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Buck converter and inductor

A switching regulator that steps a high voltage (12–24 V) down to 3.3 V efficiently, using an inductor. Needed when an LDO would get too hot.

What it does#

A buck converter switches its input on and off very fast (1.1 MHz for the AP63203) into an inductor; the inductor and the output capacitors smooth that into a steady lower voltage. Efficiency is typically 85–90 %, so 12 V → 3.3 V at 500 mA wastes about 0.25 W instead of the LDO’s 4.4 W.

The AP63203WU is a fixed 3.3 V part: 3.8–32 V in, up to 2 A out, in TSOT-23-6.

Example circuit: 12 V in on J1. 10 µF + 100 nF at IN, EN tied to IN, 100 nF from BST to SW, 4.7 µH, two 22 µF on 3V3. FB goes straight to the output.
12 V in on J1. 10 µF + 100 nF at IN, EN tied to IN, 100 nF from BST to SW, 4.7 µH, two 22 µF on 3V3. FB goes straight to the output. Open the buck_ap63203 example from this page in PCB Canvas: “Insert into my schematic”.

Layout matters more than the schematic#

  • Input capacitor right at IN and GND: the hot loop. Keep it tiny.
  • SW node short and small: it is a fast square wave that radiates.
  • FB from the output capacitor, away from the inductor.
  • A solid ground pour under the whole block.

The inductor#

Pick the value from the datasheet (3.9–4.7 µH here) and a saturation current above the peak current (the catalog’s SWPA4030 4.7 µH is rated 2 A). A small 0805 inductor will saturate and overheat at these currents.

Pitfalls#

  • Ceramic input caps lose capacitance at 12 V+: use 25 V parts.
  • Hot-plugging 24 V through a long cable can ring to twice the voltage; 32 V max leaves little margin at 24 V. Add a TVS for 24 V inputs.
  • Light load: many bucks skip pulses at light load (audible or visible ripple); that is normal.

Footprint#

All parts are SMD and JLCPCB-assemblable (extended). Hand soldering TSOT-23-6 is fine; the inductor needs a hot iron.