[SI-LIST] Re: Target impedance for power plane
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: "Lakshmi N. Sundararajan - PTU" <lakshmi.s@xxxxxxxxxxx>
- Date: Wed, 10 Mar 2010 19:46:10 -0500
Hi Lakshmi,
These are fairly basic questions and you should find plenty of free
reading material on
the internet to clarify these things.
Regards,
Istvan Novak
Oracle
Lakshmi N. Sundararajan - PTU wrote:
> Hi Brad,
> What exactly is your definition of power plane impedance?
> How do you get Voltage to be 0.05V in this case.
> I am sorry I do not follow your numbers for the case you mentioned.
> 1A draw will yield 10mV drop on the power plane.
> 3A draw will yield 30mV drop. So to average them will yield 20mV.
> But still you quote 0.01V of DC IR drop variation? I would have expected
> 0.02V.
>
> In which case, the margin would be 0.03/800mA = 37.5mohm
>
> Also, you use the term AC noise margin for computing power plane impedance.
> So this all confuses me a bit here. Can you please be more clear.
>
> Thanks,
> -LN
>
>
>
> -----Original Message-----
> From: Brad Brim [mailto:bradb@xxxxxxxxxxx]
> Sent: Wednesday, March 10, 2010 3:45 PM
> To: Lakshmi N. Sundararajan - PTU; si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Target impedance for power plane
>
> hello Lakshmi,
>
> You should set your target impedance at 0.05V/800mA = 63mOhm.
> You should also consider that your chip may have multiple modes of
> operation, which might pull different levels of DC current. For example, one
> mode pulls 1A DC and the other mode pulls 3A DC and you have 10mOhm DC
> resistance between chip and VRM. If you adjust the 1.2V DC rail at the
> device for the average of these two modes, then you have 0.01V of DC IR drop
> variation, so your AC noise margin is more likely 0.04/800mA = 50mOhm.
>
> cheers,
> -Brad
>
>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx
>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Lakshmi N.
>> Sundararajan - PTU
>> Sent: Wednesday, March 10, 2010 3:32 PM
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Target impedance for power plane
>>
>> Hi Experts!
>> Over the last week there has been so many emails on the
>> s-param and channel modeling.
>>
>> I am still in the process of digesting each of your responses.
>>
>> I would like to thank you all for those wonderful insightful
>> advices and observations.
>>
>>
>>
>> I have another question on how can I calculate what should be
>> the target impedance of power plane?
>>
>> Suppose, assume I have a V1P2V supply to a IC. V1P2V can be
>> from 1.15V to 1.25V.
>>
>> And the max current when all signals are switching is 800mA.
>>
>>
>>
>> Given this requirement, then for this IC, should the power
>> plane impedance be 1.15V/0.8A = 1.4ohms
>>
>> Or should I consider only the ripple on the power which would
>> be 0.1V and so the impedance would then be 0.1V/0.8A = 0.125ohm
>>
>>
>>
>> Please suggest some pointers incase I am way off.
>>
>>
>> Thanks,
>>
>> -LN
>>
>>
>
>
>
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