devices? - We do not know this, but should be at least 2A
    3. Did you try to replace the non-working ISOW device with a new one to see if the existing ISOW device is somehow damaged? - Yes, this still gives us the same problem
    4. When the device gets hot, do you notice an increase in the input current? If you already know the increase in current, please do let me know the value. - I'll see if I can measure this.
    5. How do you check if the ISOW device is working or not? Do you try to establish communication through ISOW device data channels and then notice no response? Or do you monitor VISO voltage? - We try to establish communication through the ISOW part. when it is off we can get comms by touching the Iso Ground, but not when it's left floating
    6. Even though the VISO of non-working device is left floating, I am assuming you still have the 10uF + 0.1uF capacitors connected at VISO. - No we do not have this, would it make a big enough difference?

    thanks,

    James

  • Hi James,

    Thank you for your quick and clear inputs for all the questions I have asked.

    Since ISOW integrates an isolated DC/DC converter and it being a power supply, a min of 10uF bulk cap is necessary for both VCC and VISO pins for the device to operate normal. Without the bulk cap, the functionality of ISOW is not guaranteed. An additional 0.1uF capacitor at both VCC and VISO would help it improve device's transient performance.

    Please use 10uF + 0.1uF at both VCC/GND and VISO/GNDISO pins of ISOW and test circuit the again. We expect it to work without any issues. After your test, please let us know if the issue is resolved or not. Thanks.


    Regards,
    Koteshwar Rao
  • Hi Koteshwar,

    Thanks for the input. I have attached 2 caps to both sides of the ISOW part and still we have no communications. Do you have any other ideas? If you want to see our schematics and layout please do not hesitate to email me.

    thanks,

    James

  • Hi James,

    Like we discussed over email, please do let us know once you have your boards working. Thanks.


    Regards,
    Koteshwar Rao