Of course you are right. Layout plays a big part as well. Normally
the vendor will have quite some good documentations to follow. Still 
you need o try it out. I agree that it is not easy especially when
the board is small.

I have played with some VRM demo boards for Pentium 4 from Intersil, IR 
and On Semiconductor. They all have piles of SMT ceramic capacitors
at the output to fulfill Intel VRM9 or VRM10 specifications. Ceramic
capacitors have much lower ESR than the tantalum capacitors. Since 
the voltage is so low, you can find a lot of good ceramic capacitors 
(10uF or bigger). That is a possibility for your application.
I think Intersil demo boards are really well made compare to the
other companies and that may be why they are leading the market of
desktop VRM. 

VRM means voltage regulator module. Normally the output voltage are of low 
voltage and high current.

There are quite some PWM IC with built-in LDOs. I have not played
with them before so I am not so sure about the performance.

Regards,
Xiaofan


-----Original Message-----
From: Vasile Surducan
Sent: Friday, July 22, 2005 1:14 PM

However, if you've built many switching supply, you've noticed that
rejecting noise and the ripple (for other than a digital load) it's a
big problem. Using very good ESR tantalum capacitors (less than 2 ohm)
in parallel with any combination of non inductive ceramic capacitors
does not help since the noise is flowing on the ground of the supply.

There is a matter of switching technology combined with the PCB topology.
On the other way, Russels ideea of using an LDO at a switching output
is not helping too much if LDO was not designed for rejecting high
frequency ripple. Usually the common LDO are rejecting up to hundreds of 
hertz so using them on a 500KHz output or on 2-4 MHz range is quite weird.

cheers,
Vasile
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