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  • Resolved

LM25575 stuck at -1.6V for a -12V rail

Prodigy 40 points
CZ

Replies: 5

Views: 528

Hello!


I have a LM25575 hooked up as an inverting power supply, giving me -12V out of +12V. I followed the Webbench Design closely and ordered the parts it recommended me (except for resistors). I attached the schematic and layout. Current draw is 300mA and my input comes from another wall wart 12V power supply. My input voltage doesn't look jagged up or anything, also not when the supply is running.

Whenever I turn on this power supply, it will go to -1.6 V and then just stay there. I have no idea how this could have happened because I had it on a prototyping PCB before I incorporated it into my actual design now and there it worked perfectly. I would be happy about any help here.

Prodigy 40 points
CZ
  • Prodigy 40 points
    CZ
    I don't know if this helps but when I remove the load from the output, I get -4,6V.
  • Prodigy 40 points
    CZ
    Okay, here is another good data point. When I change the ration of Rfbt and Rfbb, the output stays the same. Even if I remove Rfbt, the output and behaviour stays exactly the same. What can that mean? The IC is broken?
  • Genius 16025 points
    Anston Lobo

    In reply to CZ:

    Hi CZ,

    I'm looking at this right now.

    Thanks,
    Anston
  • Genius 16025 points
    Anston Lobo

    In reply to Anston Lobo:

    CZ,

    The problem is likely with your layout. Pin 11 should not be connected to the Pad under the IC.

    The exposed pad of the IC should be -12V not GND.

    Pin 11 should connect to GND not exposed PAD (-12V)

    Also, it is not recommended to Via VIN with just one VIA.

    SW node and PRE should both make contact with the trace to the inductor not just PRE.

    Let me know if this helps.

    Thanks,
    Anston
  • Prodigy 40 points
    CZ

    In reply to Anston Lobo:

    Hello Anston,


    thanks for the answer.


    Okay, so you're saying Pin 11 should be connected to GND but not the exposed pad under the IC. At the same time the exposed pad should be connected to -12V. I don't fully understand this because Pin 11 is in fact connected to GND and from what I gather from the datasheet the exposed pad is exclusively used as a heatsink - or am I wrong?

    Also, I have a prototype PCB of the same schematic - Pin 11 connected to GND and the exposed pad connected to GND as well - and it works in that setup. The layout isn't exactly the same and Pin 11 is in that case not directly connected to the exposed pad. But yet it works.


    Can you also explain why it is advantageous to connect Vin with more vias and SW node and PRE should both make contact with the inductor trace?


    Thanks a lot for your help!

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