Target Impedance Calculator
Calculate first-pass PDN target impedance from allowed rail voltage deviation and expected transient current step.
Inputs and tolerances
Calculate the maximum PDN impedance target from allowed rail deviation and the expected transient current step.
Target impedance is a first-pass power-integrity goal. The PDN impedance should stay below this value across the frequency range where the load transient has energy.
Nominal results and guaranteed range
- Design note: Low-voltage, high-current rails can demand very low impedance because a small voltage tolerance divided by a large current step produces milliohm-level targets.
- This calculator does not model capacitor ESR, ESL, mounting inductance, antiresonance, VRM response, plane spreading, package parasitics, or frequency-dependent impedance.
Turn a rail droop budget into a PDN impedance target
Target impedance is the maximum power-distribution impedance that keeps a rail within its allowed voltage deviation during a load-current transient. It is a first-pass planning value for decoupling, planes, package effects, and regulator response.
Rail deviation budget
Use the portion of the rail tolerance allocated to transient ripple or droop, not the full absolute supply tolerance unless that is intentional.
Transient current step
Use the expected load-current change that the PDN must support over the relevant time and frequency range.
Milliohm targets
Modern low-voltage digital rails often produce targets in milliohms, which makes layout and capacitor parasitics critical.
Worked example
If a 1.0 V rail can move by 50 mV during a 2 A load step, the first-pass target impedance is 50 mV divided by 2 A: 25 milliohms.
Calculation
Ztarget = deltaV / deltaI.
Ztarget = 0.05 V / 2 A = 0.025 ohm.
0.025 ohm = 25 milliohms.
Design meaning
The PDN impedance should stay below 25 milliohms across the frequency region where the load transient demands current.
Actual design still needs capacitor models, placement, planes, mounting inductance, VRM response, and measurement or simulation.
Common mistakes and limits
Using the whole rail tolerance
Static tolerance, regulator accuracy, DC drop, noise, and transient droop may need separate budget allocations.
Ignoring frequency
Target impedance is not one capacitor value. The impedance profile is frequency dependent and can include antiresonance peaks.
Forgetting layout parasitics
Capacitor ESL, via inductance, mounting loop area, spreading inductance, and package parasitics can dominate the useful response.
Related calculators and next checks
Buck converter calculator
Estimate regulator power flow and current before PDN planning.
Voltage drop calculator
Check DC path resistance and load voltage before high-frequency PDN work.
Capacitor energy calculator
Check stored capacitor energy when bulk capacitance affects hold-up or transient support.
Frequency period wavelength converter
Convert transient and switching frequencies into timing and wavelength context.
Engineering reference
Equations, assumptions, and design guidance
Calculates a first-pass PDN target impedance from allowed rail voltage deviation and expected transient current step.
Equations and variables
Ztarget = deltaV / deltaIZtarget_mohm = Ztarget * 1000- deltaV
- Allowed voltage ripple or droop (V)
- deltaI
- Transient current step (A)
- Ztarget
- Maximum PDN target impedance (ohm)
Assumptions and limitations
Assumptions
- The allowed voltage deviation is the rail budget allocated to transient ripple or droop.
- The current step represents the load transient the PDN must support.
Limitations
- Frequency-dependent impedance, capacitor ESR and ESL, mounting inductance, antiresonance, VRM response, package parasitics, plane spreading, and layout are not modelled.
Worked example and design use
50 mV droop for a 2 A transient
Inputs: deltaV = 0.05 V, deltaI = 2 A
Outputs: Ztarget = 0.025 ohm, Ztarget = 25 milliohms
Design guidance
- Use target impedance as an early PDN planning number, then verify the impedance profile with component models, layout parasitics, and measurement or simulation.
- Very low-voltage, high-current rails can demand milliohm-level impedance targets.