WEBENCH® Tools/TPS53355: divider resistor for Vout 5V is strange

JeffYC Yang

Expert 1290 points

Part Number: TPS53355

Tool/software: WEBENCH® Design Tools

Hi,

I used TPS53355 webench ti simulation the Vin = 11~13V, Vout =5V Iout =19A conditon,

But Rfbt and Rfbb is 62k and 10k, it seems only output 4.32V, and we test in actual board is also 4.3V output,

Why webench show the Rfbt only 62k? is it something wrong?

Thanks!

Jeff

over 8 years ago

0 JeffYC Yang over 8 years ago

Expert 1290 points

Attached figure

0 JohnTucker over 8 years ago in reply to JeffYC Yang

TI__Guru**** 191445 points

You are correct, there is a bug in Webench. I will notify the Webench team to get this corrected.

0 Nikhil Gupta (Product Folder Models) over 8 years ago in reply to JohnTucker

TI__Mastermind 39210 points

Jeff,

Are you using the ripple injection circuit in the actual board? That will also contribute to the FB node and the FB resistors are adjusted to take that into account.

0 JeffYC Yang over 8 years ago in reply to Nikhil Gupta (Product Folder Models)

Expert 1290 points

Hi,

You mean Rr/Cr/Cac? But it will impact Vout setting?

Thanks!
Jeff

0 Nikhil Gupta (Product Folder Models) over 8 years ago in reply to JeffYC Yang

TI__Mastermind 39210 points

Yes it does. You can look at the datasheet Page 24.

0 Marcos López - Online design tools over 8 years ago in reply to Nikhil Gupta (Product Folder Models)

TI__Intellectual 2290 points

Hi Jeff,

Nikhil's post answers the question on why the value of the feedback resistors. To expand some more on the subject, If you apply equation 14 in the datasheet

for the current design conditions, you get Vvfb ~= 0.69. Let R1 = 62k and R2 = 10k , now plugging Vvfb into equation 15:

and solving for Vout yields: Vout ~= 4.9

Best,

Marcos

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WEBENCH® Tools/TPS53355: divider resistor for Vout 5V is strange