Re: [SI-LIST] : Clean sheet of paper

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Wed Dec 22 1999 - 15:34:51 PST


Bruce Rosenquist wrote:
>
> I was skimming the latest SI news, and your final comment caught my interest.
> I am new to high-speed issues and controlling board impedances, and working
> on a design which contains a lot of parallel termination for LVPECL signals.
> This requires a termination voltage, which must, as you say, both sink and source current.
>
> How do the pros typically do this? Are there any DC-DC converters that are made
> specifically for this function? What are some of the nasties you speak of ?
> What are the tricks ?

You'll notice the deafening silence. Economical, powerful Class A
or AB amplifiers aren't exactly cheap. Switching power supplies do
a great job sourcing power but don't absorb it worth spit.

With SSTL becoming mainstream memory technology the support-chip
outfits will be coming out with some devices to address this but
it's not going to be pretty. Essentially you can do the job sort
of efficiently by putting a rail-to-rail PWM into an LC filter,
no rectifier needed.

> D. C. Sessions wrote:

[...]

> > * It should be economical. Wasted power is a Bad Thing, so low
> > swing is a must. Padrings are some of the most expensive real
> > estate around, so pincount should be minimized. Line termination
> > can dominate a PWB so KISS is the rule. Power supplies (esp.
> > ones that can both sink and source current) are expensive and
> > nasty to deal with, so do without (both for termination and
> > funny analog functions in the I/O circuits.)

-- 
D. C. Sessions
dc.sessions@vlsi.com

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