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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?
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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,
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