WEBENCH® Tools/UCC28700: UCC28700

Jim Newlon

Prodigy 100 points

Part Number: UCC28700
Other Parts Discussed in Thread: PMP9018

Tool/software: WEBENCH® Design Tools

Hello,

I have a couple questions on the formulas on P.10 of the UCC28700/701/702/703 datasheet.

1) I am using 220uA for Ivsl(run), is this correct?

2) I am using 4.05 for Vvsr, is this correct?

3) I am dividing auxilliary turns by secondary turns for Nas, is this correct?

4) I am designing a 24V output, is Vocv 24?

Thank you very much!

over 7 years ago

0 Mike O' over 7 years ago

TI__Guru* 86266 points

Hello Jim,

Please see attached PDF

Regards,

Mike

Jim Newton UCC28700 7 12 18.pdf

0 Jim Newlon over 7 years ago in reply to Mike O'

Prodigy 100 points

Okay good and thanks Mike. The problem I'm running into is that the calculations I'm doing don't match what WEBENCH comes up with.

Given RS1, I calculate Rs2 and it is off. Maybe given Rs1 = 82.5k, Rs2 = 11.74k (Turns of 1.33, Diode drop of .45V), WEBENCH gives 23.7k. I'm looking specifically at reference design PMP9018.

Any ideas?
Jim Newlon

0 Mike O' over 7 years ago in reply to Jim Newlon

TI__Guru* 86266 points

Hello Jim,

I reviewed the calculations that you used with a diode drop of 0.45V and your RS1 and RS2 values you calculated would be correct based on an Nsa = 1.33.

The higher value of RS2 that is calculated by Webench would indicate that the Nsa turns ratio is lower than the PMP9018 design and/or what you are using.

Regards,

Mike

0 Jim Newlon over 7 years ago in reply to Mike O'

Prodigy 100 points

Okay thanks Mike.

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WEBENCH® Tools/UCC28700: UCC28700