Lpc2000

----- Original Message ----- 
From: "Owen Mooney" <ojm@...>
To: <lpc2000@yahoogroups.com>
Sent: Wednesday, July 13, 2005 11:11 PM
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.

It might only be 10 MHz, but the edges will be very sharp, resulting in odd 
harmonics up into the VHF region, causing EMC problems. That is why several 
smaller capacitors are best. They also have less inductance making them more 
effective.

Leon 



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