[sdiy] How to Determine Value of Power Supply Decoupling Caps?

[ASSI]

On Wednesday 26 November 2003 16:56, john mahoney wrote:
> I have seen the ".1 uF rule" mentioned in several places. Having
> followed the links provided by Philip Gallo, however, I was left with
> the impression that .01 uF SMD caps were the best "RTBC" (rule of
> thumb bypass caps) to use on ICs. :-)

These are actually two rules of thumbs for two different situations and 
from different times. One is from a time where the thought of running 
something that was not called a radio at more than 10MHz would freak 
out most folks and the other is from a time where running something 
with almost 500 pins at 2.6GHz (please say hello to my microwave oven) 
by sinking 80A into it elicits shrugs from passers-by.

If you find explicit power bypassing recommendations in a datasheet, 
you can be sure that there is a good reason for it and you should 
follow it to the dot unless you _know_ differently. The recommendations 
on typical values also have (probably much) to do with the available 
types and ranges of values for the caps at the time when the rule was 
cast (meaning: commonly available and being as big as practically 
possible). As with everything, bigger isn't always better: if chosing 
the next value up means more series inductance (and/or having to lay 
out longer traces) it may not be a wise choice for your application. If 
inrush current from the caps kills the power regulator on power-up or 
stored energy frys some unsuspecting IC on power down, then you've 
overstepped the boundary as well.

In particular, the recommendation for using tantalum capacitors is not 
really a good one for some types of modern tantalum caps. SMD caps are 
now available in a much larger range, so you can easily find 100nF 
types that are as good as the older 10nF ones (beware, the "old" series 
are still around - they aren't bad, just different).


Achim.
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