[SI-LIST] Re: A Question about Target Impedance Threshold computation

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 13 Jun 2013 15:07:16 -0700

Steven, target impedance applies at the point of use.  In some cases, 
interconnect impedance can become significant even at supra-audio 
frequencies in the 100's of kHz, but almost always is an issue by the 
low Mhz.  The target Z relation is also just a guideline.  It assumes 
resistive Z which is rarely the case over the frequency range any given 
PDN needs to support.

Steve.
On 6/13/2013 1:32 PM, Steven J Ohlsen wrote:
> Hello,
> I am somewhat new to the world of Power Integrity.  I hope this isn't a
> dumb question, but I can not find any answer in the books I have or online
> either.  I understand that the equation for Target Impedance is (allowed
> Voltage Variation)/(max change of current).  However this seems like a
> "50,000 ft" rule-of-thumb.  If your design meets this criteria then all is
> well.  But if your design has difficulty reaching this impedance target
> then what.  My observation is that when you get down to the board level
> for example, there can be 30-40 balls on the voltage regulator (and maybe
> multiple voltage regulators), and 50-100 balls on the device under test,
> and numerous decoupling capacitors scattered around, between what points
> does the target impedance apply?  If you look at the board geometries then
> the current from Voltage Regulator to Device may travel through a 1-inch
> (or more) wide path on the power plane.  Current is also provided by the
> decoupling capacitors whose current (probably) has a different path to the
> device under test .  It seems that the current portion of the target
> impedance equation varies from point to point on the board depending on a
> host of very complicated relationships.  Simply dividing the current
> portion of the target impedance equation by the number of balls on the
> voltage regulator or by the number of balls on the device under test also
> seems incorrect.  Is there a more design specific method to compute the
> target impedance?
>
> Thank You all for your responses.
> Steven Ohlsen
>
>
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