Balajy,
It is not that straight forward.
While calculating the target impedance, I think we need to consider all the
current consumed by other devices feeding of that rail. How would you add
up the currents is a totally different story. It generally depends on the
frequency of the current drawn. This has been nicely explained by Larry
Smith (one of the guys who actually developed the target impedance concept)
in the following SI-List thread:
https://www.freelists.org/post/si-list/A-Question-about-Target-Impedance-Threshold-computation,2
However, how much minimum impedance the power ground pads at the die can
see while looking outwards will depend on several factors like on-die
capacitance, on-die resistance, on-package capacitors, package lead
inductance, etc. These parameters will put a fundamental limit on how low
PCB board level impedance actually is effective.
Hope this helps.
Binayak
On Fri, May 5, 2017 at 3:50 PM, Balajy Kumar <BKumar4@xxxxxxxxxxxxxxx>
wrote:
Hi
I am doing Power AC analysis using hyper lynx tool. l am having doubt in
calculating the target impedance using transient current for Lumped and
distributed analysis.
Here I used 50% load current (sum of all the IC loads current in the power
supply) to calculate Target impedance for lumped analysis since I didn't
find the transient current details.
I want to probe the impedance at one particular load point (IC) only so I
used distributed analysis to probe in one load IC in the power supply. My
question is which current I have to use to calculate the Target impedance ,
50% of sum of all the loads current or 50% of that particular IC load
current. Kindly clarify with details.
Also the frequency till which I need to validate the Target impedance both
in Lumped and distributed (one load IC)? Hope it will be clear.
Thanks
Balajy
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