TPS7H4010-SEP: Cff calculation guidance

Steve Widener

Part Number: TPS7H4010-SEP

Section 7.3.10 of the current datasheet describes how to calculate Cff - esp eqn (21)

When we apply that calculation to the EVM design (Figure 4-2, copied below), 12V -> 3.3V, 6A, ~150uF ceramic output cap & 100K/43.2K feedback divider, I get ~ 9nF for Cff.

The EVM design shows 22pF. Why the big discrepancy? Would you please check my calculation to see if I made a mistake somewhere?

What formula should be used to calculate Cff?

Thanks for any advice!

over 1 year ago

+1 Daniel Hartung over 1 year ago

TI__Mastermind 28615 points

Hey Steve,

It seems that the CFF on the EVM was not calculated optimally.

When I use the equation I get ~300 pF

Assuming 10 kHz for Fx:

1/2/PI()/10000= 1.6e-5

100k || 43.2k = 30.167k

SQRT(30167*100000) = ~55k

1.6e-5/55k = 300 pF

Perhaps I can get you to check my math.

Thanks,

Daniel

0 Steve Widener over 1 year ago in reply to Daniel Hartung

TI__Genius 11010 points

Daniel -

That helps - I used the Fx from equation (18)

my Fx (from the EVM design voltage and output cap) is:

24.16 / (3.3 * 150uF) = 48.8K

I think I added vs multiplying under the SQRT

I now get 66pF for the Cff, that's much closer

Steve

0 Daniel Hartung over 1 year ago in reply to Steve Widener

TI__Mastermind 28615 points

Welcome Steve!

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TPS7H4010-SEP: Cff calculation guidance