• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 64 subscribers
  • Views 244 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

LM5143: 12V-to-12V near 100% duty ratio requirement

Dave Chang
Expert 5475 points
Part Number: LM5143
Other Parts Discussed in Thread: TPS51220A,

Hello WV support team,

My customer uses TPS51220A a lot, however, which TPS51220A is old part and we are moving to new solutions.

We will suggest LM5143, but the 100% duty cycle requirement is the need for customer's request.

The input voltage will be 12V to 24V for the system and the output is 12V here.

Thus, the 100% duty requirement will happen at the 12V condition for them.

According to the minimum off time, the LM5143 is 80ns comparing to the TPS51220A's 160ns, which means when operating at low fsw, the duty is possible to reach 99% at 100kHz, and 97% at 300kHz.

We want to make sure at this condition, the 12V-to-12V scenario is able to reach within ±5% output voltage requirement?

Because TPS51220A's datasheet states that it's able to reach 99%, and based on the data, LM5143 is more well-behaved, so it seems no problem for this application, right?

Best Regards,
Dave

  • +1
    TI__Mastermind 40040 points

    Hello Dave,

    The Toff min sets the off time in a period.  At 100kHz you will have a period of 10us, so the Dmax will be ~= (10us-80ns)/10us.   this is the max possible Duty cycle that can be achieved.  note that the RDSons of the FETs and the DCR of the inductor are not taken into account here and so there will be an additional IxR drop you will need to account for.

    Please see the article below for more details.

    Understanding output voltage limitations of DC/DC buck converters

    Hope this helps.

    David.

  • +1 in reply to David Baba
    TI__Guru 74052 points

    One advantage of the LM(2)5143(A)/-Q1 family of controllers to emphasize here is that the switching frequency folds back in dropout, thereby increasing the effective duty cycle closer to 100%. Of course, the high-side FET Rdson and inductor DCR voltage drop can still be significant at load in terms of how close Vout actually gets to Vin.

    --

    Tim