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WEBENCH® Tools/LMZ14203: 400mV ripple

fremsoft
Prodigy 60 points
Part Number: LMZ14203

Tool/software: WEBENCH® Design Tools

Hi all, I have some troubles with LMZ14203 DC-DC Step-down converter.

The Datasheet is declaring 8mVpp of ripple, while in reality I am measuring 340mV over my board.

This is the WEBENCH proposal:

and this is the waveform from the simulator:

Now I show you the real schematic:

Ok, with this setup, I have the following waforms on the Oscilloscope:

  • 0
    TI__Mastermind 23815 points

    Hi,

    I will relay this information to the apps engineer for the product.

    Also, do you have the layout for the IC?

    Thanks
    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 60 points

    Thank you very much for your quick rensponse.

    This is the layout over 6 layers:

  • 0 in reply to fremsoft
    TI__Mastermind 23815 points
    Would you mind highlighting the location of the input capacitors that is near the VIN pin and its relation to GND (PAD)?

    Thanks
    -Arief
  • 0 in reply to fremsoft
    TI__Intellectual 1155 points
    Hi,

    what kind of load do you have? Did you take the measurements with or without a load? The output voltage is ramping up and down with a frequency around 350Hz (see your pictures). This is much below the switching frequency. So it might be that the "ripple" is kind of instability.

    1. Do you know if your device is in DCM or CCM? This depends amongst other factors on the inductor ripple current (see section 7.4.1. of the datasheet).

    2. Perhaps your injected voltage ripple on the FB pin is too large. When the voltage on the FB pin exceeds a certain value the device will turn off. Note: Some ripple at the FB pin is needed, but too much can make the device unstable.

    2.1. What is the ESR of C6?
    2.2. Try to run the device without C5 (the Feed forward cap). You use an electrolytic cap C6 as output cap. Electrolytic caps have a larger ESR than ceramic caps. The injected ripple to the FB pin is already high enough and you don't need C5. This will only add more ripple and therefore force the device to shut off.
    2.3. You also can try to place another ceramic cap in parallel to C6. This will lower your overall ESR and can result also in lower ripple at the FB pin.

    Please let me know, if one of the suggestions works.

    Best regards,
    Michael
  • 0 in reply to Michael Daimer
    Prodigy 60 points

    I have removed the C5 capacitor and the ripple changed in the following manner:

    Increasing LOAD on 5V this is the new ripple waveform:

    Sincerely this is solving major of issues but it is very far from 8mVpp declared in the datasheet

    Unfortunately I dont' know the ESR of C6

  • 0 in reply to fremsoft
    TI__Intellectual 1155 points
    According to your schematic, I modified a LMZ14203 EVM trying to reproduce the issue.

    I removed the Cff cap and the two ceramic output caps (1uF and 100uF) and inserted instead an electrolytic cap with 100uF, like in your application. The output ripple was large, around 200mVpp. Close to instability.
    Then I replaced the 100uF electrolytic cap with a 100uF ceramic cap, and had a look at the output ripple. It was much better, around 20mVpp.

    8mVpp output ripple for a switching regulator is hard to achieve and needs an almost perfect layout. This value can't be achieved with only electrolytic caps. The graph in the datasheet for the output ripple is also taken with a high quality X7R ceramic output capacitor.

    Nevertheless, it seems that adding a ceramic capacitor to the output helps to decrease the ripple.

    Best regards,
    Michael
  • 0 in reply to Michael Daimer
    Prodigy 60 points
    Thank you very much for your kind support
  • 0 in reply to fremsoft
    TI__Intellectual 1155 points
    Glad I could help you.
    If you have more questions or need further support, please don't hesitate to contact us again.
    In case your question about this topics is solved, please verify the answer.

    Many thanks!