Decoupling capacitor selection - Page 2

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
Re: Decoupling capacitor selection
I've had chips where the mfr has requested I put those low inductance
vias to the ground and power planes *in* the device pads.

I didn't, and everything worked fine, although I did take great care
with the decoupling. Obviously, that sort of thing has implications at
board reflow.

On another much earlier point, caps up to 100uF are available in
multilayer ceramic (although the 100uF are 6.3V, X7R,1210) which I have
used extensively with no problems. To my understanding, the capacitor
manufacturers are trying to increase the capacitance available, (with a
generally lower voltage range) for low voltage, high current systems.

Cheers

PeteS


Re: Decoupling capacitor selection
'Decoupling capacitor selection', on Sat, 17 Sep 2005:
Quoted text here. Click to load it

They've been doing that since around 1920. Equipment manufacturers
always want more uF in fewer m^3.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
We've slightly trimmed the long signature. Click to see the full one.
Re: Decoupling capacitor selection
Hello Pete,

Quoted text here. Click to load it

I have done that. But the assembly house folks are going to throw the
pots and pans at you for that.

Quoted text here. Click to load it

Past 1uF they tend to get expensive. On one design I had an electrolytic
in there and someone changed it to ceramic. Unfortunately that was at
the output of an LDO regulator and sure enough that LDO started singing
the blues. It wanted an ESR below x _but_ above y. I guess that's the
last time I have designed in an LDO regulator. Many of these things are
IMHO the pits. The amazing thing was that the LDO produced a sawtooth
from 3V to 5V and the board worked anyway with a precision this was only
slighty reduced. But it drowned out Rush Limbaugh on the radio...

Regards, Joerg

http://www.analogconsultants.com

Re: Decoupling capacitor selection
<< On one design I had an electrolytic in there and someone changed it
to ceramic. Unfortunately that was at the output of an LDO regulator
and sure enough that LDO started singing the blues.>>

I wrote on this issue on a thread some time ago on the issues of
ceramics at the output of a switcher or an LDO. It can cause grief
untold (as you found). It can be *very* difficult to get a switcher or
LDO to operate properly with ceramics on the output within perhaps 3 cm
of track (the track inductance helps to damp the low esr effect, of
course). One trick I use is a 1 ohm resistor in series with the ceramic
- seems to work well.
One can only be glad it drowned Mr. Limbaugh - I used to refer to him
as the definition of Oxymoron - self definitional, I think ;)

Cheers

PeteS


Re: Decoupling capacitor selection
Hello Pete,

Quoted text here. Click to load it

Actually I never had a problem with ceramics or for that matter any
capacitors on switcher outputs. Next time I am squeezed for dropout
voltage I am just going to design in a switcher. Then I know it works.

Regards, Joerg

http://www.analogconsultants.com

Re: Decoupling capacitor selection
<< Actually I never had a problem with ceramics or for that matter any
capacitors on switcher outputs>>

Nor I - if that is what I designed in the first place :)

I had to 'redo' a design that was clearly not right - and that was
difficult; indeed it proved impossible. The client insisted I could not
change the layout, which made it almost impossible as the device used
was designed for 1A (limited by the device used to a great extent) and
the load could have 3A pulses. I eventually told the client that what
was required (no layout changed and power did not deviate) was not
possible.

He wasn't happy - oh well.

Cheers

PeteS


Re: Decoupling capacitor selection
Hello Pete,

Quoted text here. Click to load it

Nice. <blush>

Quoted text here. Click to load it

With clients it's often the same as with patients. If someone with a
hardcore cholesterol issue is told by the doctor to cut out the cheese
burgers and then says that would be impossible, oh well.

Regards, Joerg

http://www.analogconsultants.com

Re: Decoupling capacitor selection
Quoted text here. Click to load it

Was it this one?
http://groups.google.com/group/sci.electronics.basics/browse_frm/thread/ad909bc66ac407f2/8d41faf8980b8669?q=PeteS +*-*-generated-*+*-*-*-*-*-The-Easy-Way
(Attribution is included.)


Re: Decoupling capacitor selection

Quoted text here. Click to load it

    This is a bad thing ????   :-)

Quoted text here. Click to load it

Re: Decoupling capacitor selection
Hello David,

Quoted text here. Click to load it

That can be a recipe for trouble. Mostly in the form of purchasing guys
running down the hallway towards your office because they can't get
enough of the caps or the things went on allocation. If you max out the
capacitance versus size factor at a given voltage the number of
available manufacturers can shrink dramatically. Even down to one, and
occasionally down to zero ...

Regards, Joerg

http://www.analogconsultants.com

Re: Decoupling capacitor selection
Quoted text here. Click to load it

That's always important (and often underestimated by developers...).
But that's why I wrote "in your voltage, price and *availability* range" :-)

mvh.,

David

Re: Decoupling capacitor selection
Hello David,

Quoted text here. Click to load it

Yes, then you should be fine. But the availability range can change
suddenly and without notice. Remember the days when those small Z5U were
all the rage and suddenly this ceramic material became scarce? I forgot
the details but IIRC a chemical plant in Asia went kablouie. The guys
that had settled for the larger X7R caps could continue to sleep at night.

Regards, Joerg

http://www.analogconsultants.com

Site Timeline