[SI-LIST] On-die caps for IO supply
- From: "Stephane Tremblay" <strembl1@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 9 Apr 2008 10:32:35 -0400
Hi SI-Listers,
I am looking for a rule of thumb on the required on-die capacitors
needed for proper operation of a given IO (a DDR mem IO in my case). I am
looking for a number of "x" capacitance per IO.
My IO can be programmed to 18 Ohms of drive and the measured slew-rate is
slightly higher than 6 V/ns for both rise and fall. The parasitic capacitance
of this IO is about 3.5 pF. The toggle rate could be as high as 2 Gbps so I
expect the IO rail to recover within reasonable limits within half a period.
I could always start by assuming the current I need just for the transmission
line (being a 50 Ohms one):
dI/dt = 6 V/ns / 50 Ohms = 120 mA / ns (per IO)
but this basic current demand calculation neglected the parasitic capacitance
of 3.5 pF.
Some will say it is also greatly dependant of my power-gnd loop inductance from
IO to on-package decaps. On the other end, if my noise spectrum is in the
hundreds of MHz, I can't do much at the package level to clean my die supply.
So from your experience and knowledge, what would be a good start as a
required on-die capacitance required. I want to avoid my on-die IO rail
collapsing at a frequency that my package could not keep up.
Thanks for the answers,
Steph.
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