[SI-LIST] Re: On-die caps for IO supply
- From: "Stephane Tremblay" <strembl1@xxxxxxxxxx>
- To: "'Vinu Arumugham'" <vinu@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 24 Apr 2008 16:22:08 -0400
Vinu,
I like your idea of symmetrically reference the signal to a ground and a
vddio plane. I do understand the benefits of doing so. On a typical package
stackup, it also means that instead of having (representing only 3 layers of
interest) SIG-GND-PWR, you would change to GND-SIG-PWR. Doing so will
realize your plan but it will increase the loop inductance from PWR to GND.
Can anyone comment on this practice? Anyone ever tried that?
Thanks for the help,
Steph.
-----Original Message-----
From: Vinu Arumugham [mailto:vinu@xxxxxxxxx]
Sent: Wednesday, April 09, 2008 12:26 PM
To: Stephane Tremblay
Subject: Re: [SI-LIST] On-die caps for IO supply
If you can afford to symmetrically reference the signal to a ground and
vddio plane along the entire length of the interconnect and use a
thevenin termination to vddio and ground, the die capacitance required
for signal switching is close to zero. You will only need enough I/O die
capacitance to handle the parasitic 3.5pF, any crowbar current and
pullup/pulldown asymmetry.
Thanks,
Vinu
Stephane Tremblay wrote:
> 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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