AP2112K-3.3TRG1 PCB design guide and datasheet

AP2112K-3.3TRG1 is the exact tape-and-reel SOT25 orderable for the fixed 3.3 V AP2112 CMOS LDO. It fills the modern low-dropout lane that the 1117 family does not: 600 mA minimum output capability, 250 mV typical / 400 mV maximum dropout at 600 mA, 55 uA typical quiescent current, enable control, and stability with local 1.0 uF ceramic capacitors.

The part is useful when a 3.3 V rail must follow a single-cell or low-headroom source, but the headline dropout is not universal. It changes with load, and a linear regulator still burns (VIN - VOUT) x IOUT as heat. The SOT25 package is 184 C/W junction-to-ambient without a heatsink, so high input voltage and full load can make the thermal limit arrive well before the electrical current rating.

The exact orderable matters. AP2112K-3.3TRG1 uses the SOT25 pin map VIN, GND, EN, NC, VOUT; it actively discharges VOUT through about 60 ohm when disabled and does not provide reverse protection. Those details turn a sensible part choice into a sequencing or footprint failure if they are treated as generic LDO behavior.

What breaks boards

1. 01

   Dropout is load-dependent, not a blanket Li-ion guarantee

   For the 3.3 V grade, dropout is 250 mV typical and 400 mV maximum at 600 mA, but only 125 mV typical and 200 mV maximum at 300 mA. A Li-ion rail must stay above 3.3 V plus the worst-case dropout at the real load and temperature; the family's 2.5 V recommended input floor does not let the 3.3 V version regulate from 2.5 V.

2. 02

   Use the exact SOT25 pinout for the K orderable

   AP2112K-3.3TRG1 is the SOT25 / SOT-23-5 orderable: pin 1 VIN, pin 2 GND, pin 3 EN, pin 4 NC, and pin 5 VOUT. Do not reuse a generic five-pin LDO footprint without checking the pin map; regulator pinouts are not interchangeable.

3. 03

   The characterized loop uses local 1.0 uF ceramic capacitors

   The 3.3 V table is characterized with 1.0 uF ceramic capacitors at both input and output, and Diodes recommends X7R or X5R when ceramic is used. Place both at the pins and account for tolerance and DC-bias loss; remote bulk capacitance does not replace the local loop.

4. 04

   Disabled output is actively discharged, not high impedance

   When EN is low, an internal 60 ohm typical discharge path pulls VOUT toward ground. That is useful for sequencing but can drain or contend with a downstream rail driven from another source. The manufacturer product table also says reverse protection is not provided, so do not infer bidirectional isolation.

5. 05

   The 600 mA rating still needs linear-regulator thermal math

   The SOT25 junction-to-ambient figure is 184 C/W without a heatsink. Dissipation is approximately (VIN - VOUT) x IOUT plus quiescent loss, so 600 mA is not thermally available at every input voltage and ambient. Use copper, airflow, and measured board temperature to keep junction temperature inside the design limit.

Key specifications

Verified against the manufacturer datasheet on 2026-07-13. Confirm the current revision before production use.

Alternatives

• AMS1117-3.3: Higher-current SOT-223 alternative for solid 5 V inputs, but with about 1.1 V typical dropout, 5 mA typical quiescent current, and a different capacitor/thermal problem.
• MCP1700-3302E: Much lower quiescent current for sleeping battery systems, but only 250 mA output capability and no enable pin.
• LP2985-33: A 150 mA low-noise LDO option when the load is smaller and TI sourcing or noise behavior is preferred.

Common questions

Can AP2112K-3.3 regulate a Li-ion cell to 3.3 V?

Only while the cell remains above 3.3 V plus dropout at the actual load. At 600 mA the 3.3 V grade is specified at 250 mV typical and 400 mV maximum dropout, so regulation is lost before the cell reaches the bottom of its discharge curve.

What capacitors does AP2112K-3.3 need?

The 3.3 V electrical table uses 1.0 uF ceramic capacitors on input and output, and the typical-circuit note recommends X7R or X5R ceramic. Put both at the pins and account for tolerance and DC-bias derating.

What happens to VOUT when EN is low?

The output is actively discharged through a 60 ohm typical internal path; it is not left high impedance. Check that behavior against any downstream backup source or sequencing requirement.

Is AP2112K-3.3TRG1 pin-compatible with other SOT-23-5 LDOs?

Do not assume so. This orderable maps pin 1 VIN, 2 GND, 3 EN, 4 NC, and 5 VOUT. Compare the exact footprint and pin map before substituting any five-pin regulator.

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