[SI-LIST] Re: subscribe: bulk capacitor

Pratap if you work the numbers, a 20% variation at the load will almost 
always favor placing the bulk caps close to the VRM.  The bulk caps cut-off 
well below the frequency at which the planes act as transmission lines.  Go 
ahead and apply a 1A, 2A, or even 3A step and see what you get with the 
capacitor located right at the chip versus five or even ten inches 
away.  Whatever that step is, as long as it is 20%, the 40% inductor swing 
current amounts to twice as much.  When you distribute the bulk caps you 
will eat the spreading resistance going back to the VRM as a group, and 
only gain at any given load by the ratio of the one to the total number of 
bulk caps.

Steve


At 05:33 PM 1/28/2005 +0530, Pratap Narayan SINGH wrote:
>Steve,
>         You are very right on the fact that spreading capacitors are used 
> when
>spreading resistance is high same is true with multi supply transmission 
>lines.
>but with DSPs like SHARC have very high current requirement sometimes and 
>Current
>variation may change up 20 %. It all depends on the application. In this 
>case current
>variation is on low freq. range ( and depends on the processing load on 
>DSP ).
>So we can have Bulk capacitors on supply lines closer to the loads 
>requiring high
>current , this will suppress maximum current harmonics ( Low freq. ) and 
>reduce the noise.
>A proper distribution of caps between VRM output and current sync. will 
>minimize noise.
>But as you have mentioned, it depends more on topology .
>
>pratap.
>
>steve weir wrote:
>
> >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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