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LMR38020: Different operating modes of the LMR38020

GBDH
Prodigy 90 points
Part Number: LMR38020


Hi,

We having a question about the different references of the LMR38020.
- LMR38020SDDAR   => spread spectrum version
- LMR38020FSDDAR => FPWM + spread spectrum version
- LMR38020FDDAR   => FPWM version

We using the LMR38020SDDAR, but when we measure the signals on the HS output we seen that at low load that he goes in spread spectrum mode and at Higher loads in FPWM mode.
How can that, because the LMR38020SDDAR has only a spread spectrum mode?
See datasheets page 3

  • 0
    TI__Mastermind 23905 points

    Hi GBDH,

    Spread spectrum and FPWM/PFM aren't mutually exclusive. You can still modulate the frequency between a couple of Hz with both modes.

    Thanks,

    Andrew

  • 0
    Prodigy 90 points

    Dear,
      
    if Spread spectrum and FPWM/PFM aren't mutually exclusive, what is than the difference between these part numbers?

    (Explicitly the difference between LMR38020SDDA and LMR38020FDDAR)

    Regards

  • 0 in reply to GBDH
    TI__Mastermind 23905 points

    Hi GBDH,

    The device doesn't have spread spectrum unless there's an S in the p/n. Ex:

    LMR38020FSDDAR

    LMR38020SDDA- Spread spectrum with PFM mode switching

    LMR38020FDDAR- FPWM with no spread spectrum enabled.

    Thanks,

    Andrew

  • 0 in reply to Andrew Kutzler
    Prodigy 90 points

    Dear


    This isn't what we measure for the 'S' version.

    I explain it in another way

    For the 'S' version we seen at low loads that he goes in PFM ( < 420mA) mode and at higher loads (>560mA) he goes in FPWM mode.
    (Designed power supply goes from 24V to 15V at 500mA)

    Now you say that the 'S' version is a version with Spread spectrum and works only in PFM mode.
    Why we seen than that he works also in FPWM mode for the 'S' version?

    The issue what we having is that the output ripple in PFM mode is 150mVrms and in FPWM mode 20mV and 150mV is to much for our analog application
    So which reference should we choose that the buck regulator always works in FPWM mode? (Spread spectrum doesn't matter)

    ps we how checked the component marking and it was correct for the 'S'

  • +1 in reply to GBDH
    TI__Mastermind 23905 points

    Hi GBDH,

    That is correct. The version with the S in the suffix is also a version lacking the F, so it has a PFM mode in light load, and uses the spread spectrum feature to oscillate the switching frequency on top of the PFM effect. At full loads, it goes to an FPWM mode, same as the F device.

    For your application I recommend the F device or FS device. Both will have the FPWM mode enabled.

    Thanks,

    Andrew