[SI-LIST] Re: subscribe: bulk capacitor

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
>>>
>>>
>>>
>>>__________________________________
>>>Do you Yahoo!?
>>>Yahoo! Mail - Find what you need with new enhanced search.
>>>http://info.mail.yahoo.com/mail_250
>>>------------------------------------------------------------------
>>>To unsubscribe from si-list:
>>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>>
>>>or to administer your membership from a web page, go to:
>>>http://www.freelists.org/webpage/si-list
>>>
>>>For help:
>>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>
>>>List FAQ wiki page is located at:
>>>               http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>>>
>>>List technical documents are available at:
>>>               http://www.si-list.org
>>>
>>>List archives are viewable at:
>>>              http://www.freelists.org/archives/si-list
>>>or at our remote archives:
>>>              http://groups.yahoo.com/group/si-list/messages
>>>Old (prior to June 6, 2001) list archives are viewable at:
>>>              http://www.qsl.net/wb6tpu
>>>
>>>
>>>
>>>
>>>      
>>>
>>------------------------------------------------------------------
>>To unsubscribe from si-list:
>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>>or to administer your membership from a web page, go to:
>>http://www.freelists.org/webpage/si-list
>>
>>For help:
>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>List FAQ wiki page is located at:
>>                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>>
>>List technical documents are available at:
>>                http://www.si-list.org
>>
>>List archives are viewable at:
>>                http://www.freelists.org/archives/si-list
>>or at our remote archives:
>>                http://groups.yahoo.com/group/si-list/messages
>>Old (prior to June 6, 2001) list archives are viewable at:
>>                http://www.qsl.net/wb6tpu
>>
>>    
>>
>
>The weirsp@xxxxxxxxxx e-mail address will terminate March 31, 2005.
>Please update your address book with weirsi@xxxxxxxxxx
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>http://www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
>                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
>                http://www.si-list.org
>
>List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
>or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>  
>
>  
>



------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: