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LM25085: Not Holding Regulation at Low Current

Michael Stagg
Prodigy 120 points
Part Number: LM25085

I need some help. The regulator is setup to deliver about 4A continuous current at 5V. The input can vary from ~14VDC to 30VDC. It regulates fine once the output current exceeds ~300mA. However, the output voltage drops in the output current is less than 300mA. I have attached the schematic which is really just the recommended schematic from TI Webench. Is there anything I can do to get the regulation to work when the current is low <100MA? The output inductor is SRR1208-100ML from Bourns.

  • 0
    TI__Genius 11490 points

    Hi Michael,

    How much does the VOUT drop at low loads. Some data will help.

    Regards

  • 0 in reply to Vijay86929
    Prodigy 120 points

    Basically, it stop regulating. It goes to a mean of about 2.5V but not a very good regulated signal at all. It seems to run fine with a load of at least 300ma. It regulates well at ~200mA at 24V but when the input voltage is reduced to around 22V it start to occasionally drop out. Once you drop below 22V it pretty much drops out. IAs I increase the input voltage it is able to regulate the output at lower load currents. Once my load current is 300mA or above the regulator does fine over the entire spec'd voltage range.

  • 0 in reply to Michael Stagg
    TI__Genius 11490 points
    Hi Michael,

    It will be helpful to see the waveform of the VOUT. From what I see on the schematic, the ripple injection circuit as described in the LM25085 datasheet seems to be missing. That should be added.

    Please provide/check the FET part number. The LM25085 quick-start calculator (on the LM25085 product folder under "tools and software" tab) is a good way to check the schematic component selection. I would check to see if the FET is not too big in the quick-start.

    Also, it is good to see a CIN capacitor close the LM25085 IC. I was not able to find one in your schematic.

    Hope this helps.
  • 0 in reply to Vijay86929
    TI__Genius 11490 points
    quick-start excel tool. it is on the bottom of this page.

    www.ti.com/.../toolssoftware
  • 0 in reply to Vijay86929
    Prodigy 120 points
    Vijay,

    I changed the Cin and also added the bypass capacitor. This resolved the issue. Additionally, the inductor was recommended to be 16uH min and I was using 10uH. This change improved the quality of the output as well. Thanks!
  • 0 in reply to Michael Stagg
    Prodigy 120 points

    Vijay,

    I now have a slightly different issue. When using an Iron rod core inductor like RC-11 by Triad the regulator works fine. However, when I use a Bourns inductor SRR1208-220ML or really any other inductor we have in our lab the regulator will not turn on with a load unless the input voltage is <19VDC. It will turn on without any load. Once the regulator turns on then it runs fine. I can increase or decrease voltage across the entire range with or without a load. Any suggestion? I have tried a wide range of inductor values from 22uH to 100uH. I can use the RC-11, however, it is more costly and I do not understand why the regulator does not come on?

    Thanks!