Parallel Equivalent for Bypass Caps?

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tblough
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Posted by tblough: Sat. Mar 21, 2015 - 03:32 PM
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I'm playing with a ATmega32u2 and I'm trying to layout the bypass caps for the supply pins on the board.  All three supply pins (VCC, AVCC, UVCC) are within a few millimeters of each other.  Can I place a single 300nF bypass cap instead of three individual 100nF ones?

 

Cheers,

Tom

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Learning and Information, General Electronics
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ka7ehk
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Posted by ka7ehk: Sat. Mar 21, 2015 - 04:13 PM
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Hope you have an assembly house that can deal with components on both sides of the board. Personally, I avoid that.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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david.prentice
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Posted by david.prentice: Sat. Mar 21, 2015 - 04:15 PM
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You can do if you want.   You will get marginally more noise on AVCC.

You seem to have the capacitors on the underneath of the board.    Can you not re-arrange them to fit on the same side as the AVR?

 

With your two-sided components,   you have plenty of board space.    Separate capacitors will not cost much money.

 

I am assuming that you want everything to run at 5V.

 

David.

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tblough
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Posted by tblough: Sat. Mar 21, 2015 - 04:28 PM
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David and Jim,

 

I'm the assembly house and the boards are going to be hand soldered.  The connector board at the bottom flips up and solders at right angles to the main board.  I've placed components on both sides of the board to try to get it as small as I can and not have to go small than 0805 components if I don't have to.

 

I'm not concerned about the cost of two extra chip caps, and I could easily add them to the board but their traces would be much longer.  I'm not using the ADC so I'm not really concerned about a little extra noise.  I guess my biggest question about the capacitor is if I do "combine" them, do I substitute with a larger cap, or keep just one 100nf?

 

Cheers,

Tom

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david.prentice
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Posted by david.prentice: Sat. Mar 21, 2015 - 05:50 PM
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100nF should be fine.    220nF might be marginally better.

 

David.

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DocJC
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Posted by DocJC: Sun. Mar 22, 2015 - 01:35 AM
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It doesn't help when you get differing answers... sorry.

 

If you are building a hobbyist project, or a prototype, then one, 0.1 uF cap would be fine.

 

If its a real project, and you want high reliability, I'd go with three, 0.1 uF caps.

 

Look at your layout and see if you can fit them in.

 

JC

 

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Brian Fairchild
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Posted by Brian Fairchild: Sun. Mar 22, 2015 - 08:45 AM
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It's the layout that matters not the actual value (within reason).

 

Three correctly routed and placed 22n capacitors will perform much better than one 300n badly placed and routed capacitor.

#1 This forum helps those that help themselves

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#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Torby
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Posted by Torby: Sun. Mar 22, 2015 - 12:46 PM
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I find 0603 parts pretty easy to deal with. Mistakenly ordered 0402 caps once. Those are insidious. 

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