The AP2112 is a 600 mA CMOS low-dropout regulator (LDO) from Diodes Incorporated with an enable pin, built for clean local or secondary rails in consumer electronics — the datasheet names laptops, LCD monitors, and portable DVD players as target applications. It ships in fixed output options of 1.2, 1.8, 2.5, 2.6, or 3.3 V with ±1.5% accuracy, runs from a 2.5–6.0 V input, and starts up in about 20 µs typical with no load. It's a general-purpose on/off-controlled rail LDO — not a wide-input buck converter, and not a nano-power part at 55 µA typical quiescent current. Available in SOT25, SOT89-5, and SO-8 packages, it's a common choice wherever a design needs a small, simple, enable-controlled linear rail.
Treat this like a structured bring-up: run the checks in order, before you ever power up and again after the board comes back. This post is that checklist — five phases, from "what is this part" to "why is my rail dead on the bench."
What the AP2112 Is
The AP2112 (Diodes Inc.) is a 600 mA CMOS LDO with an enable pin, aimed at clean local/secondary rails in consumer electronics — the datasheet names laptops, LCD monitors, and portable DVD players. Key facts:
- Fixed outputs: 1.2, 1.8, 2.5, 2.6, 3.3 V, with ±1.5% accuracy
- 600 mA min guaranteed output
- Input range 2.5–6.0 V (abs-max 6.5 V)
- PSRR 65 dB at 100 Hz and 1 kHz (at 100 mA load); 50 µV_RMS output noise (no load, 10 Hz–100 kHz)
- Packages: SOT25, SOT89-5, SO-8
- Enable with auto-discharge and fast 20 µs typ no-load startup
It's a general-purpose local rail LDO with on/off control — not a wide-input buck, and not a nano-power part (55 µA typ quiescent).
Check 1: Wiring It Up (Before Layout)
The baseline BOM is minimal:
- C_IN = 1.0 µF ceramic, C_OUT = 1.0 µF ceramic. Every electrical spec is characterized with exactly these values, and the typical applications circuit shows the same. No high-ESR output cap needed. Keep both close to their pins.
- EN pin: no external pull-down required — there's an internal 3.0 MΩ pull-down, so floating EN keeps the part off by default. Drive EN ≥ 1.5 V (VIH) to turn on; it's off below 0.4 V (VIL) and rated up to 6.0 V. A 3.3 V GPIO is fine. Add a pull-up only if you want default-ON and your control source can't hold the rail.
- Package choice: pick by thermal. θJA (free air, no heatsink) is 184 °C/W (SOT25), 114 °C/W (SO-8), 120 °C/W (SOT89-5); θJC is 96/75/47 °C/W respectively. Don't mix the two tables — use θJA for temperature-rise estimates, θJC for heatsink design. For high-power dissipation, SOT89-5 has the best θJC/case heat path (47 °C/W) — but only when heat flows to a case or heatsink. In free air, use θJA, where SO-8 is actually the lowest at 114 °C/W.
- Input protection: the abs-max VIN is 6.5 V against a 6.0 V recommended ceiling — only 0.5 V of margin. If your source can overshoot (hot-swap, charger ripple), add a TVS or clamp.
Check 2: Watch-Outs Before You Commit
Known limitation (datasheet self-contradiction): the dropout spec is a mess — read carefully. The EC table lists dropout as typ 1000 mV / max 1300 mV at 10, 300, and 600 mA — flat across load. But the datasheet's own characteristic curve shows dropout scaling from 0 to ~270 mV at 600 mA (~25 mV at 100 mA), and the front-page feature bullet claims 250 mV typ @ 600 mA. Two of three sources say dropout rises with current; the flat-1 V table row is likely a documentation error. Until you measure your own unit, size your VIN headroom conservatively to the table's worst case (max 1300 mV at full load) — designing to the ~250 mV curve/bullet before bench verification risks non-functional hardware if the real dropout is higher. Verify on the bench (see Check 3) before relaxing the headroom budget.
Foldback can block startup. The short-circuit limit is typ 50 mA at VOUT = 0, and the datasheet's application note explicitly warns: if a load is already present on the output before EN goes high (multiple/negative rails, pre-biased test fixtures), the foldback limit may prevent startup. The root cause: during power-up VOUT sits at 0 V, where foldback caps the regulator's current near 50 mA — so any pre-existing load that sinks more than that (or rails pulling the output toward ground/negative) stalls the ramp. Don't let a load draw from VOUT before enable.
EN-low means actively loaded, not open. A 60 Ω internal discharge resistor pulls VOUT to ground when disabled. Great for repeatable power-down sequencing; bad if another device sources into that rail while the part is off.
