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Become a Decoupling Capacitor Network Guru, Part 1

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Max The Magnificent
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Cost vs Characteristics?
Max The Magnificent   12/11/2013 1:12:29 PM
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Is the reason ceramic capacitors are traditionally used for decoupling purposes (at least on lead-through-hole boards) due to cost or superior characteristics?

Max The Magnificent
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Are SMT Decoupling Capacitors Ceramic
Max The Magnificent   12/11/2013 1:14:30 PM
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This question simply never struck me before -- but what type of device are SMT capacitors (ceramic, polymer...). Also, can you get electrolytic SMT capacitors?

MeasurementBlues
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Don't forget resistors
MeasurementBlues   12/11/2013 3:20:27 PM
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@Adam, nice article and I mostly agree with it but I'm sure I can find someone who won't. Be sure to see Kenneth Wyatt's Resistors aren't resistors at EDN. It's a good complementary article to yours.

MeasurementBlues
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Don't forget inductance
MeasurementBlues   12/11/2013 3:27:40 PM
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It really comes down to the inductance of the decoupling path between the decoupling cap and the IC, espiecally at high switching speeds. Of course, power-plane inductance figures in as well.

Adam-Taylor
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Re: Don't forget inductance
Adam-Taylor   12/11/2013 3:29:33 PM
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You are correct this one is setting the scene, the next one looks at mounting and placement of the components ;) 

Max has it so hopefully it will be posted soon 

MeasurementBlues
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Re: Don't forget inductance
MeasurementBlues   12/11/2013 3:34:31 PM
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"Max has it so hopefully it will be posted soon"

And then you can do one on measuring power integrity for Test & measurement DesignLine, right? :-)

 

Adam-Taylor
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Re: Don't forget inductance
Adam-Taylor   12/11/2013 3:45:24 PM
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of course ;)

Howard.Lankford
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Don't Forget ALL Those Other Important Characteristics!
Howard.Lankford   12/11/2013 7:09:16 PM
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Your article has started out with good information but please don't forget to talk about the following:

1) change in capacitance of ceramic caps versus bias voltage or the DC voltage across them

2) connection impedances to the ground plane and power plane and to the device you're trying to bypass

3) performance of ceramic caps versus electrolytics as bypass elements

4) location of bypassing caps versus location of the part you're trying to bypass

5) change in capacitance versus bias voltage for other types of capacitors other than ceramic caps

6) amount of charge storage or capacitance required for various current transients

7) package resonances when using several caps to get low impedance over wide frequency range

8) when to use broad frequency range decoupling versus narrow frequency range decoupling

There are more but I suspect you don't have the time or space available to talk about all of them.

 

Adam-Taylor
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Re: Don't Forget ALL Those Other Important Characteristics!
Adam-Taylor   12/12/2013 3:03:40 AM
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Hi Howard,

Thank you for the comment, there is limited space for a blog so a number of your points will be addressed in part 2 which looks at how and where we mount the devices and what the driving requirements are.

Hopefully max will be posting it soon

Adam

bk11
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Re: Are SMT Decoupling Capacitors Ceramic
bk11   12/12/2013 12:37:29 PM
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Most capacitor types are available as either SMT or TH.  Oftentimes ceramic caps are used int SMT as decoupling due to low cost and low ESR.  As others have noted, temperature and voltage operating conditions need to be accounted for.

(Aluminum) electrolytics are commonly available, as are tantalum.  The Al electrolytics tend to be relatively large depending on voltage and capacitance.  This can lead to mechanical shock and vibration problems, as the only thing holding them on the board is the glue between the pads and the PCB material.

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