From: Christian S. Rode ([email protected])
Date: Wed Nov 03 1999 - 20:26:04 PST
D. C.:
A question: Is it really practical to assume the benefits of reduced
package crosstalk with push-pull drivers? I can see that you could
tune the delays in open-collector-like technologies to maintain
a nearly continuous current flow (at least in the nominal case) but
with CMOS drivers and cross-coupled break-before-make pre-drivers
won't you always get a period where both drivers are off?
The spike in that case would be due to currents dropping, rather
than increasing and creating an initial on-chip voltage rise, rather
than a fall. David didn't characterize the nature of his spikes.
David?
Chris Rode
"D. C. Sessions" wrote:
>
> David Haedge wrote:
> >
> > Fellow SIers,
> >
> > I am working on an SSO analysis that involves large numbers of LVDS outputs
> > switching on a die. One of the reasons to use LVDS is because the driver is
> > basically just redirecting a 4mA current in the output loop, hopefully
> > eliminating large di/dt's on power and ground. However, my SPICE analysis
> > shows a 20-25mA current spike on VDD and VSS with a rise/fall time of about
> > 230ps each time the device switches (375MHz rate). It was assumed that we
> > could get by with a lot less VDD and VSS pads due to the expected low di/dt's.
> > It appears now that with this large unexpected current spike, we need to triple
> > the number of powers and grounds to achieve an acceptable voltage drop/ground
> > bounce. Has anybody out there seen this sort of behavior in LVDS circuitry?
> > Or is this just perhaps a quirk in SPICE (or my chosen vendor)?
>
> Assuming that you're not running the predriver on the output rails (BAAAAAD idea)
> then it sounds like you have some rise/fall assymetry. Step one is to fix it,
> since power noise is the least of the troubles it causes.
>
> Beyond that, you may not have a big problem Oddly enough, in a differential
> current environment supply inductance can actually be a Good Thing since it
> effectively forces rise/fall symmetry. Put some package inductance into that
> simulation (with or without mutual inductance, although ignoring mutual
> inductance is kinda silly) and see what happens. Betcha you like the results.
>
> --
> D. C. Sessions
> [email protected]
>
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