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LDO voltage regulator

A linear regulator turns a higher voltage (USB 5 V, a LiPo) into a steady 3.3 V. Simple and quiet; the difference is burnt as heat.

What it does#

An LDO (“low dropout”) keeps its output at 3.3 V as long as the input is a little higher: the dropout voltage. Everything above that is burnt: P = (V_in − 3.3 V) × I. From USB at 200 mA that is 0.34 W, fine for a SOT-23-5. From 12 V at 500 mA it would be 4.4 W: use a buck converter instead.

The catalog’s choices#

Part Current Dropout Quiescent Use
ME6211C33 (SOT-23-5) 500 mA 120 mV at 100 mA, 260 mV at 200 mA 30 µA USB-powered boards
AP2112K-3.3 (SOT-23-5) 600 mA 250 mV at 600 mA 55 µA same pinout
RT9080-33 (TSOT-23-5) 600 mA 310 mV at 600 mA 2 µA battery boards
AMS1117-3.3 (SOT-223) 1 A ~1.1 V ~5 mA 5 V only; not for LiPo

All three small ones share a pinout: 1 VIN, 2 GND, 3 EN (CE on the ME6211, active high), 4 not connected, 5 VOUT. Dropout and quiescent figures are typical values at 25 °C.

Example circuit: VBUS in, 3V3 out. EN tied to VIN keeps it always on. 1 µF on each side, plus 10 µF bulk on 3V3 (an ESP32 wants it).
VBUS in, 3V3 out. EN tied to VIN keeps it always on. 1 µF on each side, plus 10 µF bulk on 3V3 (an ESP32 wants it). Open the ldo_3v3 example from this page in PCB Canvas: “Insert into my schematic”.

Battery boards#

A LiPo runs from 4.2 V down to ~3.3 V. With 0.25 V dropout the 3.3 V rail starts sagging below ~3.55 V of battery. The RT9080’s 2 µA quiescent current matters when the board sleeps for months; AP2112’s 55 µA alone would drain a 1000 mAh cell in about two years.

Pitfalls#

  • EN must not float. Tie it to VIN or drive it from a GPIO/switch.
  • Output capacitor: these parts are stable with ceramics (1 µF X5R/X7R in the datasheets); some AMS1117 clones want a capacitor with some ESR (tantalum) on the output.
  • ESP32 Wi-Fi peaks need ≥ 500 mA: ME6211 is the minimum, add bulk capacitance.

Footprint#

SOT-23-5 is hand-solderable; JLCPCB parts in the catalog (all extended except AMS1117, which is basic).