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LM51772: PSM entry threshold

Masashi Mineda
Intellectual 695 points
Part Number: LM51772

Tool/software:

Dear Sirs and Madams,

Please tell us about the "PSM Entry Threshold" of LM51772.

I am referring to Figure 7-10 in the datasheet, but the PSM entry threshold (Red line) in this graph is in a region higher than the inductor peak current (Brack line). Is this correct?

Please let us know if there is a way to calculate the PSM enter threshold.

I understand that the discontinuous mode (PSM) occurs at about half of the inductor peak current ripple.

Isn't the red line I added in the figure below the PSM entry threshold?

Also, the horizontal axis is stated to be the input voltage, but I believe it should actually be the duty cycle written in blue.

Regards,

Masashi

  • 0
    TI__Mastermind 23880 points

    Hi Masashi,

    Thanks for using the e2e forum.
    Our expert for the LM51772 device is out of office today.
    We will into your question and get back to you within 1-2 days.

    Thank you very much for your patience.
    Best regards,
    Niklas

  • +1 in reply to Niklas Schwarz
    TI__Guru 50861 points

    Hi Masashi,

    for figure 7-10 please differ between inductor ripple and inductor peak current.

    In the graph the two curves do not scale but do show the typical shape of the curves.

    See also

    Figure 5-22. Buck Minimum Duty -cycle for PSM Operation versus Switching Frequency (SYNC_OUT = Enabled)

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Intellectual 695 points

    Hello Stefan,

    I had misunderstood the inductor ripple current to be the peak inductor ripple current.

    I apologize for the inconvenience.

    We understood that the timing to transition to PSM mode also depends on the load current which is approximately half the peak inductor ripple current, and on the minimum ON/OFF times.

    Regards,

    Masashi

  • 0 in reply to Masashi Mineda
    TI__Guru 50861 points

    Hi Masashi,

    yes, this is right.

    The dependency between peak inductor ripple current and load current is only a very, very rough estimation as this depends on the used inductance.

    Best regards,

     Stefan