LM51551-Q1: question about duty cycle and ripple ratio

Hi Jay,

Thank you for using E2E forums, Regarding your questions:

Q1: The duty cycle or the D defined in the datasheet is for a Boost converter and in the app note AN-1484 the D defined is for a SEPIC converter.

Q2: The ripple ratio parameter is usually suggested to be around 50 to 70% as you can read in the explanation in the yellow box. It basically shows the amount of ripple on the inductor current, so as long as it is not larger than 70% and not too small it should be alright. Rule of thumb is usually the value that is suggested in the excel sheet comment box.

BR,

Haroon

hi Haroon

thanks for your reply. I make a mistake about Q1.

another question, as shown in yellow box, how can i understand "the max ripple ratio occurs when D=0.33"
Why max ripple ratio occurs in D=0.33?

Thanks

Hi Jay,

Thank you for your reply, regarding your question, in CCM mode the maximum ripple ratio occurs at duty cycle 33%. You can read more about this here https://www.ti.com/lit/an/snva941a/snva941a.pdf?ts=1675252413953&ref_url=https%253A%252F%252Fwww.google.com%252F 

I hope this helps.

BR,

Haroon