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LM3478: Output ripple

Sandra Pradeep
Intellectual 300 points
Part Number: LM3478


Tool/software:

Hi Team,

We designed the LM3478 Boost circuit based on the Webench simulation (attached simulation report)

WBDesign8-LM3478MMNOPB 2.pdf

Below are the inputs:

Vin(min): 4.9V

Vin (Max): 5.1V

Current: 0.5A

Vout: 48V

But the output Generated with zero load has a ripple of 2V. Attached is the output image..

Could you please help to solve this issue.. what will be causing this issue??

  • 0
    TI__Mastermind 31800 points

    Hi Sandra,

    Thanks for using the e2e forum.

    This voltage ripple might come from the skip mode operation.
    At light load, the device operation in DCM mode with decreased duty cycle. If the required on-time of the duty cycle is smaller than the minimum on-time of the device, Vout will slowly rise until the overvoltage protection triggers. Then the device stops until Vout drops and the process repeats.

    To avoid this behavior, it would be required to either add a dummy load to artificially increase the duty cycle, or use a larger inductance to shift the entrance point of skip mode operation to a smaller Iout value. However, when there is no load at all, even a higher inductance cannot avoid pulse skip operation.

    Please let me know if you have additional questions on this topic.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 300 points

    Thanks for your reply Niklas..

    i did measurement once again with load..

    below is the observation.

    The simulation is done for 500mA, but with a load current more than 18mA I'm getting ripple around 2V.

    Cont. Load Voltage
    Vmax Vmin Ripple (mV)
    0A 48.62 48.43 185.55
    18mA 48.6 48.39 214.85mV
    20mA 49.37 47.68 1.68V
    50mA 49.46 47.473 2V
  • 0 in reply to Sandra Pradeep
    TI__Mastermind 31800 points

    Hi Sandra,

    Thanks for the quick feedback.
    Are the values from the table simulated, or measured on bench?
    They show a behavior that is the opposite of what I would expect based on pulse skip behavior...

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 300 points

    Hi Niklas,

    I measured these values from our board.

    In our board, I've isolated the input and output of LM3478 from other circuits.. applied load at the output using a DC loader and supplied input 5V using an external DC supply

  • 0 in reply to Sandra Pradeep
    TI__Mastermind 31800 points

    Hi Sandra,

    Thanks for the confirmation.
    It is interesting that the threshold where the ripple drastically increases is at 20mA load.
    Based on calculation, the design should work in DCM in both conditions and the duty cycle should be around 50% at that time.

    Would it be possible to make a measurement of the switch node and the output voltage at 18mA and 20mA load and share the waveforms?
    Based on this we can check the switching behavior and check if the device is resetting or behaving strange.

    Can you also share what output capacitance is used?
    The webench design shows one 100uF bulk capacitor, but we recommend low ESR ceramic capacitors in parallel, which are better at filtering switching noise.

    Thanks and best regards,
    Niklas