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How to properly shutdown multiple DC-DCs - LM25575 doesn't shutdown

Intellectual 680 points

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We use multiple DC-DCs and one of them (LM25575) doesn't shutdown properly.

We use the LM25575 to generate a Vout=-11V from a Vin=12V. The SD (Shutdown) pin is controlled by a transistor from a microcontroller. The same control signal goes to other DC-DC converters. I can turn all the DC-DCs on with a enable pin voltage of 2.7V but when turning OFF the LM25575 doesn't shutdown even though the voltage on its SD pin is around -0.5V. The voltage at the output of LM25575 is still -11V and all the other DC-DCs will have some output voltage but bellow 1V.

For LM25575, I can see a 500KHz switch frequency on SW pin even when the SD pin is at -0.5V. I was thinking the LM25575 could be powered from the load but couldn't find anything in that direction.

So my questions are:

a) Is there any application note that describes how to properly shutdown multiple DC-DCs? Any pitfalls?

b) How do you troubleshoot a "power from the load" issue?

Thanks,

Valentin

  • Hi Valentin-

    The EN pin needs to be level-shifted and referrenced to -11Vout since you're using it in a negative buck-boost configuration. Please take a look at Application Note 2027 (AN-2027) for some examples of EN pin level shifting circuits. Here's the link to this App Note: www.ti.com/.../snva425a.pdf

    Let us know if you have further questions.

    Best Regards,
    -JP
  • In reply to Juan Paulo Fung:

    JP, Having EN pin level-shifted and referenced to -11V was my issue.

    The app note gave me exactly the info I was looking for and I can shutdown everything properly now.

    Thanks a lot for your quick answer,

    Valentin

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