Decoupling Capacitor Calculator Formula
Understand the math behind the decoupling capacitor calculator. Each variable explained with a worked example.
Formulas Used
Minimum Capacitance
min_cap_uf = transient_current_a * duration_s / droop_v * 1000000Minimum Capacitance (nF)
min_cap_nf = transient_current_a * duration_s / droop_v * 1000000000Charge Needed
charge_uc = transient_current_a * duration_s * 1000000Variables
| Variable | Description | Default |
|---|---|---|
transient_current_a | Transient Current(A) | 0.5 |
transient_duration_us | Transient Duration(µs) | 10 |
max_droop_mv | Maximum Voltage Droop(mV) | 100 |
duration_s | Derived value= transient_duration_us / 1000000 | calculated |
droop_v | Derived value= max_droop_mv / 1000 | calculated |
How It Works
Decoupling Capacitor Sizing
Formula
C = I x dt / dV
Where I is the transient current, dt is the duration of the transient, and dV is the maximum allowed voltage droop. Place decoupling caps as close to the IC power pins as possible.
Worked Example
0.5 A transient for 10 us, maximum 100 mV droop.
- 01dt = 10 / 1000000 = 0.00001 s
- 02dV = 100 / 1000 = 0.1 V
- 03C = 0.5 x 0.00001 / 0.1 = 0.00005 F = 50 uF
Frequently Asked Questions
Why use multiple capacitor values?
Different capacitor sizes cover different frequency ranges. Use 100nF for high frequency, 10uF for low frequency, together for broadband decoupling.
Does ESR matter?
Yes. Capacitor ESR limits high-frequency effectiveness. Ceramic caps have much lower ESR than electrolytic.
How close to the IC?
As close as possible. Every millimeter of trace adds inductance that degrades decoupling at high frequencies.
Ready to run the numbers?
Open Decoupling Capacitor Calculator