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Decoupling Capacitor Calculator
This calculator estimates the required decoupling capacitance, target PDN impedance, and approximate number of parallel MLCCs needed for IC power rail design. It is useful for digital ICs, microcontrollers, FPGAs, RF circuits, power rails, and high-speed PCB decoupling analysis.
Input Parameters
Results
This calculator gives a first-pass estimate. Real PDN performance depends heavily on package ESL, mounting inductance, PCB planes, via layout, capacitor placement, and IC current spectrum.
Typical Decoupling Capacitor Combinations
| Application | Typical Frequency Range | Common Starting Combination |
|---|---|---|
| General Digital ICs | 1 MHz – 100 MHz | 100 nF 1 µF 10 µF |
| MCU / DSP / Logic Rails | 10 MHz – 300 MHz | 10 nF 100 nF 4.7 µF |
| FPGA / ASIC / High-Speed Digital | 100 MHz – 1 GHz | 1 nF 10 nF 100 nF 10 µF |
| RF / Microwave Bias Decoupling | 500 MHz and above | 100 pF 1 nF 100 nF |
| Power Supply Bulk Decoupling | Low frequency / load transients | 10 µF 47 µF Polymer / Electrolytic |
Equations Used
Required Decoupling Capacitance:
C = Istep × Δt / ΔV
Target Impedance:
Ztarget = ΔV / Istep
Single MLCC Effective Capacitance:
Ceff = Cnominal × Capacitance Remaining %
Capacitive Reactance:
Xc = 1 / (2πfCeff)
Approximate Single MLCC Impedance:
Zsingle ≈ √(ESR² + Xc²)
Parallel MLCC Estimate:
N ≈ max(Crequired / Ceff, Zsingle / Ztarget)
Where:
Istep = transient current step
ΔV = allowed voltage droop
Δt = transient duration
f = target noise or switching frequency
ESR = equivalent series resistance of one capacitor
Frequently Asked Questions (FAQ)
Q1: What does this decoupling capacitor calculator do?
It estimates the required capacitance, target impedance, and approximate number of parallel MLCCs needed to support an IC power rail during current transients.
Q2: What is target impedance in PDN design?
Target impedance is the maximum acceptable power rail impedance, usually calculated from allowed voltage droop divided by transient current.
Q3: Why include DC bias remaining percentage?
MLCC capacitance can drop significantly under DC voltage. A 10 µF X5R capacitor may provide only a fraction of its nominal capacitance in real operation.
Q4: Why use multiple capacitor values?
Different capacitance values and package sizes cover different frequency ranges. Small capacitors usually perform better at higher frequencies, while larger capacitors support lower-frequency transients.
Q5: Does this calculator include ESL and PCB layout?
No. This is a first-pass calculator. Real high-frequency decoupling performance depends strongly on ESL, mounting inductance, via placement, power plane design, and capacitor location.
Q6: Can this replace PDN simulation?
No. It is useful for early component estimation, but high-speed FPGA, ASIC, RF, and processor designs should be verified with PDN simulation or measurement.
