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Original question:

ISOW1432: Heating issue

ISOW1432: What is the expected current/dissipation?

Ian Gregg
Prodigy 10 points
Part Number: ISOW1432

Tool/software:

Similar to others I have ISOW1432 correctly wired, DE is low, EN is high, mode pin floating for 3.3V output.  Isolated output measures 3.3V

There is no external connection, just 120R termination.

It draws 220mA from the 5V supply when idle and the thermal camera shows a hot spot near pins 11 and 12 that is at 55 degC which is more than 30 deg above ambient.

This seems wrong to me but I have spent a lot of time looking for errors in the layout and disconnecting the outputs.

  • 0
    TI__Expert 7325 points

    Hi Ian

    Thanks for reaching out.

    Can you please share a schematic and layout for us to better review this. 220mA seems to be on the higher side indeed.

    Can you please make Mode Pin as low and check again.

    Regards
    Varun

  • 0 in reply to Varun Kumar
    Prodigy 10 points

    Bad turn on isolated Vout pin12/pin11.  5V input current 220mA

    Good turn on, 5V input current 14mA

  • 0 in reply to Ian Gregg
    Prodigy 10 points

    The good turn on was after a lot of hacking to try different things including lifting the whole isolated side of the IC off the board, still had the problem.

    After the hacking the 2 ferrite beads were removed and the pads shorted.

  • 0 in reply to Ian Gregg
    Prodigy 10 points

    Board A004 draws 440mA! Isolated Vout looks like

    Note different time scales.

  • 0 in reply to Ian Gregg
    Prodigy 10 points

    I have checked that MODE pin is low with both multimeter and oscilloscope

  • 0 in reply to Ian Gregg
    Prodigy 10 points

    I can hear it buzzing when it is drawing a lot of current.

  • 0 in reply to Ian Gregg
    TI__Expert 7325 points

    Hi Ian,

    Thanks for the details. Couple of points and questions here:

    1. Questions on the data provides by you:
      1. Can you please specify clearly what changed for Good Turn ON after a Bad Turn ON?
      2. Was the VIO powered ON while observing ?
        1. If NO, then VIO needs to to be powered ON for Voltage Regulation and Current Draw from Power Converter as well.
      3. The VISOOUT and VISOIN also should be shorted all the time for regulation to work.
        1. Shorting via Ferrite Bead is also good.
      4. What is the VISOOUT value that you're seeing when the IDD current is on the higher side?
    2. Feedback on Schematics :
      1. The Supply Decaps at VDD and VISOOUT are not as per the minimum recommended decaps mentioned in datasheet, especially the 10nF cap is missing on both ends
        1. Ferrite bead(FB6) should not come between the intended decaps for Pin12(VISOOUT), but rather between VISOOUT and VISOIN Decaps.
        2. Please also follow strict layout guidelines given in datasheet for optimal performance of ISOW1432

    Regards
    Varun

  • 0 in reply to Varun Kumar
    Prodigy 10 points

    1a I have no idea why one device started working correctly

    1b VIO is always on

    1c isolated supply output is normally connected to the isolated power input and the 2 output grounds are connected

    1d VISOUT is usually 3.3V as you can see from the oscilloscope screenshots

    2a I have experimented with extra decaps across pins 11 and 12 without resolving the issue.

  • 0 in reply to Ian Gregg
    TI__Expert 7325 points

    Hi Ian,

    Thanks for the details. From What you said it looks like that Output Voltage is able to reach 3.3V but It's only the IDD that is of problem here.

    Does ABA replacement solve the issue on the same board?

    How Many Discrete Boards are showing this issue at your end?

    Can you please keep EN as HIGH and check again? I think that somehow the decaps on VDD and VISOOUT are not able to deliver the current required for Power Operation.

    Regards
    Varun