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Original question:

LM3281: Why should I use a buck converter when I can use an LDO to power my WIFI Module and circuit?

LM3281: Some question about LM3281

Lumina Gui
Genius 3680 points
Part Number: LM3281

Hi Sir,

We have a request about the LM3281YFQR DCDC buck converter. 

We have faced the situation when ripple voltage from it interferes with the BLE function of our MCU (STM32WB55RGV6TR from ST). This default ripple shape on 3.3v line after DCDC converter as follows. 

According to the datasheet, the ripple of ~60mV is the normal way how LM3281YFQR operates in the ECO mode. But the problem is that we cannot have this level of ripple because our MCU requires more clean power for it's BLE connectivity. So we try to make the voltage ripple of DCDC more smooth by adding those resistors and additional inductor (see read arrow below) before MCU VDD input:.

After this the output ripple becomes smaller (yellow line) and our MCU can work normally.

Our question is if it's okay for LM3281YFQR to have those additional components after it's output because this schematic design is different from the one recommended by the datasheet. Can u help to confirm it?

Thanks,

Best Regards

  • 0
    TI__Mastermind 21580 points

    Hi Lumina,

    Thanks for checking with us about this. Running LM3281 in FPWM would be the easy and best way to reduce the ripple, if the higher quiescent current is acceptable.

    Adding a second LC to the output has the potential to make the loop unstable. If the additional poles from this filter are within the loop bandwidth, the device can become unstable. This may not be evident when operating in PFM mode. I would suggest to run a loop stability measurement in PWM mode by injecting a signal into the FB pin. This can give an idea by how much you need to shift the poles of your 2nd LC to higher frequencies. But keep in mind the effectiveness of the filter to filter the PFM ripple reduces then.

    You can find more information about designing a second stage LC in this blog 2nd stage LC.

    Best regards,

    Varun

  • 0 in reply to Varun John
    Genius 3680 points

    Hi Varun,

    Thank u very much for your suggestion, and we will try it.

    Best Regards,

    Lumina

  • +1 in reply to Lumina Gui
    TI__Mastermind 21580 points

    Hi Lumina,

    Let me know if you need any further help from us. I am closing this thread for now.

    Best regards,

    Varun