Author Topic: Why three output capacitors after the LDO  (Read 535 times)

Tonn

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Why three output capacitors after the LDO
« on: December 28, 2017, 07:46:54 am »
Hi Andy,
I'm reading your NanoCounter article. It's an excellent article. But something puzzles me.

The LDO (LP5907) that powers the Connor Winfield M100 has three output capacitors: 1uF, 100n and 10n.
The 1uF capacitor is straight from the datasheet.
Why the other two?

I'v seen this construction before in high quality designs. Often the capacitors are mounted next to each other and of the same type. I can't seem to find out why this is done.
Thanks for your explanation.

Regards, Tonn

Andy Brown

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Re: Why three output capacitors after the LDO
« Reply #1 on: December 28, 2017, 10:29:34 am »
Hi Tonn,

C20 (100n) and C21 (10n) are really for the M100 - take a look at "M100 - M200 Test Circuit" in the M100 datasheet. It's just a happy co-incidence that together with C18 (1u) the 3 of them are capable of providing decoupling across a wider range of frequencies than just a single one could provide.

Regards
Andy
It's worse than that, it's physics Jim!

Tonn

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Re: Why three output capacitors after the LDO
« Reply #2 on: December 29, 2017, 08:30:36 am »
Hi Andy,

Thanks. That clears it up.

I recently discovered your site and I think it's fantastic.
Best wishes for 2018.

Regards,
Tonn (The Netherlands)