Target

The target impedance is set in the Target line. Two parameters are used. Target impedance (enter a value in milliohm) and frequency (enter a value in Mega Hertz).

See the target line in the graph as the orange curve. When using the optimizer, it will try to push the impedance curve under the target line.

Target Impedance

The target impedance is the required maximum impedance the PDN is designed for. Simply put, this comes from Ohms law by dividing the maximum voltage variation (ripple) tolerated by the maximum current change seen.

Target Frequency

As the inductance and capacitance involved in getting from the board planes in through the package to the IC die effectively forms a low-pass filter, there is an upper frequency to which bypassing at the board level makes sense. Above that frequency, there is very little you can do as a board-level designer. This is the frequency set under Target. The maximum frequency where the impedance target is relevant.

2 thoughts on “Target

  1. I’m curious why you roll off the target impedance as you approach 100MHz? I believe Lee Ritchey advocates for simply “keep Z below the target up to 150MHz” and several other sources I’m familiar with suggest the same to 100MHz.
    The exact f_max I’m not concerned with, to me that’s a design choice, but I’m interested to know why you added a roll off.
    Thanks

    • You set the frequency, so that is up to you. The roll-off itself is pretty steep (so almost a sharp corner) and not directly rooted in any specific theory. It is clear that the “correct” (whatever that means in this situation) requirement cannot be a sharp corner. If you look at energy over frequency for digital signals there will be some roll-off. If you look at the L’s and C’s of going from board to die in through the package etc., that will also form a filter with a roll-off. What this all adds up to is often not simple to describe. A steep roll-off as used here is likely every bit as good as a sharp corner requirement for all practical purposes. Hope this helps.

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