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LM25576: Unable to change output voltage regardless of feedback

Habibi
Prodigy 70 points
Part Number: LM25576

Hello,

I am trying to adjust the output voltage of the LM25576, circuit attached. It runs off +/-16V, and boosts up to a high voltage, however the regulator main output needs to be 12V

Unfortunately the output is fixed to around 5.6-5.9V no matter how much I change the feedback voltage divider. HV SMPS0.4.pdf

Have I done something stupid?

Thanks in advance

Charlie

  • 0
    TI__Guru 57555 points

    It looks like you are trying to convert +16V to a -16V using the LM25576 as an inverting buck boost; true?

    You also have some kind of charge pump on the switch.

    Your FB divder is set to give an output of 6.95V=1.22*(1+4.7K/1k)

    You will need 16V=1.22(1+12.1K/1k)

  • 0 in reply to Frank De Stasi
    Prodigy 70 points

    Hello,

    Sorry the schematic was not very well labelled.

    The regulator is referenced to -16V, so I am trying to get -4V from +16V (+12V referenced to -16V). The attached was my original design that I used to make 6V output, but just wanted to increase this. The circuit off the switch creates a HV 1mA supply for a tube.

    However, no matter what feedback resistor I put into the circuit, the regulator output is always around 5.5-6V. I tried values all the way up to 20K and there was no change.

    I guess if there is nothing essentially wrong with the topology I must have done something wrong with the build or PCB...?

    Thanks for your help.

  • 0 in reply to Habibi
    TI__Guru 57555 points

    I would double check you values with the calculator tool on the TI website.  

    And also check your PCB layout.

    If you are still having trouble you could temporarily disconnect the charge pump and see if that changes anything.

  • 0 in reply to Frank De Stasi
    TI__Guru 57555 points

    Hello

    I am closing this post due to inactivity.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 70 points

    Hello,

    The issue was with my build. I used a 1000uF cap instead of the 470uF I planned to as the initial load of the regulator.

    I checked in the datasheet and didn't see anything about a maximum load capacitance, did I miss something?

    Thanks for your help

  • 0 in reply to Habibi
    TI__Guru 57555 points

    Hello

    Thanks for the update.

    I don't think that the maximum output capacitance is explicitly stated in the data sheet.

    However, 1000uF is a bit large for most DC/DC converters.

    Thanks