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LMR51450: LMR51450 Unstable

Zheng Hao Chin
Prodigy 50 points
Part Number: LMR51450

Tool/software:

Hi, I am using the LMR51450 as a 12V buck converter for a 4S li-ion battery (12.8V-16.8V). I followed the schematic and recommended component values as mentioned in the datasheet, but the output of the buck converter is really unstable and noisy and hovers around the 11V range when the input supply is in the range of 15-16.8V. I am drawing roughly 3A of current at 12V.

I would like to request for a review of my schematics and layout. Thank you!

6281.schematic.pdf

  • 0
    TI__Mastermind 22260 points

    Hello,

    The inductance is low for this application. For a 12V output, the datasheet recommends 8.2uH.

    Also, what are the voltage ratings of your output capacitors and the specific part numbers? It may be that the output capacitors are de-rating under 12V bias.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 50 points

    Based on this table isn't the recommended inductance 6.8uH?

    The output capacitors used are these. They are rated for 50V, so it should be fine right?

  • 0 in reply to Zheng Hao Chin
    TI__Mastermind 22260 points

    Hello,

    Yes that is correct. I misread the schematic as 4A. Your inductor is OK for this 5A design.

    For the capacitors, refer to the capacitance vs. DC bias curve on the manufacturer website: https://www.murata.com/en-us/products/productdetail?partno=GRM21BR61H106KE43%23

    According to the manufacturer website, those 50V capacitors are derating by 73% under 12V bias, which means your output capacitance is low. This low capacitance may be causing instability. Also, depending on your ambient temperature, the capacitors may be de-rating even more.

    Try the following:

    1. Add 2 or 3 more capacitors to raise the effective capacitance after de-rating 
    2. Use X7R capacitors, which will usually have a softer de-rating curve than the X5R capacitors.

    Replace the capacitors with either, or both, of those options and retest.

    With the new test, please provide oscilloscope images of Vin, Vsw, Vout, and VCC.

    Best regards,

    Ridge