[SI-LIST] Re: On-die caps for IO supply

Steph,
Since the main benefit of this configuration is reduction of switching 
current drawn from the PWR/GND structure, the increased loop inductance 
is not a concern. This is assuming of course PWR is dedicated I/O power.

Thanks,
Vinu

Stephane Tremblay wrote:
> 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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>>
>>   
>>     
>
>
>   



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