FreeLists / si-list / [SI-LIST] On-die caps for IO supply

["Stephane Tremblay" <strembl1@xxxxxxxxxx>]

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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