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LMR51450: Should I be worried about the low frequency output ripple

Gavin Jones
Prodigy 70 points
Part Number: LMR51450

Tool/software:

Dear TI,

I have designed a 24V to 6V buck converter using the LMR51450SDRRR. The schematic is shown below. The converter is typically supplying 2A to the load.

I have assembled the boards and they work great. However, I have observed a low frequency ripple on the output of the converter, as shown in the oscilloscope trace below. The ripple has a frequency of around 7.9kHz and an amplitude of around 20mV pk-pk. I have confirmed that this is not a result of the oscilloscope aliasing the switching ripple (500kHz) - the low frequency ripple really is present. I have also confirmed that the 24V input rail is well regulated.

This ripple is not a problem in my application, but I am concerned that it might suggest a stability problem. Is this ripple expected, or should I be worried about it?

Thanks for your help.

Best regards,

Gavin.

  • 0
    TI__Guru 73515 points

    Hello

    You are using the device that incorporates spread spectrum, so you may be seeing the effect of that in the output voltage.

    You are also using an aluminum electrolytic on the output.  The ESR of that capacitor may be affecting the loop stability.

    If you perform a load transient, you may be able to tell if the loop is stable.  You can also try removing the aluminum electrolytic

    and replacing with an extra ceramic cap.  

    If you can provide your load transient image, we will try to troubleshoot further.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 70 points

    Hi Frank,

    Thanks for the response. I tested the load transient with a 2A load in series with a mechanical switch. The switch closes with a characteristic chatter. It opens cleanly. There is also a 100mOhm connection resistance between the regulator and my measurement point, which explains most of the DC step change in output voltage. Zero to 2A transient (with switch chatter) is as follows:

    The 2A to zero transient (on CH1) is as follows:

    Based on these transient responses, the stability looks good to me.

    Best regards,

    Gavin.

  • 0 in reply to Gavin Jones
    Prodigy 70 points

    Hi Frank,

    I remembered that I have an LMR51450EVM board. I set it up with 24V input, 5V output, 3.3Ohm load (1.5A) and I see the same ripple:

    The EVM board has only ceramic capacitors on the output.

    Is it safe to assume that the 8kHz ripple is just a characteristic of the LMR51450 and I do not need to worry about it?

    Best regards,

    Gavin.

  • +1 in reply to Gavin Jones
    TI__Guru 73515 points

    Hello

    Yes; it appears to be an artifact of the spread spectrum feature, as mentioned.

    Thanks