Target Impedance

Target Impedance

Target impedance is the maximum PDN (Power Delivery Network) impedance allowed to keep voltage ripple within spec.

Z_target = VDD × (ΔV/VDD) / I_max

Where:

• VDD = supply voltage (V)
• ΔV/VDD = allowed ripple as a fraction of VDD (e.g., 0.05 = 5%)
• I_max = maximum current transient (A)

Example: A 1.0V core, 5% ripple, 10A transient: Z_target = 1.0 × 0.05 / 10 = 5 mΩ

You must keep the PDN impedance below Z_target across the entire frequency range of interest.

Selecting Decoupling Capacitors

Once you know Z_target, you choose capacitors so that their impedance stays below Z_target near their resonant frequency. The required capacitance to achieve Z_target at frequency f is:

C_min = 1 / (2π × f × Z_target)

Your Task

Implement:

• targetImpedance(VDD, rippleFraction, I_max) returning Z_target in milliohms
• minCapacitance(Z_target_mohm, f_MHz) returning required capacitance in μF