Lpc2000

In a perfect world, 5 x 0.1 uF caps = 1 x .5uF cap.  But that doesn't
take into consideration the inductance of the leads and the traces.
With a dedicated ground and power plane, the inductance is less of an
issue, but still comes into play.  

 

This is the reason you don't simply put one 1000 uF cap on the board and
forget it.

 

Many designs require 0.01uf and 0.1 uf scatter across the board (one on
each Vcc pin).  

 

  _____  

From: lpc2000@yahoogroups.com [mailto:lpc2000@yahoogroups.com] On Behalf
Of Owen Mooney
Sent: Wednesday, July 13, 2005 3:11 PM
To: lpc2000@yahoogroups.com
Subject: [lpc2000] Re: Two layer PCB and LPC2000 and EMC

 

Why number ? 

A single 0.5 uf cap will do exactly the same job as 5 x 0.1 uf caps
provided the distances remain short, and the capacitor ESR
characteristics are the same.

Its not the number of capacitors but the capacitance and distance that
matters. Capacitance and distance also have a relationship with
frequency.

The choice for decoupling is multilayer ceramics becase of their low ESR
at high frequencies. Someone correct me if I'm wrong, but I think the
reason that designers scatter 0.1 caps every where, is that once that
was the largest size available. You can now get them in up to 10uf size
in 0805 packages.

I am only clocking at 14 MHz with no appreciable loads on the pins. I
use a 0.1 cap under the micro and a 10 uf ceramic at the power supply
nearby (about 3 cm). I examined the specs of the caps. The 10uF has an
ESR of 1 ohm at 100Mhz, 0.15 at 10 Mhz. The addition of the 0.1 was just
paranoia.

If you are driving leds etc Alexandre, use 1 or 10 uf ceramic caps in
the same layout.

Don't forget that at 100Mz has a wavelength of 3m, so a short stub of
about 3mm has very little ablity to radiate EMC. I think harmonics will
drop off rapidly after 100 Mhz.

Owen Mooney 



The number of despiking caps you need is affected by issues such as 
how much current you are drawing from the pins to drive things like 
led's and other low-impedance loads.  You might get away with less 
than one cap on each power pin when the I/O pins are lightly loaded, 
but for safety don't count on it.

I like empirical experimentation.  If you're interested in finding 
out what's really needed set up a proto board with the kind of loads 
you anticipate using, then start pulling off despiking caps until the 
system gets erratic.

Bill T.
http://www.kueprcontrols.com



--- In lpc2000@yahoogroups.com, "Alexandre Guimaraes" <listas@l...> 
wrote:

>Hi, Owen
>
>    Nice layout...  Thanks..
>
>    Is just one 0.1uf cap enough for decoupling the LPC2138 ?? 
>  
>
Wouldn't it 

>be necessary to use one cap for each power pair ?
>
>Best regards,
>Alexandre Guimaraes
>
>
>
>----- Original Message ----- 
>From: "Owen Mooney" <ojm@s...>
>To: <lpc2000@yahoogroups.com>
>Sent: Monday, July 11, 2005 6:45 PM
>Subject: [lpc2000] Re: Two layer PCB and LPC2000 and EMC
>
>
>  
>
>>Uploaded as pdf to the files section crt diagram + images.
>>
>>Owen
>>
>>
>>Hi, Owen
>>
>>   Would it be possible to have a image of this part of the 
>>    
>>
board ? It

>>would help a lot many of us that are starting to work with the 
>>    
>>
LPC's.

>>Best regards,
>>Alexandre Guimaraes
>>
>>    
>>



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