TPS54531: Questions on the current ripple and how to get it down and remain within the phase margin.

Robert Freehart

Part Number: TPS54531

I am working on a design using a TPS54531. the input voltage is 12V the output requirement is 6V at 700mA. The Lmin calculated is 25uH. So I decided to go with a 33uH inductor but when I calculate the Iripple its 200mA. Is that what I'll actually see on the output? I calculated again using 330uH and get 20mA, better. I go into Webench and put in my values, it likes the 33uH but 330uH destroyus the phase margin so it won't work. If the Iripple is an output result I need to drop it significantly but stay within the phase margin! So explain the iripple to me please.

over 4 years ago

0 Zhao Ma over 4 years ago

TI__Genius 15135 points

Hi, Robert

when calculate the L, usually use the max output current capability(5A) for Iout, not the max application loading(700mA).

And the Kind is usually set to 0.3, which means the ripple current is 5*0.3=1.5A, then the calculated L is 3.5uH, we can select 3.3uF or 4.7uH, both are okay.

A certain ripple current is not a bad thing, it can improve the phase margin and good load transient performance.

However, if you want small Vout ripple, you can decrease the ripple current, but not make the ripple smaller than 5*0.1(Kind)=500mA.

0 Robert Freehart over 4 years ago in reply to Zhao Ma

Prodigy 80 points

Well I know how to calculate the Lmin and the Iripple. My question is whether this Iripple will actually be seen on the output?

0 Zhao Ma over 4 years ago in reply to Robert Freehart

TI__Genius 15135 points

Hi, Robert

No, the ripple current will not flow into the output, it will flow into output capacitors and generate the ripple voltage.

Enlarging the Cout can reduce the ripple voltage.

0 Robert Freehart over 4 years ago in reply to Zhao Ma

Prodigy 80 points

Thanks, that's what I needed.

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TPS54531: Questions on the current ripple and how to get it down and remain within the phase margin.