Thermal shutdown at 160 °C with 25 °C hysteresis. At 600 mA with a 1 V drop (0.6 W), a SOT25 in free air rises ~110 °C — uncomfortably close. Design for junction temperature, not ambient.
Don't overstate the noise/PSRR specs. The 50 µV_RMS noise figure is a no-load number; the 65 dB PSRR is only at 100 Hz/1 kHz and 100 mA load.
Battery life: standby with EN off is typ 0.01 µA / max 1.0 µA vs. 55 µA typ / 80 µA max quiescent when on. Drive EN low to shut down rather than idling.
Check 3: Bench Verification (After the Board Comes Back)
Setup: programmable supply on VIN, electronic load on VOUT, scope with current probe. Test in this order:
- Power-on/enable. EN low → off (standby ≤ 1.0 µA max). Float EN → confirm it stays off. Drive EN high → VOUT ramps in ~20 µs (typ, no load); confirm monotonic ramp and that the EN source never exceeds 6.0 V. On EN-low, confirm the 60 Ω active discharge pulls VOUT down.
- Accuracy. At 1–30 mA, a 3.3 V part must read 3.2505–3.3495 V. Re-check at your temperature extremes (±100 ppm/°C drift, −40 to +85 °C range).
- Regulation. Sweep load 1→600 mA (±1 %/A max — and confirm the rail actually reaches the guaranteed 600 mA). Sweep VIN 2.5–6 V (±0.1 %/V max).
- Dropout — the critical test. At fixed load, lower VIN until VOUT drops ~2%. Record VIN−VOUT at light, medium, and full load. This is how you resolve the datasheet's internal contradiction for your unit — and confirm VIN stays above VOUT + dropout under worst-case sag.
- Short/foldback. Verify ~50 mA into a dead short and recovery after removal. Then the startup-under-preload test: apply a load before raising EN and confirm the part still starts. If your system can load or bias VOUT before enable, this test catches the failure.
- Thermal. Worst-case VIN/load/ambient, thermocouple on the part. Confirm Tj = TA + P·θJA stays well below the 160 °C shutdown.
- Noise/PSRR (only if feeding sensitive analog): ~65 dB rejection at 100 Hz/1 kHz at 100 mA; 50 µV_RMS at no load with an LNA.
- Abs-max/ESD. Confirm worst-case VIN overshoot never touches 6.5 V. The part is rated 4 kV HBM / 400 V MM — run unit ESD if the rail is connector-exposed.
Check 4: Troubleshooting — Symptom Triage
No output at all:
- Measure EN at the pin (not at the GPIO): it must exceed 1.5 V. A floating or weakly driven EN sits behind the 3 MΩ pull-down and stays off.
- VIN below ~2.5 V under load — measure with the part actually loaded.
- Hot board: 160 °C shutdown with 25 °C hysteresis — let it cool and re-test.
Output low or sags under load:
- Headroom. Measure VIN under load. If the table's flat dropout applies, a 3.3 V rail needs VIN ≥ ~4.6 V even at light load; if the curve applies, the requirement scales with current. Either way, VIN sag is the most common cause.
- Load above 600 mA — the guaranteed max.
Stuck off even with EN high:
- Foldback startup lockout — during power-up VOUT is at 0 V, where foldback caps current at ~50 mA. Any pre-existing load sinking more than that (another rail, a biased fixture, or a rail pulling VOUT toward ground/negative) traps the regulator in foldback. Sequence EN only after the pre-existing load is removed.
Rail won't stay up when disabled / discharges fast:
- That's the 60 Ω discharge working as designed. If something else sources into the net while off, it's fighting that 60 Ω load.
Excess off-state current:
- EN isn't actually below 0.4 V — check the driver pulls the pin low rather than relying on the 3 MΩ pull-down with a leaky source. If intentionally on, expect 55–80 µA quiescent, not nano-amps.
Accuracy out of spec:
- Check temperature (±100 ppm/°C), line (±0.1 %/V), and load (±1 %/A) conditions before blaming the part.
Final Checklist
Quick triage order: ① EN at the pin ≥ 1.5 V → ② VIN under load ≥ VOUT + dropout → ③ load ≤ 600 mA → ④ temperature below shutdown → ⑤ no pre-existing load on VOUT before enable.
The two items most likely to bite: the dropout discrepancy (measure it yourself at all load points, and size headroom to the 1300 mV worst case until you have bench data) and the foldback startup lockout with a pre-biased output. Test both explicitly before you ship.
Part page: AP2112.