[SI-LIST] Re: Fw: Decoupling capacitor,
- From: Bill Owsley <wdowsley@xxxxxxxxx>
- To: weirsi@xxxxxxxxxx, eddyvk@xxxxxxxxx
- Date: Sun, 8 Jul 2007 19:47:46 -0700 (PDT)
"Eddy how far away a capacitor can be depends on the interconnect between
the capacitor and the load."
Well distance does seem like a factor. The cap is meant to be close enough
to maintain the voltage at an acceptable level and to do this it has to be
within a distance determined by the speed of propagation in the substate of
interest in order to supply charge an enough to maintain the voltage within the
defined levels/limits, say for example 1/20 of the supply.
So how far away is 1/20, or some fraction of the voltage, given a speed of
propagation, then the caps should not be any further away than that distance,
and large enough to supply the charege needed.
steve weir <weirsi@xxxxxxxxxx> wrote:
Eddy how far away a capacitor can be depends on the interconnect between
the capacitor and the load.
Ideally this is something that we engineer
by design. Unfortunately it is often something that is handled by rote
practice. In the past rote practice often overengineered the power
system. These days more often than not it underengineers it. I suspect
that failure analysis will find power and thermal issues at the heart of
Microsoft's current $1.15 billion recall.
Steve.
Eddy wrote:
> "a temporary power feeder in case of power shortage"
> I think that is nice job description for a decoupling
> capacitor. :-)
> The problem is impedance of power lines. Distance
> means inductance and inductors resist fast changes of
> current. When a CMOS buffer changes state, it goes
> together with a current spike in the power supply.
> Depending upon the impedance of the power supply,
> there will be a "negative spike" (dip) in the power
> supply voltage. This dip not only slows down the
> transition of the CMOS buffer itself but also affects
> other circuits tied to the same power supply nearby. A
> current spike is not just "a frequency" but rather a
> wide spectrum of frequencies. Most chips have lots of
> different circuits all creating total chaos ("noise")
> on the power supply. For most chips it is vital to
> have a decoupling capacitor as close as possible
> between the power supply pins of your circuit.
> Sometimes 10mm distance is already too far. The most
> used decoupling capacitor value has to be 0.01uF or
> 10nF.
>
> Eddy
> Fremont CA
>
> --- M Sridhar wrote:
>
>> I have a doubt about Decoupling capacitor, I
>> understand that decoupling capacitor is used to
>> decouple power supply to the device, so it acts as a
>> temperery power feeder in case of power shortage.
>> My doubt is how to know at what frequency the power
>> fluctuation would happen?
>> From were we may get this information.
>>
>> Thanks,
>> Sridhar.
>>
>>
>
>
>
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