Chris Collins

Tools · electronics

Power supply design

Size the inductor, capacitors, and currents for buck, boost, and inverting buck-boost converters, check linear regulator heat, and budget power, injection, and wiring for WS2812/NeoPixel LED strips.

Added

How to use it

Values accept engineering notation: 500k, 4u7, 10m, 3V3.

  • Buck (step-down), following TI application note SLVA477B:

    • Duty cycle D = Vout/(Vin·η).

    • Inductor ripple ΔI = ripple × Iout.

    • L = Vout·(Vin,max − Vout)/(ΔI·f·Vin,max).

    • Peak current Ipk = Iout + ΔI/2. The inductor’s saturation rating must exceed this.

    • Output capacitance C = ΔI/(8·f·ΔV), with the maximum ESR for the same ripple, ΔV/ΔI.

    • Input capacitance C = Iout·D·(1 − D)/(f·ΔVin).

  • Boost (step-up), following TI application note SLVA372C:

    • D = 1 − Vin,min·η/Vout.

    • ΔI = ripple × Iout·Vout/Vin.

    • L = Vin·(Vout − Vin)/(ΔI·f·Vout).

    • Ipk = Iout/(1 − D) + ΔI/2. This must stay under the converter’s switch current limit.

    • Cout = Iout·D/(f·ΔV).

  • Inverting buck-boost:

    • D = |Vout|/(Vin·η + |Vout|).

    • Average inductor current Iout/(1 − D).

    • The switch and diode see Vin + |Vout|.

  • Linear regulator:

    • Dissipation P = (Vin − Vout)·I + Vin·Iq.

    • Junction temperature Tj = Ta + P·θJA.

    • The tab compares the loss with a switching module at a stated efficiency, and warns when the headroom is below dropout.

  • LED strip power:

    • Current per LED is its full-white current (60 mA for WS2812B), scaled by brightness and colour mix, plus about 1 mA idle. Full white on 300 LEDs is 18.3 A.

    • The supply recommendation adds 25% headroom.

    • Voltage drop: a uniformly loaded strip fed from one end drops R′·i·L² across both rails, where R′ is the resistance per metre of one rail and i is the current per metre. Feeding from both ends quarters the drop. The tab gives the longest run per feed point for your allowed drop, the number of injection points, and the copper gauge for the feed wires.

The LED tab also covers the data line. WS2812 needs a logic high of 0.7 × VDD: 3.5 V at 5 V. So a 3.3 V ESP32, RP2040, or Pi output is out of specification. Use a 74AHCT125 or 74HCT245 level shifter.

The converter results are a starting point. Integrated converter ICs often fix the frequency and recommend specific inductors — follow the datasheet where it differs.

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