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LM61460-Q1: detail of Auto PFM mode

Yusuke Mukuno
Expert 7395 points
Part Number: LM61460-Q1
Other Parts Discussed in Thread: LM61460

Hi expert,

My customer have a question about mode transition from PWM to PFM of LM61460.

Could you please let me know the transition threshold? What parameter will the device monitor for PFM?

 

I found the apps note for LMR336xx device. Does LM61460 have same operation?

https://www.ti.com/lit/an/snva857/snva857.pdf

Thanks

Muk

  • 0
    TI__Guru 50120 points

    Hi Muk,

    I don't think the LM61460 employs the same kind of scheme as the LMR336xx device. I will have to look into this a little further to see if there are any documentation on this transition.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    TI__Expert 7395 points

    Jimmy-san,

    Do you have any update?

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Guru 50120 points

    Hi Muk,

    You can refer to Section 8.4.3.2.1 (Diode Emulation) in the datasheet to get a rough estimate of the device entering PFM.

    "Once output current is less than half of inductor ripple in CCM, the part operates in DCM".

    PFM occurs when the output current continues to reduce pass the DCM point, and the characteristic of this is that the switching frequency will reduce to maintain output regulation while reducing switching losses to achieve higher efficiency at light load. 

    If you calculate half of the inductor ripple current of the application, then this is a good starting point to estimate the PFM transition point. Otherwise, the best way to determine this per a specific application is to take the EVM with the AAS variant and lower the output current until the SW node no longer is a fixed square wave. 

    Regards,

    Jimmy