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Buck converter power stage

Size the inductor and output capacitor of a continuous-conduction buck converter and inspect its triangular inductor current.

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Published 28. October 2026 · version 1

How it is calculated

The duty-cycle estimate is $D = V_{out}/(V_{in}\eta)$ . Following TI SLVA477B, the ripple target is $\Delta I_L=rI_{out}$ , the minimum inductance is $L=V_{out}(V_{in}-V_{out})/(\Delta I_L f_{sw}V_{in})$ , and the ideal-capacitor ripple estimate is $C_{out}=\Delta I_L/(8f_{sw}\Delta V_{out})$ .

Assumptions

Ideal buck power stage in continuous conduction with a triangular inductor-current ripple; constant input and output voltage; efficiency used only in the duty-cycle estimate; capacitor ESR, DC bias, tolerance, inductor saturation, switching losses, control-loop stability, and load-transient requirements are not included.

Where it applies

Use for a diode-rectified or synchronous step-down stage when $0<D<1$ and the operating load is above $\Delta I_L/2$ . Enter the worst case, as SLVA477B does: the maximum input voltage, the minimum switching frequency of the controller and the maximum output current. The output capacitor estimate is for externally compensated converters; an internally compensated controller needs the inductor and capacitor its data sheet recommends. Select practical components with margin above the calculated minima and verify them against converter and component data sheets.

References

Texas Instruments application report SLVA477B, Basic Calculation of a Buck Converter's Power Stage, revised August 2015, https://www.ti.com/lit/an/slva477b/slva477b.pdf

Inputs

Input	Symbol	Unit	Default
Input voltage (the maximum: worst case for L and the switch current)	`Vin`	V	12
Output voltage	`Vout`	V	5
Output current (maximum)	`Iout`	A	3
Switching frequency (the minimum the controller guarantees)	`fsw`	kHz	500
Inductor ripple ratio ΔIL/Iout	`ripple_pct`	%	30
Allowed output voltage ripple ΔVout	`ripple_v`	mV	20
Efficiency estimate η	`eta_pct`	%	90

Results

With the default inputs above, the calc gives:

Result	Value
Duty cycle	0.463
Minimum inductance	6.481 uH
Peak switch current	3.45 A
Minimum output capacitance	11.25 uF
Inductor RMS current	3.011 A
CCM/DCM boundary load current	0.45 A

Checked against sources

Each case below was calculated by this calc and agrees with the value from its source (within the tolerance where one is shown).

12 V to 5 V, 3 A at 500 kHz

Source: Hand calculation: D = 5/(12·0.90) = 0.462963; ΔIL = 3·0.30 = 0.9 A; L = 5·(12−5)/(0.9·500000·12) = 6.48148 µH; C = 0.9/(8·500000·0.020) = 11.25 µF; Ipk = 3 + 0.9/2 = 3.45 A.

Inputs: the default inputs above.

Result	Source	This calc
Duty cycle	0.463	0.463
Minimum inductance	6.481 uH	6.481 uH
Minimum output capacitance	11.25 uF	11.25 uF
Peak switch current	3.45 A	3.45 A

24 V to 12 V, 5 A at 400 kHz

Source: Hand calculation: D = 12/(24·0.95) = 0.526316; ΔIL = 5·0.20 = 1 A; L = 12·(24−12)/(1·400000·24) = 15 µH; C = 1/(8·400000·0.030) = 10.4167 µF; IL,rms = √(5² + 1²/12) = 5.00833 A; boundary current = 1/2 = 0.5 A.

Inputs: Input voltage (the maximum: worst case for L and the switch current) = 24 V; Output voltage = 12 V; Output current (maximum) = 5 A; Switching frequency (the minimum the controller guarantees) = 400 kHz; Inductor ripple ratio ΔIL/Iout = 20 %; Allowed output voltage ripple ΔVout = 30 mV; Efficiency estimate η = 95 %; the others as above.

Result	Source	This calc
Duty cycle	0.5263	0.5263
Minimum inductance	15 uH	15 uH
Minimum output capacitance	10.417 uF	10.417 uF
Inductor RMS current	5.008 A	5.008 A
CCM/DCM boundary load current	0.5 A	0.5 A