LMZ23605: SNVA489C application report question (AN-2162)

Jim Kulyk

Part Number: LMZ23605
Other Parts Discussed in Thread: LMZ14202

Tool/software:

Equation (5) used to calculate Cfb does not reconcile with the values obtained in the two design examples: Ex1 LMZ23605 Cfb = 2.5 uF and Ex2 LMZ14202 Cfb = 6.4 uF. The calculations from equation (4) Cfa yield the same values listed in the example designs. It appears there is an error in equation (5). Please confirm.

Thanks,

Jim

11 months ago

0 Rahil Ajani 11 months ago

TI__Genius 9460 points

Hi Jim,

Can you show me your calculations here? I'm not seeing anything wrong in the equation.

You use cfa because it is the greater of the 2 values.

Regards,

Rahil

0 Jim Kulyk 11 months ago in reply to Rahil Ajani

Prodigy 30 points

Hi Rahil,

I used a spreadsheet to calculate the values. The Cfb calculation is the following:

=(1/D28)*((10^(C11/40))/(2*PI()*D19))^2 where D28 is the inductor, Lf, C11 is the attenuation, !Attn!dB, and D19 is the switching frequency, Fsw. This is equation (5) in the SNVA489C document. In both example designs the Cfb values listed are different from the calculations made using equation (5).

Br,

Jim

0 Rahil Ajani 11 months ago in reply to Jim Kulyk

TI__Genius 9460 points

Hi Jim,

What value are you using for Attn (dB)?

Regards,

Rahil

0 Jim Kulyk 11 months ago in reply to Rahil Ajani

Prodigy 30 points

Hi Rahil,

40dB as in the examples.

Br,

Jim

0 Rahil Ajani 11 months ago in reply to Jim Kulyk

TI__Genius 9460 points

Hi Jim,

I just confirmed that the formula is correct but the values in the table are wrong.

Regards,

Rahil

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LMZ23605: SNVA489C application report question (AN-2162)