LM5157-Q1: The sepic design tool of LM5157 of Cout

Lin Vast

Prodigy 250 points

Part Number: LM5157-Q1

Here is the design value.

My design of the outout current is only 0.42A.

The output ripple of Vout is ±7.5%.

But the value of Cout is 32.25uF.

Cout = I * dt / dv = 0.42 * 0.385 / 440k / 0.15 = 2.45uF.

I think there is something wrong.

over 2 years ago

0 Bryan Radix over 2 years ago

TI__Expert 3340 points

Hi Lin,

Thanks for reaching out. Determining the output capacitance can be done in multiple ways. Using the output voltage ripple is not the best way to do so.
Most probably, our SEPIC Quick Start Calculator determines the output capacitances based on the load step response voltage deviation, i.e. that when a load step happens, the voltage only changes by x%. If one does it in this way, one can also make sure the voltage ripple in normal operation remains within the designated range.

Thank you and best regards,
Bryan

0 Lin Vast over 2 years ago in reply to Bryan Radix

Prodigy 250 points

Hi Bryant,

I'm designing a gate driver pre-regulator. I believe the load will vary between light and full load. If I apply compensation frequency for Cout, would it be suitable ?

0 Bryan Radix over 2 years ago in reply to Lin Vast

TI__Expert 3340 points

Hi Lin,

Thanks for the feedback.
I'm not sure what you mean by compensation frequency. It would be great if you could a different explanation.
Furthermore, we've given you some answers in your other thread, maybe that already answers your question.

Thank you and best regards,
Bryan

0 Stefan Schauer over 2 years ago in reply to Lin Vast

TI__Guru 63101 points

Hi Lin,

it looks like this thread is identical to this one:

(+) LM5157-Q1: Sepic converter design - Power management forum - Power management - TI E2E support forums

So, i will close this one an put my response on the other thread.

Best regards,

Stefan