[SI-LIST] Re: PDN related

  • From: Istvan Novak <istvan.novak@xxxxxxx>
  • To: steve weir <weirsi@xxxxxxxxxx>
  • Date: Mon, 15 Mar 2010 23:57:39 -0400

Lynne and All,

One more caveat: the metric to judge the PDN performance is very 
important.  There are applications
where the noise power matters more, in those applications widening the 
PDN bandwidth with Q>1
capacitors works very well.  There are other applications, where the 
worst-case instantaneous
noise voltage matters, and in those cases any impedance profile that 
deviates from a smooth R-L
equivalent impedance will result in an increase of noise. 


Istvan Novak

steve weir wrote:
> Lynne, there has been for many years an incorrect notion that bypass 
> capacitors are not effective past their mounted SRF.  This is at best a 
> truism.  The fact is that bypass capacitors work just fine decades 
> beyond their mounted SRF.  Istvan's DMB/DMI scheme that you will find 
> detailed on his web site that you've referenced relies on this fact.
> At the beginning and end of the key issue in power delivery is 
> inductance.  If you are concerned with either or both cost and 
> performance inductance of the components and interconnect is the 
> limiting factor to both.  Inductance drives basic impedance.  Inductance 
> drives Q in resonances. 
> While many pole networks can be very useful and Sigrity's Optimize 
> PI(tm) customers are generally getting good results with that tool's 
> approach, beware the rat hole of a many pole FD impedance plot:  Those 
> plots reflect voltage / current relations for circulating currents.  
> This is Bruce's big caveat and one of the big reasons why he is so 
> insistent on TD analysis. 
> Best Regards,
> Steve.
> Lynne D. Green wrote:
>> Hi, Steve and Rajesh,
>> Enough caps of one size will work, until your signal content goes too 
>> far past the resonance frequency.
>> And yes, different packages have different parasitics and different 
>> resonance freqs.
>> Using a variety of (carefully selected) caps covers a wider frequency 
>> range (several decades).
>> At the same time, this reduces the total number of caps needed, and 
>> quite possibly total cost.
>> These days, there are EDA tools to make this easier.
>> And I agree wholeheartedly with Lee that there are plenty of good 
>> books out there.  Istvan Novak's
>> home page lists several of them.  http://www.electrical-integrity.com/
>> Lynne
>> steve weir wrote:
>>> And yet there are any number of boards and systems that have been 
>>> built using mostly one capacitor value such as 0.1uF that work fine.
>>> Steve.
>>> Scooby Doo wrote:
>>>> Hi Lynne,
>>>> This is what exactly what is wanted to ask. If the slew rate is 
>>>> 1v/ns, then my 0.1uF capacitor value (designers blindly using value) 
>>>> will not support due to existance inductive nature at that frequency.
>>>> So kindly give some reference / notes so that choosing right value.
>>>> --- On Mon, 3/15/10, Lynne D. Green <lgreen22@xxxxxxxxxxxxxx> wrote:
>>>> From: Lynne D. Green <lgreen22@xxxxxxxxxxxxxx>
>>>> Subject: [SI-LIST] Re: PDN related
>>>> To: "Scooby Doo" <si.scooby@xxxxxxxxx>
>>>> Cc: "SI LIST" <si-list@xxxxxxxxxxxxx>
>>>> Date: Monday, March 15, 2010, 5:20 PM
>>>> Depends on the frequency range where you need PDN clean.
>>>> If it is a wide range (generally true) then one would use
>>>> more than one value of capacitor.  Remember, capacitor
>>>> impedance increases at high frequency due to inductive
>>>> package effects.
>>>> Lynne
>>>> "IBIS training when you need it, where you need it."
>>>> Dr. Lynne Green
>>>> Green Streak Programs
>>>> http://www.greenstreakprograms.com
>>>> 425-788-0412
>>>> lgreen22@xxxxxxxxxxxxxx
>>>> Scooby Doo wrote:
>>>>> Hi SI Experts,
>>>>>     I have a question reg PDN.
>>>>>   Suppose an FPGA has 100 I/O pins with 10 VCCO/Gnd pairs. So an 
>>>>> average of 10 I/O pins for 1 VCCO/Gnd pair. Assume each I/O pin 
>>>>> driving 10pf load.
>>>>>   My qn is, to make the clean PDN, is it enough to provide 100pf of 
>>>>> capacitor to each Vcc/Gnd pair?
>>>>>   I am not convinced with blindly provinding 0.1uF cap to Vcco pins.
>>>>>   kindly clarify.
>>>>>   Thanks in advance for your valuable feedback.
>>>>>   Rajesh

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