• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 75 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3481: Pulse skipping mode

Austin Do
Intellectual 2795 points
Part Number: LM3481
Other Parts Discussed in Thread: LM5155

Tool/software:

Hello team,

My customer is using LM3481 as SEPIC.

LM3481 goes into PSM when light load condition. The customer does not want switching frequency to be changed. 

Is there new device which does not have pulse skipping mode?

Best Regards,

Austin

  • 0
    TI__Mastermind 26930 points

    Hi Austin,

    Thanks for using the e2e forum.
    All asynchronous devices have this PSM feature to avoid Vout runaway in light load operation.
    We have newer devices like LM5155, which allow smaller minimum on-time, so the device operates longer with PWM at constant fsw.
    However, it is also possible through external to design to keep the LM3481 device in PWM at smaller load condition.

    The equation for the duty cycle in DCM operation is shown below:

    larger duty cycle means longer on-time and therefore less problems with minimum on-time restriction.
    VIN, VOUT and IOUT are fixed by design, but they can try to increase the inductance or the switching frequency.

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

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Intellectual 2795 points

    Hi Niklas,

    Thanks for reply.

    Smaller minimum on-time means the device can operates in higher frequency and larger inductance can reduce the slope of inductor current.

    Both can help to operate with PWM at constant fsw. Is this correct?

    Also, how about synchronous devices?

    Best Regards,

    Austin

  • +1 in reply to Austin Do
    TI__Mastermind 26930 points

    Hi Austin,

    You want to increase the slope of the inductor to achieve a larger duty cycle and therefore longer on-time. This is so the on-time becomes larger than the minimum on-time limit of the device and no Vout runaway or pulse skipping is happening.

    Synchronous device do not have this issue. They can allow negative current in the inductor, which means there is no risk of Vout runaway and no pulse skipping is necessary even at zero load. This mode is called FPWM (forced pulse width modulation)

    Best regards,
    Niklas