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Original question:

LMR14006: Light load operation of converter

LMR14006: Minimum on time calculation in DCM mode

vaibhav waman
Intellectual 280 points
Part Number: LMR14006

Hello,

In light load condition LMR14006 enters into DCM mode. It has minimum on time period of 95ns.

I am using at 12V input to 5V output configuration. What formula I should use to calculate minimum on time in DCM mode ?

From this I want to verify in my application output voltage will not rise above 5V due to given minimum on time mentioned.

Thank you.

Best Regards,

Vaibhav Waman

  • 0
    TI__Guru* 76805 points

    Hello Vaibhav,

    For the buck converter, the DCM duty cycle is dependent on the load. Specifically,

    where

    • t1 is the on time
    • Iout the load current
    • L1 the buck inductor's inductance
    • Vout the output voltage
    • Vf the rectifier diode forward voltage drop,
    • Vin the input voltage
    • Fswitch the switching frequency.

    If the calculated t1 is shorter than the min on time, you should consider to add a dummy load prevent the Vout from rising above the regulation level.

    Wish this helps.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Intellectual 280 points

    Hello,

    In this formula if I consider scenario of light load that is Iout is very small in that case only I can see need of external dummy load ? am I correct or Iout should always be maximum output current ?

  • 0 in reply to vaibhav waman
    TI__Guru* 76805 points

    Hello Vaibhav,

    Iout in the equation is the small light load which causes the IC running into DCM.  At your max output current, your circuit should be designed to operate in CCM and the duty cycle is determined by (Vo+Vf)/Vin.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Guru* 76805 points

    Hi Vaibhav,

    May I assume the issue is resolved so I can close the thread here?  You are welcome to re-open it by adding a new post.

    Thanks,

    Youhao