Buck Converter Calculator — inductor, ripple and duty cycle

Duty cycle, inductor ripple current, output capacitor ripple and the CCM/DCM boundary for a step-down converter.

Example: Stepping 12 V down to 5 V at 2 A needs about 10.8 µH at 46% duty, with 2.3 A peak inductor current.

Inputs

Input voltage(V)

Output voltage(V)

Output current(A)

Switching frequency(Hz)

Inductor ripple ratio

0.3 means peak-to-peak ripple is 30% of output current.

Efficiency

Output capacitance (optional)(F)

22u is 22 µF.

Result

Inductor

10.8 µH

Duty cycle: 46.3%
Inductor ripple: 600 mA
Peak inductor current: 2.3 A
Average input current: 925.9 mA
Output cap RMS current: 173.2 mA
Power lost: 1.111 W
Output ripple (ideal C): 6.818 mV

Continuous conduction down to 300 mA of load. Below that the converter enters discontinuous mode.
6.818 mV of ripple assumes an ideal capacitor. Real ESR adds ΔI × ESR on top — at 600 mA ripple, even 20 mΩ of ESR contributes another 12 mV, which usually dominates.

Inductor value against switching frequency

Frequency	Inductance	Peak current
100 kHz	54.01 µH	2.3 A
250 kHz	21.6 µH	2.3 A
500 kHz	10.8 µH	2.3 A
1 MHz	5.401 µH	2.3 A
2 MHz	2.701 µH	2.3 A

Formula

D = V_out / (V_in × η)    ΔI_L = (V_in − V_out) × D / (L × f)

Worked example

Stepping 12 V down to 5 V at 2 A needs about 10.8 µH at 46% duty, with 2.3 A peak inductor current.

D = V_out / (V_in × η)
5 V / (12 V × 0.9)
46.3%
ΔI_L = ripple ratio × I_out
0.3 × 2 A
600 mA
L = (V_in − V_out) × D / (ΔI_L × f)
7 V × 0.463 / (600 mA × 500 kHz)
10.8 µH
I_peak = I_out + ΔI_L / 2
2 A + 300 mA
2.3 A
The inductor's saturation current rating must exceed this, not just its RMS rating.

Frequently asked questions

How do I choose the inductor for a buck converter?

Pick a ripple ratio first — 30% of output current is the usual starting point — then L = (V_in − V_out) × D / (ΔI × f). Smaller inductors mean more ripple, higher peak current and more capacitor RMS current; larger ones cost size and DC resistance.

What is the difference between saturation current and RMS current?

Saturation current is where the core stops being an inductor and inductance collapses; RMS is the thermal limit from winding resistance. Your peak current must stay below saturation, which is why the peak rather than the average is the number that matters when choosing a part.

What is discontinuous conduction mode?

When the inductor current falls to zero before the next cycle, which happens at light load. The transfer function changes — output voltage starts depending on load current — and the control loop behaves differently. Most converters do this at light load by design.

Why is my output ripple higher than calculated?

ESR. The ideal formula only accounts for charge moving in and out of the capacitor, but real capacitors have series resistance, and ΔI × ESR is often larger than the capacitive term. Ceramic capacitors have very low ESR; electrolytics do not.

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