Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast

D. C. Sessions ([email protected])
Thu, 15 Jul 1999 09:27:00 -0700

Mike Degerstrom wrote:
> Raymond,
> I'd also like to know more about the edge-rate control with use of
> a series resistor as has been mentioned is used with the Analog
> Devices 21020. I have seen mention of parts that use a reference
> resistor to control output impedance, i.e., drive strengths such
> as in the HP article by Esch, and Manley. I also noticed that
> IBM sells SRAMs with programmable impedance control drivers.

In principal, you can control edge rates by setting the predriver
currents. Output edge rate is mainly dictated by the risetime
of the final driver devices' gate voltages, and that in turn is
dictated by the Ids of the predriver transistors. If you provided
the predriver with a bias voltage to a two-transistor stack (or
three with tristate) such that the switching transistors just
gated the current from the bias transistor the result would be
a controlled dv/dt at the final drive gate and indirectly a
controlled edge rate.

Since the gate capacitance and transconductance of the final devices
are roughly proportional with process, this approach would give
a reasonably stable output edge rate. Of course there would be
timing impact, and if you controlled the turnoff transistors you'd
have crowbar current. (If you don't control them, then the turnoff
transition could get VERY abrupt, making the net output a bit odd.)

So it *could* be done, albeit with tradeoffs in power, noise, and
layout complexity.

D. C. Sessions
[email protected]

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