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LM25117: Output ripple with strange waveform

Francis
Prodigy 90 points
Part Number: LM25117


Hi, I’m testing my buck converter design with the LM25117 controller

The operating conditions are
Vin = 24 V
Vout = 3,3 V
Iout = 4 A  (resistive load)
fsw = 250 kHz

The design use those components

Inductor = 10 uF 

Output capacitor (all the below components in parallel) 

  • Two 820 uF ( aluminium polymer capacitor,  ESR = 11 mOhm )
  • Four 10 uF 0805 MLCC
  • Two 100 nF 0805 MLCC

Cout = (2 * 820 uF + 4 * 10 uF + 2 * 100 nF) = 1680 uF

I check the stability of the converter with the loop gain measurement (injection transformer in parallel with 20 Ohm) and I find
Bandwidth = 3,5 kHz
Phase margin = 97 degree

I check also the load step response and it is ok

The output voltage ripple has a strange waveform  that I never see before.
I post a screenshot  (the output voltage ripple is take with the oscilloscope in AC coupling)
The voltage probes are used with low inductance connection (pig tail)

My questions are:

- Why the output voltage ripple has that waveform (that differs from the canonical waveform), what can be the cause?
- It may be an indication of a malfunction of the converter ?

Thanks

  • 0
    TI__Mastermind 22302 points

    Hi Francis,

    Your converter looks stable. Ultimately the LM25117 can only control output voltage with the MOSFETs and SW node. 

    Since the SW node and inductor current look good, I suspect its the output capacitor or load.

    Probably the 820µF capacitors do not have the frequency response to generate a sine wave in the output voltage ripple.

    It could be parasitic inductance in those large capacitors or even some inductance in the load.

    If the switching frequency is consistent and load transients are OK then I wouldnt worry about the shape of the output ripple, I think it is cosmetic.

    Hope this helps,

    -Orlando