[SI-LIST] Re: subscribe: bypass capacitor

Zhao, it is helpful to think about this in the frequency domain.

A tantalum capacitor of any size will reach its lowest impedance in the 
1000's to 10,000's of Hz.  It will support the PDS impedance out to a point 
where the ceramic capacitor network takes over, typically between a few 
hundred KHz to no more than 10MHz.  At 10MHz the spreading inductance of a 
plane pair even on a thick PCB is still quite low.  As a result, we can 
pretty much locate the bulk capacitors anywhere on the board that is 
convenient as far as the IC load current is concerned.  This usually makes 
it a good idea to concentrate them close to the voltage regulator module.

Using a number of smaller capacitors rather than a few large capacitors can 
result in a lower cost network due to the mounted inductance of the 
devices.  Spreading devices around is only useful if there is significant 
spreading resistance.  This should be evaluated on a design by design basis.

The VRM maintains a low impedance out to a frequency that varies widely by 
the design of the VRM.   A bypass network should be engineered and not 
built by cookbook rules.  That means you need to know how the VRM response 
is defined, with or without a particular bulk capacitor.  Then from your 
requirements, you can synthesize the rest of the network.

Steve




At 11:09 PM 1/26/2005 -0600, Zhao Yu wrote:
>
>dear sir,
>  I confuse to use the bulk bypass capacitor. It is sai= d that the bulk
>capacitor is not to cancel the switch noise,but to eliminat= e the low
>frequency noise such as refresh of DRAMs. So i think it does not = need to
>place the bulk capacitor near power pins of chips, Is it right?(que= stion
>1)
>         And i see some mater= ials which recommed to place lots of small
>distributed bulk capacitor on th= e board instead of using a few large bulk
>ones. So if this idea is true, wh= en we choose the bulk capacitor,we only
>need to calculate all the current i= n the circuit and choose the
>corresponding bulk capacitors whose sum is equ= al to the request, then
>distribute them on the board. That is to say we nee= d not to choose bulk
>capacitors according to precise requirement of every c= hips and assign
>everychips a bulk capacitor.Is that right?(question 2)
>=         At very low frequecy, up to seve= ral KHZ typically, the voltage
>regulator module supplies enough low impedan= ce and the ripple voltage is
>small enough. So we need no bypass capacitors = in this bandwidth. Compared
>with a actual system, i concider the power chip= s as the voltage regulator
>module, but as we know the power supply circuit = contain a large value
>capacitor used to keep stable voltage, can i concider= it as a bulk
>capacitorand subtract its value from the requested bulk capa= citance by all
>the circuit? (question 3)
>      &= nbsp;
>
>BEST WISHES
>--=20
>
>___________________________________________________________
>Sign-up f= or Ads Free at Mail.com
>http://www.mail.com/?sr=3Dsignup[1]
>
>--- Links ---
>    1 3D"http://mail01.mail.com/scripts/payment/adtracking.cgi?bannercode=
>------------------------------------------------------------------
>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: