Buck converter calculator
Power stage values for a step-down converter in continuous conduction: duty cycle, inductance for a ripple target, peak current against the switch limit, output ripple and input capacitor RMS current.
- Duty cycle
- 30.6 %
- Ripple current with chosen L
- 532 mA 27 % of output current
- Peak inductor and switch current
- 2.266 A
- Maximum output current at switch limit
- 2.73 A
- Output ripple from capacitance
- 6.04 mV
- Output ripple from ESR
- 2.66 mV
- Output ripple, sum (worst case)
- 8.7 mV The two parts are out of phase, so the real peak to peak is lower.
- Input capacitor RMS current
- 921 mA
- Average input current
- 611 mA
How this is calculated
D = V_OUT / (V_IN(max) · η) ΔI_L = (V_IN(max) − V_OUT) · D / (f_S · L) L_min = V_OUT · (V_IN − V_OUT) / (ΔI_L · f_S · V_IN) I_SW(max) = I_OUT + ΔI_L / 2 I_MAXOUT = I_LIM(min) − ΔI_L / 2 ΔV_C = ΔI_L / (8 · f_S · C_OUT) ΔV_ESR = ESR · ΔI_L I_CIN(rms) = I_OUT · √( D · (1 − D) )
- Mode
- Continuous conduction only. Below I_OUT = ΔI_L / 2 the inductor current reaches zero (SLVA057 section 2.3) and the result is marked.
- Worst case
- Evaluated at the maximum input voltage and minimum switching frequency, where ripple and switch current are highest.
- Inductance and ripple
- Minimum inductance follows SLVA477 Equation 5, which has no efficiency term, while the ripple uses the efficiency-adjusted duty cycle from Equation 1. Choosing exactly the minimum inductance therefore gives a little more ripple than the target: 33 % for a 30 % target at the default inputs. Pick the next standard value up.
- Output ripple
- The capacitive and ESR parts are shown separately and summed. They are not in phase, so the sum overstates the real peak to peak.
- Revision note
- SLVA477C section 2 gives D = V_OUT / (V_IN · η); its Appendix A Equation 15 prints V_OUT · η / V_IN. The section 2 form, changed in revision B, is used here.
- TI SLVA477C, Basic Calculation of a Buck Converter's Power StageRevision C, revised September 2026, Equations 1 to 6, 12 and 13
- TI SLVA057, Understanding Buck Power Stages in Switchmode Power SuppliesMarch 1999, section 2.3
- TI SLTA055, Input and Output Capacitor SelectionFebruary 2006, example ceramic calculation (12 V to 3.3 V, duty 0.3)
- TI LM5177 datasheet, input capacitor RMS currentSNVSBU4F, January 2025, Equation 28
Questions
About 0.31 at 90 % efficiency: 3.3 / (12 × 0.9). TI SLTA055 rounds the same case to 0.3. The ideal lossless figure, 3.3 / 12, is 0.275.
TI SLVA477 suggests 20 to 40 % of the maximum output current. For 12 V to 3.3 V at 2 A and 500 kHz, 30 % ripple needs about 8 µH.
I_OUT · √(D(1 − D)), which peaks at half the output current when D is 0.5. The TI LM5177 datasheet gives 4.0 A for an 8 A output.
The inductor, switch and diode all carry I_OUT + ΔI_L / 2. If that exceeds the IC's minimum switch current limit, the converter current-limits before full load, so raise L or f_S or pick a larger part.