[SI-LIST] Re: subscribe: bulk capacitor

Pratap, the only justification for spreading bulk capacitors that I have 
ever seen is if the spreading resistance is high.  If you doubt this, 
construct a bedspring model and then move the capacitors around with 
respect to the VRM and the loads. In a properly designed bypass network, 
the bulk capacitors support current only up to the low MHz range.  The 
device attached inductance is an order of magnitude greater than the 
spreading inductance even if across the whole PCB, making it almost always 
a non-factor between the bulk caps and the IC noise sources.. Spreading 
resistance can be a different issue and depends on both the loads and the 
plane topology.

If you spread the devices around then you place spreading resistance and 
inductance between the caps and the VRM increasing VRM output switching 
noise, but decreasing noise EMF across the spreading resistance of the 
planes.  In a forward topology, the VRM inductor current swing is typically 
40% of the DC load.  Whereas device load current variation is typically ( 
but not always ) much less.  So if you spread the devices evenly, with the 
VRM at one end of the board, you shunt the inductor swing current with an 
RC ladder network.  OTOH, if you put the caps all right at the switcher 
output, there is very little R between that 40% inductor swing current and 
the capacitors.  If you use a flyback, or boost type topology, the 
situation moves even further in favor of the caps at the VRM output.

Steve.
At 11:35 AM 1/28/2005 +0530, Pratap Narayan SINGH wrote:
>hi dave,
>                 It's better to spread the bulk capacitors on the board.
>It will
>construct a multi supply transmission line which is more better to
>satisfy the high current requirements from the heavy loads.
>please see multi generator transmission lines for more details
>in this regard.
>
>Regards,
>Pratap
>
>dave_zhao wrote:
>
> >i'm confused by location of the bulk capacitors. I
> >shall place them near the voltage regulator module or
> >spread them around to supply enough charge for the
> >heavy loads. I use the ADI's tigersharc DSPs in my
> >design, each of them need to consume 3A current, so i
> >have to choose enough bulk capacitors for them. And
> >where should i locate those capacitors, near the power
> >chip or near the dsp chip?
> >  best wishes
> >
> >=====
> >best regards
> >                                                dave zhao
> >
> >
> >
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