[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <weirsp@xxxxxxxxxx>, <cpad@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 25 Aug 2004 21:42:30 -0400
Steve,
You make good points, but as always, it depends...
For instance, it is true that point of load converters
should not be connected to their load with a plane
larger than necessary, not only because larger planes
tend to resonate more severely, but also because if
the load is a high-speed core, the leakage of the clock
onto the PCB will create more EMI problems. But
this is just one aspect in the complex design scenarios.
However, there are situations when the large planes
are justified. Imagine a large PCB filled with. for
instance. switch ASICs, all requiring the same IO
voltage. All of these IO signals can rightfully reference
the IO plane, so it does not make much sense (though
one could do it) to feed the ASICs with small islands.
A note about patents: I am not aware of anyone paying
SUN royalty fee on the PDN resonance-reduction
patents. If you know of any such case, let me know.
Besides, other companies beyond Hitachi and SUN
also have granted patents on similar solutions.
Regards,
Istvan Novak
SUN MIcrosystems
----- Original Message -----
From: "steve weir" <weirsp@xxxxxxxxxx>
To: <cpad@xxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, August 25, 2004 5:43 PM
Subject: [SI-LIST] Re: Bypass vs Decoupling capacitors
> Chris,
>
> What I am saying is that we don't want signal current exciting the
cavities
> formed by the Vcc / Gnd planes. So, we want to insert a series (
> decoupling ) impedance between the power distribution, ( even if it is
> planar ) and local Vcc islands for ICs. The price that we pay for this is
> that the decoupled Vcc planes are no longer suitable as signal image
> current returns which is precisely the point. So just like "the old
days",
> early logic, or as Mike brought up vacuum tubes, we keep the signal
> currents out of the global power supply interconnect, and instead restrict
> return images to ground.
>
> So, now the decoupled power islands need bypassed power across a frequency
> range drops to that of what is needed to support PDS requirements, rarely
> anything over 100MHz, because of the device package inductance. That is
> equivalent to a 3ns Tr, stuff we haven't seen since nearly 12 years ago,
> when ironically, Rogers still made good money selling their bus-bar
> products for use on PCBs. Given that more and more power is locally
> regulated, the existance of global Vcc planes is diminishing. So more and
> more we see the questions about crossing plane splits. Former first lady,
> and noted signal integrity expert Nancy Reagan declares "Just say
> no!" Transport signal return images on ground planes as God
> intended. Decoupling costs go through the floor, as do headaches
> associated with trying to calm resonant cavities being pumped at
> frequencies well above their SRFs and without paying Hitachi or Sun money
> for their patents to try and make planes look like Tx lines.
>
> Regards,
>
>
> Steve.
> At 12:20 PM 8/25/2004 -0700, Chris Padilla wrote:
> >Steve,
> >
> >You've piqued my interest here since I'm an EMC guy. Are you saying that
> >because of low-impedance planes, we get too much energy sloshing around
and
> >not getting absorbed so you like to put in some kind of R value in some
> >fashion (expound here, please) to absorb that energy.
> >
> >Okay, I just read a lot into the below two paragraphs so set me
> >straight. I'd like to understand better how I can use Rs in a decoupling
> >network.
> >
> >Thanks----->Chris
> >
> > >The advent of low-impedance planes did away with the need for series
> > >isolation in most digital circuits, so with the series element gone,
all
> > >that was left of the decoupling network was the shunt capacitor on the
load
> > >side which looks and acts like a bypass capacitor, because it is one.
But
> > >the decoupling term got carried forward.
> > >
> > >Since, I am a fan of putting the series impedance back in decoupling
> > >networks as a way to dramatically reduce cost and improve EMC
performance,
> > >I like to use the terms the way they were 30 - 35 years ago.
> > >
> > >Regards,
> > >
> > >
> > >Steve.
> >
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