PDN / Decoupling Target Impedance
Target PDN impedance, how many decaps you need, capacitor SRF and plane capacitance.
Keep the rail stiff at every frequency
A load that switches current draws it in fast steps; for the voltage to stay inside its ripple budget, the power distribution network must look low-impedance to that current across a wide band. That gives a single number to design to — the target impedance:
For a 1.0 V rail at 5 % ripple absorbing a 5 A transient, that's 10 mΩ — and you need to hold
it up to the highest frequency in the current step. Above its self-resonant frequency a
capacitor is inductive, its impedance rising as 2πf·ESL (the
mounting + package inductance), so a single decap can't stay low. You parallel many, which
divides the inductance:
The lesson PDN teaches every layout: mounting inductance, not capacitance, sets the high-frequency floor. Short, fat connections to the planes and many small caps beat a few big ones. The power planes themselves add a final, very-high-frequency capacitor.