AW: [SI-LIST] : Effect of low Zo for unterminated lines

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From: Andreas Lenkisch (Trenew) (a.lenkisch@trenew.de)
Date: Mon Oct 25 1999 - 03:55:44 PDT


Raymond,

it seems that we think very close to each other, but it was expressed by different words. You favorite the current into the line capacitance (including loads) for an estimation, I believe that the current into the line impedance, assumed as a ohmic load may used for an estimation. But on my simulations, I look at first for the waveforms and when I'm happy, I make a cross check for the current absorbed from the clamps to be absolutely sure not to overstress a component and reduce the live span.
Thank you for your discussion.

regards
Andreas

----------
Von: Raymond.Leung@qsa.idt.com[SMTP:Raymond.Leung@qsa.idt.com]
Gesendet: Montag, 25. Oktober 1999 01:13
An: si-list@silab.eng.sun.com
Betreff: AW: [SI-LIST] : Effect of low Zo for unterminated lines

Andreas,

It is very true that a responsible chip designer should handle the
reliability issue very conservatively in order to keep your product
robust. Besides, electromigration is a life span problem. The
conservative figure of the running current is trying to assure the
mean time of failure which for a ordinalry chip should be higher
than 10 years. The other thing is if the ambient temperature is alway
kept much lower than the max. limit, say, 70C, the life time is much
longer.

Regards,
Raymond
---------------------- Forwarded by Raymond Leung/QSA/AU on 25/10/99 09:09
---------------------------

"Andreas Lenkisch (Trenew)" <a.lenkisch@trenew.de> on 23/10/99 23:59:16

Please respond to si-list@silab.eng.sun.com

To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
cc: (bcc: Raymond Leung/QSA/AU)

Subject: AW: [SI-LIST] : Effect of low Zo for unterminated lines

Raymond,

I understand your addressed concern, by assuming a lumped load the amout of
charge driven at each circle can be taken to estimate the
power to dissipate by the driving IC. From my experiance it seems to me that
when the line is longer and acts as a transmission line, the assumption of an
ohmic load (loaded impedance) may represent the situation may be better.
Therefore I send with my earlier email a table of loaded backplane impedances.

When I started to deal with higher data rates on (VME-) backplanes, I measured a
strong encrease of IC temperature with encreasing frequency and I assumed that
the power dissipation will become the bottleneck on parallel
multipoint/multiline buses. But with progress in work, I detected a short to GND
on one or two lines (I don't remember exactly) and with removing the short also
the encrease of heat was removed. The IC is still alive today!, it is an
ABTE16245. This may indicate something about the robustness of output stages,
but I'm not sure if it is also valid for other families or manufacturers.

regards
Andreas

----------
Von: Raymond.Leung@qsa.idt.com[SMTP:Raymond.Leung@qsa.idt.com]
Gesendet: Freitag, 22. Oktober 1999 00:37
An: si-list@silab.eng.sun.com
Betreff: Re: AW: [SI-LIST] : Effect of low Zo for unterminated lines

Andreas,

Of course I am well aware of the transmission line effects of nowadays'
board design. What I am suggesting is a rule of thumb to look at the
chip reliabilty problem, as a response to the original conern of
the driving chip from Chris. Even if the there is transmission line effect
in reality the lump estimation is a good guideline to look at the heat/current
issues, in terms of reliability.

Raymond

"Andreas Lenkisch (Trenew)" <a.lenkisch@trenew.de> on 23/10/99 00:21:48

Please respond to si-list@silab.eng.sun.com

To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
cc: (bcc: Raymond Leung/QSA/AU)

Subject: AW: [SI-LIST] : Effect of low Zo for unterminated lines

Raymond,

a longer trace appears more than a transmission line than a lumped element, it
depends from the ratio of signal speed and rise/fall time. When you have nearly
the same voltage along the line during output switching you can deal with lumped
elements (trace length << speed *trise), otherwise transmission line effects
must be taken into consideration.
In this case the output edge will not be smoothed down by capacitive loads,
because the driver "sees" only a small part of the distributed line capacitance,
the remaining part is decoupled by the distributed line inductance. In this case
the output voltage step is reduced in high, but not in edge rate. The step high
depends from the ratio of driver output impedance and line impedance

Andreas

----------
Von: Raymond.Leung@qsa.idt.com[SMTP:Raymond.Leung@qsa.idt.com]
Gesendet: Donnerstag, 21. Oktober 1999 05:23
An: si-list@silab.eng.sun.com
Betreff: Re: [SI-LIST] : Effect of low Zo for unterminated lines

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 <cbobek@cadence.com> on 20/10/99 03:08:52

Please respond to si-list@silab.eng.sun.com

To: si-list@silab.eng.sun.com
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:hycheng@3pardata.com]
> Sent: Tuesday, October 19, 1999 9:12 AM
> To: si-list@silab.eng.sun.com
> 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: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]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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