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LMR33630: LMR33630 PFM mode for lower currents is too noisy

D B
Prodigy 70 points
Part Number: LMR33630

Hi,

Is there a way to force PWM mode for the LMR33630 for lower current applications?  Or, get much higher quality filtering of the PFM mode?

We have used the LMR33630 for high frequency (~10 GHz) applications with appropriate filtering; however, we thought to use it for one of our lower power boards (~100 mA @ 5V) and noticed large sub-harmonic noise in the microwave components.  It was found to be caused by the LMR33630 in PFM mode due to the lower current draw.  We have been unsuccessful at filtering this noise as well as the PWM noise by following the recommendations in the datasheet.

Any suggestions?

Thanks!

  • 0
    TI__Expert 3835 points

    Hi D B,

    Can you explain to me the LMR33630 in the high frequency application? What kind of filtering are you doing? Can you provide any waveforms of the noise you are seeing compared to the correct operation you are expecting. Can you explain the sub-harmonic noise (<2.1MHz) and how you are noticing them in the microwave components?

    Unfortunately there is no FPWM mode in the LMR33630. You would have to use a difference device with a MODE/SYNC pin. 

    Regards,

    Ethan

  • 0 in reply to Ethan T
    Prodigy 70 points

    Hi, here are more specifics.

    We have a 24V supply going to 5V and we're using the LMR33630CDDA using the exact components from Table 3 in the data sheet.  Layout is almost exactly what datasheet shows, too.  We do have a CLC (150uF, 1.4uH) filter earlier on the 24 V line and a fuse.

    The output SW pin shows the following in PFM mode:

    On the output of the filter, it is much reduced:

    However zoomed in shows ~ 200 Mhz noise:

    This noise is what makes it into our microwave components and exacerbated by an amplifier to show sub-harmonic (relative to our microwave signal) noise all over the spectrum.  The bypass caps on the microwave components don't do much.

    We attempted to use a linear regulator after the switching supply, but that noise goes right through it. 

    We've also been attempting additional analog filter techniques like addition LC components, but this particular noise has been quite stubborn.  We're not filter experts and would be grateful for any suggestions.

    Thanks!

    David

  • 0 in reply to D B
    TI__Mastermind 25590 points

    David,

    Let me rephrase your issue to confirm we are on the same page:

    You use LMR33630 in a 100mA application using the values from table 3. You have a microwave circuit on the output which is corrupted by the noise on the output of the LMR33630 but only when the LMR33630 is in PFM mode. You want to reduce the noise of the LMR33630 by bringing it out of PFM or by adding filtering.

    Please confirm that this is all correct. Please also share a schematic and layout so we can better align on the circuit.

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 70 points

    Yep.  That's exactly it.  Here is a schematic (it includes a linear regulator):

    Thank you

  • 0 in reply to D B
    Prodigy 70 points

    I forgot the layout.  See below:

  • 0 in reply to D B
    TI__Mastermind 25590 points

    David,

    There appears to be a cut in the GND plane between the PGND pin and the negative terminals of the input capacitors. This will create a large loop with lots of parasitic inductance in the path of this discontinuous current. This will cause excessive ringing on SW which will shoot through the parasitic capacitance of your inductor. This is probably why you are seeing so much noise on the output.

    Try adding a wire connecting directly from the PGND pin to the negative terminal of the C_HF cap. This should help greatly.

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 70 points

    That helped significantly.  Noise was reduced by 20 dB! 

    I'll pay more attention to layout instructions from now on...

    Thanks!