Buck & Boost Converter Calculator

About the Buck & Boost Converter Calculator

Work out the power stage of a switching converter the way the Texas Instruments application notes do. Choose a buck (step-down), a boost (step-up) or an inverting buck-boost (a negative output), type the input range, the output, the current, the switching frequency and the efficiency, and the calculator gives the duty cycle at the worst-case input, the inductor for your ripple-current target (rounded up to a standard value) and the ripple and peak switch current it gives, with the inductor’s RMS and saturation current.

It then sizes the output capacitor for a ripple target with its ESR (and the largest ESR the target allows), estimates the input capacitor and the RMS current it carries, gives the load below which the converter leaves continuous conduction (CCM) for DCM, the diode’s current, loss and reverse voltage, the switch voltage, and — with the IC’s switch current limit — the most output current it can deliver. A table shows the duty cycle, ripple and peak current across the input range, and a chart the inductor current over two switching periods.

Without a pass, the free preview works on your own values and shows the verdict of every check, the names of the table’s first rows and the chart under a watermark, without its numbers; the inductor, the currents, the capacitors, every other figure, the working, Copy result and the CSV come with a Pro pass.

Duty cycle and the worst case

• Buck: D = Vout ÷ (Vin × η). The ripple and the peak current are largest at the highest input, so that is where TI calculates them (SLVA477C).
• Boost: D = 1 − Vin × η ÷ Vout. The duty cycle and the currents are largest at the lowest input (SLVA372D).
• Inverting buck-boost: D = (|Vout| + VF) ÷ (|Vout| + VF + Vin), again worst at the lowest input (SLVA721A).
• The efficiency η goes into the buck and boost duty cycles because the converter must also supply its own losses.

The inductor

TI estimates the ripple current as 20–40% of the output current (of Iout × Vout ÷ Vin for a boost, of the average inductor current Iout ÷ (1 − D) for the inverting stage) and sizes the buck and boost inductors at the typical input, the inverting one at the lowest: buck L = Vout × (Vin − Vout) ÷ (ΔIL × fs × Vin), boost L = Vin × (Vout − Vin) ÷ (ΔIL × fs × Vout), inverting L = Vin(min) × D ÷ (ΔIL × fs). The calculator rounds up to the next standard value, so the ripple stays at or below the target, and then works out the real ripple and the peak current ISW = IL(average) + ΔIL ÷ 2 that the inductor, the switch and the diode must take. Choose an inductor whose saturation current is above the peak (TI asks for 20% more for the inverting converter); a larger inductor lowers the peak and raises the current an IC can deliver.

Capacitors

• Buck output: Cout ≥ ΔIL ÷ (8 × fs × ΔVout), and the ESR adds ESR × ΔIL; the calculator sizes C for what is left of the target after the ESR.
• Boost and inverting output: the output capacitor supplies the whole load while the switch is on: Cout ≥ Iout × D ÷ (fs × ΔVout), with an ESR term of ESR × (Iout ÷ (1 − D) + ΔIL ÷ 2).
• Buck input: Cin ≥ Iout × D × (1 − D) ÷ (fs × ΔVin), carrying Iout × √(D(1 − D)) RMS, half the output current at 50% duty (TI SLTA055).
• Boost input sees only the inductor’s ripple; inverting input: CI ≥ IL × D ÷ (fs × (ΔVin − ΔIL × ESR)).
• Ceramic capacitors lose much of their capacitance under DC bias: use X5R or better and the maker’s data at your voltage.

Continuous and discontinuous conduction

The equations hold while the inductor current never falls to zero (continuous conduction, CCM). At light load it does, and the converter enters discontinuous conduction (DCM): for a buck below Iout = ΔIL ÷ 2, for boost and inverting converters below Iout = ΔIL × (1 − D) ÷ 2 (SLVA721A, equation 13). Synchronous converters often keep CCM by letting the current go negative; check the IC’s light-load mode.

Sources

• Texas Instruments SLVA477C, Basic Calculation of a Buck Converter’s Power Stage
• Texas Instruments SLVA372D, Basic Calculation of a Boost Converter’s Power Stage
• Texas Instruments SLVA721A, Basic Calculation of an Inverting Buck-Boost Power Stage
• Texas Instruments SLTA055, Input and Output Capacitor Selection

Limitations

• The results are a first sizing in continuous conduction from the efficiency you enter; switching losses, the inductor’s winding resistance and the control loop (compensation and stability) are not modelled.
• Internally compensated ICs need the inductor and output capacitor values their datasheet recommends; use the calculator to check the currents and ripple with those values.
• Load transients usually need more output capacitance than the steady-state ripple does.
• Layout, EMI and the IC’s own limits (maximum duty cycle, minimum on-time, thermal limits) must be checked in its datasheet.

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