[SI-LIST] Re: On-die caps for IO supply
- From: "Chris Cheng" <Chris.Cheng@xxxxxxxx>
- To: "Stephane Tremblay" <strembl1@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 9 Apr 2008 10:31:44 -0700
I don't think you will like my answer but my experience is fill it up =
with as much capacitor your technology people allow.
If you are core limited, you are SOL to begin with.
If you are pad limited, just fill the gap between the core and pad area =
with decoupling cap before the yield police or the core power =
distribution guy wants to take it for himself.
It will take a lot more than "XXX from Si-list recommended this" to grow =
your die to fit anything more than the above.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Stephane Tremblay
Sent: Wednesday, April 09, 2008 7:33 AM
To: si-list@xxxxxxxxxxxxx
Cc: 'Stephane Tremblay'
Subject: [SI-LIST] On-die caps for IO supply
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 =3D 6 V/ns / 50 Ohms =3D 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.
=20
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