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LM3481: Question about pulse skipping

Austin Do
Intellectual 2775 points
Part Number: LM3481

Tool/software:

Hello team,

Is there a specific condition that the device enter to pulse skipping mode?

For example, Isen pin detect the voltage and then does logic block control this?

The customer is using LM3481 with SEPIC topology, but sometimes the device enter into pulse skipping mode, so that they want to understand the exact operation.

Best Regards,

Austin

  • 0
    TI__Mastermind 26710 points

    Hi Austin,

    Thanks for using the e2e forum.
    The LM3481 device tries to maintain PWM for as long as possible.
    The condition to enter into pulse skip mode comes the FB pin.
    If the output voltage and FB are rising, the device will reduce the duty cycle and the on-time of the FET.
    At some time, the device will reach the minimum on-time limit, which is given in the datasheet:

    If the duty cycle with minimum on-time is still to high and the output voltage starts to run away, the device will stop switching and enter pulse skip mode instead.

    Please let me know if there are additional questions on this.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Intellectual 2775 points

    Hi Niklas,

    I'm still not understanding exactly.

    Let's say Vin = 20V, Vout = 80V @0.1A. 

    In this case, duty cycle is 0.8, so it seems difficult to reach to on-time limit.

    Also, It seems it is affected by Isen voltage also. Can you explain this in detailed also?

    Best Regards,

    Austin

  • 0 in reply to Austin Do
    TI__Mastermind 26710 points

    Hi Austin,

    Thanks for the feedback.
    For a boost ratio from 20V to 80V, you would indeed expect a duty cycle of ~80% at CCM (continuous conduction mode) operation.
    However, if the load is small, the asynchronous boost controller will operate is DCM (discontinuous conduction mode) instead. This will lead to a smaller actual on-time.
    More theory on this can be found in papers:
    https://www.ti.com/download/trng/docs/seminar/Topic_3_Lynch.pdf


    You can use our power stage designer tool to quickly calculate duty cycle and expected on-time for a boost design.
    If the calculated on-time is smaller than the minimum on-time of the device, it will operate in pulse skip mode.
    https://www.ti.com/tool/POWERSTAGE-DESIGNER

    Best regards,
    Niklas