WEBENCH® Tools/UCC28700: UCC28700

Jim Newlon

Prodigy 100 points

Part Number: UCC28700

Tool/software: WEBENCH® Design Tools

Hello,

I have a couple questions on a formula in the UCC28700/701/7022/703 datasheet, Page 20, Section 9.2.2.7 shown below.

1) How do you determine Fmax?

2) Is Tr 1 uS or 2 uS?

I see discussion of assuming 500 kHz resonant frequency and therefore 1/2 of the DCM resonant period or 1 uS. Since I'm not clear on this, it appeared maybe Tr was the DCM resonant period and the formula took care of dividing this in half (Tr/2 x fmax)

Thank you very much!

Jim Newlon

over 7 years ago

0 Emily Kong1 over 7 years ago

TI__Expert 3350 points

Hi Jim,

The maximum switching frequency for this controller is 130 kHz. Tr is the resonant frequency during the DCM time (this you assume to be 500 kHz).

Best,
Emily

0 Jim Newlon over 7 years ago in reply to Emily Kong1

Prodigy 100 points

Emily,

I don't think that can be right. Plugging that into the equation you get 1 - (500k / 2 x 130k) - .425 which is a negative number.

Jim

0 Emily Kong1 over 7 years ago in reply to Jim Newlon

TI__Expert 3350 points

Hi Jim,

So because Tr is actually going to be measured in seconds (although it is provided above, assumed in kHz) and fmax needs to be multiplied in like terms, there is some conversion that needs to happen here. The actual equation is going to look like this:

Dmax = 1 - (((1 / 500k) / 2) * (1 / 130k)) - 0.425 = 0.575

Please let me know if this is still confusing!

Best,
Emily

0 Jim Newlon over 7 years ago in reply to Emily Kong1

Prodigy 100 points

Emily,

Thank you very much. This helped tremendously! I fully understand now! Good work!

Jim

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WEBENCH® Tools/UCC28700: UCC28700