Buck/Boost Converter Calculator
Duty cycle, inductor, and ripple sizing
Required Parameters
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Quick Answer
Buck duty cycle: D = Vout/Vin. Boost duty cycle: D = 1 - Vin/Vout. Minimum inductance: L = Vout(1-D)/(2·f·ΔIL).
Design Notes
Switching converters are far more efficient than LDOs (85-95% typical). The inductor must handle peak current without saturating. Output capacitor ESR dominates ripple voltage. Use ceramic caps for low ESR. Higher switching frequency allows smaller inductors but increases switching losses.
Common Mistakes
- 1
Using ideal duty cycle without accounting for diode/MOSFET drops and DCR losses.
- 2
Selecting an inductor based only on inductance — saturation current rating is equally critical.
- 3
Ignoring input capacitor requirements — buck converters draw pulsed current that causes input ripple.
Knowledge Base
What is a buck converter?
A buck (step-down) converter efficiently converts a higher DC voltage to a lower one using a switching transistor, inductor, and diode/synchronous FET. Duty cycle D = Vout/Vin. Unlike LDOs which waste excess voltage as heat, buck converters achieve 85-95% efficiency by rapidly switching and storing energy in the inductor.
What is a boost converter?
A boost (step-up) converter increases DC voltage using the same basic topology rearranged. Duty cycle D = 1 - Vin/Vout. The inductor stores energy when the switch is on and releases it at higher voltage when off. Common use: 3.7V Li-ion to 5V USB output. Efficiency typically 80-90%.
How do I choose the right inductor?
Key specs: (1) Inductance: use L = Vout(1-D)/(ΔIL×fsw) for buck. (2) Saturation current: must exceed peak inductor current by 20%. (3) DCR (DC resistance): lower = less loss. (4) Size: higher inductance or current = larger package. Typical values: 4.7µH-22µH for 500kHz-2MHz converters.
What switching frequency should I use?
Higher frequency = smaller inductor/capacitors but more switching losses. Common ranges: 100-300kHz (older/high power), 500kHz-1MHz (most designs), 2-3MHz (compact/mobile). Above 2MHz, switching losses become significant. Match your frequency to the converter IC's recommended range.
What causes output voltage ripple?
Two components: (1) ESR ripple from capacitor's equivalent series resistance (V_ripple_ESR = ΔIL × ESR). (2) Capacitive ripple from charge/discharge (V_ripple_C = ΔIL / (8×f×C)). Use multiple low-ESR MLCC ceramics in parallel. Typical target: <1% of Vout for digital loads, <0.1% for analog/RF.
What are popular buck/boost converter ICs?
Buck: MP2315 (3A, 500kHz), TPS54331 (3A, 570kHz), LM2596 (3A, 150kHz, classic). Boost: MT3608 (2A, 1.2MHz, cheap), TPS61023 (1A, ultra-low input). Buck-Boost: TPS63000 (1.2A, Li-ion to 3.3V), LTC3780 (high power). Modules: LM2596 boards and MP1584 boards are popular for prototyping.
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