Re: [SI-LIST] : Power Plane for Internal Device Power?

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From: D. C. Sessions (
Date: Thu Jan 27 2000 - 12:18:52 PST

"phelan, tony" wrote:
> If I have a device whose I/O are 3.3V and internal logic is powered from
> 1.8V do I need a power plane to supply that 1.8V or will a power strip to
> that device suffice? Another way of asking this I guess would be: Are all
> return path currents for the internal 1.8V signals handled internally to the
> device. If so then there would be no advantage to making the 1.8V supply a
> plane?

The answer is (as usual) "It depends."

Depending on the design of the I/O predrivers, there may be some pretty large
transient currents between the core and I/O supplies that show up as honking
high-frequency spikes on the core supply.

Another source of core transient current is power management. Although
indidual clock transients have to be supplied from on-chip capacitance,
power-management and to a lesser extent ordinary data-dependent variations
can cause variations in core current large enough to make trouble if they
show up on long signal-type trace inductances.

D. C. Sessions

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