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ISOW7840: Paralleling output of 2 ISOW7840; Latchup issue

Jerry Smith55
Prodigy 40 points
Part Number: ISOW7840
Other Parts Discussed in Thread: SN6505B, SN6501

Using ISOW7840 with LMV7235M7 comparators on isolated side. The comparators sometimes draw extra current at power-up (latch-up?) causing ISOW7840 to go into current limit. If sufficient current is applied (from additional bench supply), power comes up and stays up after extra supply removed. I've got 2 of the ISOW7840's and shorting their power outputs together to put them in parallel seems to provide adequate current to power up OK. Does placing outputs in parallel cause problems?

  • 0
    TI__Genius 17100 points

    Hi Jerry,

    Thank you for posting to E2E! Viso is not intended to be tied together for multiple ISOW78xx devices. Details on this can be found on the E2E thread below:




    It seems the comparators draw excessive current during power up, which ISOW7840 cannot provide, but this demand is reduced afterwards such that the devices stay powered up after the extra supply is removed.

    Can you share a schematic of the ISOW7840 devices showing which capacitors are placed on Vcc1 and Viso pins? What are the operating voltages of ISOW7840 and the expected current load on Viso pins?


    Thank you for your time,
    Manuel Chavez

  • 0
    TI__Mastermind 49495 points

    Hi Jerry,

    Thank you for considering ISOW7840 in your application. Like any power supply or DC/DC converter, ISOW7840 outputs of two devices cannot be directly tied together. Since the absolute output voltage practically cannot be the same, the one that has lower output voltage will sink a large current causing the other ISOW7840 to go in current limit mode. I believe that is what you are seeing.

    This is also true to most other power supplies and DC/DC converters. You need external ORing / load-sharing controllers or DC/DC with load such features. ISOW7840 doesn't support such operation. If possible, please do split the load between the two ISOW7840 devices so that they can still be used in the application without connecting them together. Thanks..

    Regards,
    Koteshwar Rao

  • 0 in reply to Manuel Chavez
    Prodigy 40 points

    Thanks for your quick reply!

    I'm using +5V on input, and +5V on isolated side.

    Here's the schematic showing the caps on the ISOW7840 and the comparator circuits:

  • 0 in reply to Jerry Smith55
    TI__Genius 17100 points

    Hi Jerry,

    No problem! It seems the schematic image did not upload correctly; you may attempt to re-upload it, but I did only want to confirm the bulk capacitance values placed close to Vcc and Viso are each ~11.1uF or greater.

    As Koteshwar and I mentioned above, the Viso pins of two ISOW78xx devices should not be tied together since one of the two devices will not be sourcing current, and the one sourcing current may go into "current-limit mode."

    Koteshwar also mentioned solutions for your consideration, and another option for isolated power supply needs is to generate an isolated power supply rail using a simple transformer-driver like SN6501 or SN6505B. These solutions are capable of isolating greater amounts of power than ISOW7840.

    Please let me know your thoughts on these responses.


    Thank you,
    Manuel Chavez