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TPSM33615: Conditions for switching to PFM

Dice-K
Mastermind 7703 points
Part Number: TPSM33615
Other Parts Discussed in Thread: LMZM23601, TLVM23615,

Tool/software:

Hi support team,
I am considering changing from LMZM23601 to TPSM33615 or TLVM23615.

I will use it under the following conditions.

Vin: 24V
Vout: 5V
Iout: 0.6A

The inductor has been reduced from 10μH to 2.2μH.
What are the conditions for switching to PFM?
Is it when the inductor valley current is less than 0A?

Regards,
Dice-K

  • 0
    TI__Expert 5520 points

    Hi Dice-K,

    The engineer responsible for the part is out of office this week and will be back next week.

    Thanks,

    Nitya

  • 0
    TI__Mastermind 26810 points

    Hi Dice-K,

    By inductor do you mean input inductor?

    In regards to your PFM questions, yes you are correct. PFM mode will come into effect essentially when the inductor valley current drops below 0A. For more information please see section 7.4.3.2 of the datasheet.

    Please keep in mind as well that the recommended switching frequency for a 5 V output with this device is 1000 kHz. Table 7-1 gives various parameters for the device given a particular output voltage that we recommend following for the best results.

    Thank you,

    Joshua Austria

  • 0 in reply to Joshua Austria
    Mastermind 7703 points

    Hi Joshua,
    Thank you for your reply.

    The inductor refers to the internal inductor.

    At 1000kHz, the inductor ripple current is 1.8A.
    Therefore, PFM will occur when Iout is less than 0.9A.

    In the case of the LMZM23601SILR, the inductor ripple current is 0.4A.
    Therefore, PFM will occur when Iout is less than 0.2A.

    Is this consideration correct?

    Best regards,
    Dice-K

  • 0 in reply to Dice-K
    TI__Mastermind 26810 points

    Hi Dice-K,

    That is a decent approximation of the PFM transition in theory. In practice, other aspects such as efficiency will have a role in the PFM operation point. However, for the means of approximating that the PFM transition point is a higher load in the TPSM33615 than the LMZM23601SILR, your analysis is valid. 

    Thank you,

    Joshua Austria