[SI-LIST] Re: subscribe: bypass capacitor
- From: steve weir <weirsp@xxxxxxxxxx>
- To: tianfeiyu@xxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Wed, 26 Jan 2005 23:12:02 -0800
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
>
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