RE: [SI-LIST] : Decoupling between non-ground power rails, yes or no??

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From: Daniel, Erik S. (Daniel.Erik@mayo.edu)
Date: Tue Jan 23 2001 - 05:02:04 PST


I'd tend to be more concerned about the potential feedback loops in the
active powered circuitry than in the voltage regulators. For modern CMOS
processes, the transistor Fmax can be in the many tens of GHz, with even
higher Fmax values for bipolar and GaAs transistors, leading to much higher
gain bandwidth products than for voltage regulators. Active circuits such
as these with unintentional feedback loops resulting from power-power
coupling and package inductances (or any number of other parasitic feedback
mechanisms) can easily lead to nasty oscillations (unfortunately, we have
seen some wonderful examples of this as Pat mentioned).

In any event, I'd tend to agree with the "average" SI-list response which
seems to suggest caution when introducing power-power bypass, although
sometimes it is unaviodable (e.g., in the case mentioned in an earlier post
regarding traces crossing a power-power split).

                                        - Erik

==================================================================
Erik Daniel, Ph.D. Voice: (507) 538-5461
Mayo Foundation Fax: (507) 284-9171
200 First Street SW E-mail: daniel.erik@mayo.edu
Rochester, MN 55905 Web: www.mayo.edu/sppdg/
==================================================================

> -----Original Message-----
> From: Ingraham, Andrew [mailto:Andrew.Ingraham@compaq.com]
> Sent: Monday, January 22, 2001 9:16 PM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Decoupling between non-ground power
> rails, yes
> or no??
>
>
> > I think it is a matter of proportion. ...
>
> > I don't think that planes fall into this category (not enough C).
>
> I agree.
>
> Discrete bypass capacitors might be 0.01 to 0.1uF each, times
> a few dozen.
>
> Intrinsic plane capacitance is perhaps two to five orders of
> magnitude less.
>
> Voltage regulators likely respond in the timeframe of
> microseconds; i.e.,
> loop dynamics in the under-to-low MHz range. Above the
> loop's bandwidth,
> it's pretty much a don't-care; it doesn't affect the closed
> loop stability.
> But add enough capacitance in the right place, and the phase
> response in the
> low-MHz range IS affected, and the response blows up.
>
> Andy
>
>
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