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LM5166: LM5166 output noise

Paul Dye
Prodigy 180 points
Part Number: LM5166

Hi,

I have recently powered up a new PCB and have checked the stability of the 5V output.

The design is the same as the Webench simulation. Cin and Cout are made up of MLCC caps.

I expected the output ripple to be around 90mV but actually it is 190mV.

Also the frequency of the ripple is 45kHz but the simulation shows the switching frequency should be around 350kHz.

Would you expect to see the 350kHz frequency at the output?

I can't attach the Webench pdf report to this post but here is the circuit. Also i've attached a scope trace of the 5V output.

Do you have any suggestions I can try to reduce the noise?

Thanks

  • 0
    TI__Mastermind 23835 points
    Hi Paul,

    Do you have the waveform for the SW node of the device?
    Yes, i would expect to see when the part is at full load, it will switch at 350kHz.
    Have you try to increase the load current and see if the switching frequency increases?

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 180 points

    Hi Arief,

    I am attaching a waveform for the SW node.

    I added 100R to the output and it did not increase the frequency.

    Is there anything that can be done to reduce the noise? 

    Thanks

    Paul

  • 0 in reply to Paul Dye
    TI__Mastermind 23835 points
    Hi,

    From the switch waveform, it does looks normal because the part is operating in pulse frequency mode.
    You can also try to reduce the value of RESR to be 1 ohm instead of 2.4 ohm.

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 180 points
    Hi,
    Thanks for the response.
    What is the minimum value of Resr for the circuit configuration shown in the first post?
    The datasheet doesn't seem to show a representative scenario in Table 2. Ripple generation methods.
    I presume if Resr is too small the regulator output could become unstable?

    Thanks
    Paul
  • 0 in reply to Paul Dye
    TI__Mastermind 23835 points
    Hi Paul,

    In general the rule of thumb is that we want to have a peak to peak ripple voltage of 25mV at the FB node of the IC.
    Now if you use a fixed output voltage then you have to scale up based on your output voltage.

    In your case you have an output voltage of 5V and the internal FB reference is at 1.223V, therefore you are 4 times larger at the output and that means you need around 100mV output voltage ripple roughly

    Thanks
    -Arief