[ibis-macro] What I learned at DesignCon about PDN simulations.
- From: Walter Katz <wkatz@xxxxxxxxxx>
- To: "IBIS-ATM" <ibis-macro@xxxxxxxxxxxxx>
- Date: Mon, 3 Feb 2014 13:35:14 -0500 (EST)
All,
There were a large number of papers at DesignCon about PDN, and all pretty
much said the same thing - it is all now a Science Experiment. Although
the composite current now in IBIS is part of the solution, the real
problem is how to apply composite current on all the buffers to determine
the current requirements on chip. One needs both the spectral content of
the total current on each rail and the phase (i.e. what buffers are
transitioning when). A SerDes bus will normally have all Tx transitioning
at more or less random times. This is assured by the data patterns being
8B10, or scrambled. DDR DQ memory busses can have more regular patterns
(e.g. sending lots of 0 data with bursts of all 1's or random data. This
gets even more complicated for chips used in Mobile devices where whole
sections of a chip can be turned on or off to conserve power. These
problems need to be addressed by having sufficient on-die rail capacitance
and sufficient package capacitance. In addition, there needs to be
sufficient capacitors on the board. This on-die and package capacitance
requirements need to be addressed by the IC Vendor, requiring very
detailed knowledge of the current requirements for each buffer (e.g. IBIS
composite current), detailed knowledge of the spectral density and phase
of the buffer switching, and detailed power models on-die and in package.
The systems integrator will not have access to this type of information or
the simulation tools required to do these types of analysis. The systems
integrator does need to insure that he has sufficient bypass capacitors
and power distribution on the board to supply current to the package. This
is a ~25MegHz problem, not a ~GHz problem required by the chip/package
tool.
The bottom line is that we should not confuse the PDN modeling
requirements of the Chip/Package tools and the PDN modeling requirements
for the System Integrator.
Walter
Walter Katz
<mailto:wkatz@xxxxxxxxxx> wkatz@xxxxxxxxxx
Phone 303.449-2308
Mobile 303.335-6156
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