From: [email protected]
Date: Wed Oct 20 1999 - 20:23:00 PDT
I think the main issue as far as the driver is concerned is its reliability at
high current/temperature level. I would stick to the 50PF loading spec.
then there should not be any problem of overheat at high speed or
electromigration at high current level. Thus the estimated max. line length
should be 9.2 inches for 33 ohm and 16.7 inches for 60 ohm case.
Please be reminded the loading of the inputs of the other chips has not
been considered.
Raymond
Chris Bobek <[email protected]> on 20/10/99 03:08:52
Please respond to [email protected]
To: [email protected]
cc: (bcc: Raymond Leung/QSA/AU)
Subject: Re: [SI-LIST] : Effect of low Zo for unterminated lines
This is a little off subject, but I made a slight mistake, the Zo is 33ohms.
Here is a stripline portion of my stackup:
1 oz copper plane
4mil FR-4 (er = 4.5)
1 oz signal
4.4mil Prepreg
1 oz copper plane
When I plug those values into Specctraquest, I get 33 ohms. When I use Howard's
equations from his book, I get roughly 30 ohms or so (I kinda forget). So, I
believe this is correct.
Chris
Dave Hoover wrote:
> All the PCB's I've built using 29 Ohms (GTL and Ram something)
> was .018" lines .003" above a plane (microstrip).
>
> ??????? .006" was using a high Er' dielectric??????
>
> Dave
> -----Original Message-----
> From: Chris Cheng [mailto:[email protected]]
> Sent: Tuesday, October 19, 1999 9:12 AM
> To: [email protected]
> Subject: RE: [SI-LIST] : Effect of low Zo for unterminated lines
>
> are you sure you have a 29ohm line. its very hard to believe
> you can achieve 29ohm with 6 mil traces on PCB.
> chris
>
> > -----Original Message-----
> > From: [email protected]
> > [mailto:[email protected]]On Behalf Of Chris Bobek
> > Sent: Monday, October 18, 1999 5:24 PM
> > To: Si-list
> > Subject: [SI-LIST] : Effect of low Zo for unterminated lines
> >
> >
> > Hi,
> >
> > Because of several constraints on our stackup and line width, a 6mil
> > stripline trace yields a Zo = 29ohms. When I simulate a 29ohm,
> > unterminated line on our board (@ 10MHz), I see a pretty nice waveform,
> > albeit with slower rise/fall times (which is ok for the nets in
> > question). When I increase the Zo to 60 ohms, for example, I see a lot
> > of ringing on the same net.
> >
> > >From a theoretical point of view, my guess is that this happens because
> > as you widen the trace, there is more capacitance, which slows down the
> > rise/fall times. Is this a correct assumption? Are there other factors
> > as well that are going on?
> >
> > What I am most concerned about is the effect of a low Zo net on its
> > driver. For example, if I have an LVCH245 driver (3.3V) driving a 29ohm
> > line, will the low Zo "hurt" the driver? Or, is the only negative
> > effect the slower rise time? This question is a general question for
> > most ICs.
> >
> > Thanks for your help,
> >
> > Chris
> >
> >
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> >
> >
>
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