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LM73605-Q1: Reason why auto mode efficiency curves differs by F_sw

Yuya Shoji
Expert 3880 points
Part Number: LM73605-Q1

Tool/software:

Hi team,

I got customer question and am looping in the customer via BCC.

To my knowledge, in PFM mode at light load, the frequency varies with depending on load, not depending on RT --F_sw setting. Is this correct?

I'm thinking the curve difference doesn't stem from RT pin setting, but stems from inductor selection difference (e.g. inductor DCR) as the PFM block is decoupled from RT logic stream. Do you agree with this?

Best Regards,

Shoji

  • +1
    TI__Mastermind 18295 points

    Hi Shoji,

    Your understanding of PFM is correct. During light load operation, the frequency of the buck converter will change with load. Refer to Section 8.4.3.3 of the datasheet for details.

    Since the peak current of the buck is kept constant in PFM, the inductor size does have an impact on how the device is regulated in PFM. In those curves, the difference in efficiency at heavy loads, when the device operates in FPWM, will be more dependent on the difference in switching losses at 400kHz vs. 2.2MHz. At the light loads, it could be due to the inductor size, DCR, along with other factors. 

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    TI__Expert 3880 points

    Hi Ridge,

    Thank you for your quick answer! It's very clear to me. Out of my curiosity, where and what is the auto mode exact threshold?

    Best Regards,

    Shoji

  • +1 in reply to Yuya Shoji
    TI__Mastermind 18295 points

    Hi Shoji,

    There is not an exact threshold for the change from FPWM to PFM mode which is simple to calculate. The device enters PFM during discontinuous conduction mode. When the device begins to operate in discontinuous conduction mode depends on the current ripple, inductor size, and a variety of other factors. 

    If the application requires a constant frequency at light loads, then FPWM mode is the better option.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    TI__Expert 3880 points

    Hi Ridge,

    Thank you for your additional answer:)

    Best Regards,

    Shoji