FreeLists / si-list / [SI-LIST] Re: Questions about interplane capacitance

["Joel Brown" <joel@xxxxxxxxxx>]

Istvan,

When I consider the PDN it does not seem like a transmission line because it
consists of planes with many lumped L and C elements, compared to a uniform
trace with very little lumped L C. When I look at the charactersitics of a
single bypass capacitor it is resistive only at the SRF. How do you make a
PDN resistive over a wide range of frequencies? What is meant by putting the
bypass components "further" away? One thing I read regarding plane
capacitance is that there is an effective bypassing radius which is related
to the rise time of the bypass currents and propagation velocity of the
medium. If the the bypassing is too far away it will have little effect
because the device requiring the bypass current will finish switching before
the current wave even reaches the bypass cap.

Joel

 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Istvan Novak
Sent: Tuesday, March 11, 2008 5:15 AM
To: Joel Brown
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Questions about interplane capacitance

Joel,

Just one quick comments to the good summary from Steve:

While considering planes and bypass capacitors in terms of effective
capacitances and inductances is a valid approach, we need to keep in mind
that focusing on the capacitive or inductive nature of parts without looking
at the wider picture misses a very important and useful class of solutions,
namely that of matched transmission lines.  As it was pointed out earlier
several times on the SI list, the best
(self) impedance for a power distribution network is a resistive one,
neither capacitive, nor inductive.
We can get resistive impedance from a matched transmission line, regardless
of its capacitance and inductance, and in such cases the notion of 'service
area' of parts become meaningless: you can put bypass components further
away from the active devices without sacrificing performance. 

Regards,

Istvan Novak
SUN Microsystems

Joel Brown wrote:
> Interplane capacitance is frequently cited as the only effective 
> bypass capacitance on a PCB at frequencies above 200 MHz.
> I am currently working on a design which brings up some questions 
> regarding interplane capacitance.
>  
> 1. Power planes normally carry "standard" voltage rails that are used 
> throughout a board such as +5V and +3.3V.
> High speed ICs usually have core voltages that are local to the IC and 
> are provided by a local regulator which converts the standard rail to 
> the core voltage (example 3.3 to 1.8V).
> The local core voltage is distributed on a plane area that is local to 
> the IC and therefore is small in area (0.25 sq in or less) which 
> results in a very small amount of interplane capacitance.
> Is this very small amount of capicitance effective for bypassing the 
> IC? I am sure it depends somewhat on the current waveform being drawn 
> by the IC but this can only be estimated because semiconductor 
> manufacturers do not provide current consumption profile as a function 
> of frequency. To make matters worse, some ICs have several different 
> VCC pins which the manufacturer recommends connecting to separate 
> networks of bypass caps and ferrite beads. This cuts the power 
> distributuion up even more resulting in less (practically zero) 
> interplane capacitance. It is somewhat ironic that the the voltages 
> such as +5V and +3.3V which are required at points across the whole 
> board and therefore have the most interplane capacitance are also the 
> voltages which have least requirement for interplane capacitance because
they do not directly supply high speed rails.
>  
> 2. There has been a lot of emphasis on reducing the mounted inductance 
> of bypass capacitors. Even with this reduced inductance they are still 
> only effective up to several hundereds of MHz at which point the 
> interplane capacitance becomes the only bypass capacitance mechanism. 
> However there is inductance between the connection of the IC to the 
> planes. This inductance consists of vias and package inductance. I did 
> look for some numbers for package inductance and did not find much, it 
> seems to be a closely held secret. Also it is unknown how much bypass 
> capacitnace is internal to the IC package. Just for example if we 
> assume 250pH for the vias and 500 pH for the package, then the 
> impedance at 500 MHz would be 2.36 Ohms. This seems rather high for the
interplane capacitance to be of much benefit.
>  
> In summary how much interplane capacitance is needed to be beneficial, 
> and why is it beneficial given the inductance in the vias and package?
>  
> Thanks - Joel
>  
>  
>  
>
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