[sdiy] bypass caps, subbing MLCC for Electrolytic

[Harry Bissell]

I have seen an app note for a switching power supply chip which
said that electrolytics might be preferred because of their
inherently higher ESR. too low of a value would make their
chip unstable.  OTOH they did say you could arrange to have higher ESR by adding a series
resistor or equivalent length of circuit trace (which might be bad for
other reasons)

I forget whether it as a Linear Tech part, or Maxim. I used the Linear Tech with the
ceramic caps.

Ceramic caps could be an advantage for board manufacture, there is no polarity to
get wrong (and explode - tantalum :^)

H^) harry

----- Original Message -----
From: Tony Rolando <tony at makenoisemusic.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: David G. Dixon <dixon at interchange.ubc.ca>, synth-diy at dropmix.xs4all.nl
Sent: Tue, 05 Oct 2010 14:24:34 -0400 (EDT)
Subject: Re: [sdiy] bypass caps, subbing MLCC for Electrolytic

  Thanx for the insights guys. David, it is the diameter. I could put a 
1206 MLCC in the same spot.

Harry, I was going to use either X7R or X5R, 1206 MLCC. The ap note for 
the IC (PCM1807 ADC) did specify Electrolytic, but did not say why... I 
suppose I am wondering what the electrolytic could offer that the MLCC 
could not. Perhaps as you noted the lower ESR could be "bug" in this 
case. I just prefer MLCC since they are longer life and not polarized.

Texas Instruments had the following to say this morning when I ask this 
same question:

"They (electrolytics) are used in relatively high-current and 
low-frequency electrical circuits, particularly in power supply filters, 
where they store charge needed to moderate output voltage and current 
fluctuations in rectifier output. They are also widely used as coupling 
capacitors in circuits where AC should be conducted but DC should not.

So, they were particularly recommended for their ability to better block 
DC, and moderate output voltage. If you aren't worried too much about 
these characteristics, then it should be fine to use the ceramic versions."


I've never heard anybody say electrolytic blocks DC better?


Tony



On 10/5/10 12:37 PM, Harry Bissell wrote:
> The ceramic caps are just fine for bypassing, except in some
> critical switching power supply designs that might specify to
> USE ONLY electrolytic or USE ONLY MLCC (depends on whether the ESR
> is a "bug" or a "feature"
>
> Ceramics are nice in that they take high SMT temperatures, and are
> inherently non-polar.  Watch out for the dielectric... some like
> Y5V (there is a cap in the circuit, its size is basically unknown and you don't care...)
> are really crap (Z5U is not much better). I prefer X7R if they would
> be small enough...
>
> H^) harry
>
>
> ----- Original Message -----
> From: David G. Dixon<dixon at interchange.ubc.ca>
> To: 'Tony Rolando'<tony at makenoisemusic.com>, synth-diy at dropmix.xs4all.nl
> Sent: Tue, 05 Oct 2010 12:26:34 -0400 (EDT)
> Subject: Re: [sdiy] bypass caps, subbing MLCC for Electrolytic
>
>>    Hello, Has anybody ever had any problems subbing a MLCC type cap for
>> an electrolytic in a bypass situation? Low ESR is not required. 16V 10uF
>> Electrolytic was called, I'd like to use 16V 10uF MLCC in order to save
>> space.
> I use 10uF electrolytics in this application which are only about 0.2" in
> diameter and about 0.5" high, and have 0.1" lead spacing.  Is it the
> diameter or the height that you are worried about?
>
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-- 
Harry Bissell & Nora Abdullah 4eva