FEDEVEL
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Pak00 , 05-22-2020, 12:26 AM

Hi,
I would like to ask about power plane and filtering capacitors. I have dc/dc converter with output capacitor (C2) placed on power plane with inductor L1. I want to deliver from here voltage for the circuit. And there is my question, Should I connect the +3V3 after the capacitor or can I connect +3V3 at any point on power plane and capacitor will filter? Does the power track must pass directly through the capacitor pad for it to filtering ? From the electrical point of view this is the same point. Witch layout is better ?

robertferanec , 05-22-2020, 11:11 PM

I always try to take power out after the capacitors. I would go with the second picture.

chitransh92 , 05-23-2020, 04:04 PM

Hello @Pak00 ,
I would also agree to @robertferanec on taking power out after the capacitors.

For your point on It depends on what you are powering.
On the schematic it may look like a single node but when you are on PCB the load and its speed determine several parameter of components placement.

Thanks.

Pak00 , 05-26-2020, 12:52 PM

Thank you for your answers !

I haven't used power plane until now and I'm trying to learn to do it correctly, Picture shows Texas Instruments eval board witch similar dc/dc converter which I showed in the first post. Output capacitors are placed on power plane. Is the current flow on power plane always incudes output capacitors and it filtering power corectly ? Until now, I've always placed the track through the capacitor pad and it was more clear to me. Is this layout correct?

chitransh92 , 05-27-2020, 02:05 AM

Yes, the current flow on the power plane always include at least one capacitor and there could be more as per requirement.

There are 2 application of the capacitors i.,e Decoupling and Bypass..
Decoupling Caps are used to provide low impedance source of energy to High frequency change in load current. These are comparatively high value caps and typical values 1uF, 4.7uF, 10uF, 22uF..
Bypass caps are used to provide a low impedance path to the high frequency noise signals.These are low values caps Typical values 100nF, 680nF etc.

Your application may require utilization of either of these or both in conjunction with each other and this solely depends on your application.

For this eval board, C1,C4 are bypass caps and C2. C3. C5, C6 are decoupling caps.
At times these function as both depending on frequency...

Thanks..

robertferanec , 05-28-2020, 07:07 AM

- very simply to say, that inductor is placed much further from the output connector than the caps, means, capacitors are in the way of output and they are placed correctly.