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Q: Is the AMS1117 tab connected to ground?

No — the tab is VOUT. The datasheet prints TAB IS OUT precisely because designers assume ground. Tying the tab to the ground plane shorts the output. Solder it to the output copper pour, where it also sheds heat.

Q: Can I use an AMS1117-ADJ where a fixed version was?

Only with a divider. On the adjustable part, pin 1 is the feedback node: Vout = 1.25 × (1 + R2/R1), R1 from VOUT to ADJ, R2 from ADJ to ground. Keep R1 at 120Ω or less — the divider doubles as the ~10mA minimum load.

Q: Why does my AMS1117 oscillate with a ceramic output capacitor?

The control loop expects 0.1–10Ω of capacitor ESR. Ceramics measure milliohms — effectively no ESR — so the loop loses its compensation and the rail becomes a 100–500kHz sawtooth. Use 10–22µF tantalum or electrolytic, or add 0.5–2Ω in series with a ceramic.

Q: Why does my AMS1117 get hot on a 12V rail?

Linear math: Pd = (VIN − VOUT) × Iload. At 12V in, 3.3V out, 500mA, that’s 4.35W — the SOT-223 handles roughly 1.2W bare and ~2W with generous copper. Either drop the input with a pre-regulator or lighten the load.

Q: What’s the minimum input voltage for AMS1117-3.3?

About 4.5V at light load, 4.6V at full 1A. Regulation needs VOUT plus dropout — 1.3V max at 1A per the datasheet, less at lower current. A 3.7V LiPo is already below that; a sub-300mV-dropout LDO is the right tool there.

Q: Can I swap AMS1117 into a TO-252 footprint from SOT-223?

Not without a layout change. Same pinout order and same tab = VOUT, but the packages differ physically. The TO-252’s larger tab is the upgrade when thermals push the SOT-223 out — it is a board change, not a drop-in.

Q: Which fixed voltages does the AMS1117 come in?

1.5V, 1.8V, 2.5V, 2.85V, 3.3V, and 5.0V in the standard set per the datasheet, plus an adjustable version covering 1.25V to 13.8V with a divider. The pinout never changes between them — only pin 1’s meaning flips on the ADJ part.

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AMS1117 Pinout Guide — SOT-223, Fixed & Adjustable

2026/8/13 13:54:03

AMS1117 Pinout Guide — SOT-223, Fixed & Adjustable | ICMASS

The AMS1117 is a 1A low-dropout linear regulator with three pins plus a tab — and two of those four pieces of metal catch people: pin 1 changes meaning between fixed and adjustable versions, and the tab is VOUT, not ground.

This guide covers the pinout, the standard circuit, and the layout and capacitor rules that keep a 3.3V rail from becoming a 100–500kHz oscillator.

The SOT-223: pins 1-2-3 left to right, tab on the wide end. The tab carries VOUT — the datasheet spells it out because everyone assumes it’s ground.

AMS1117 Pinout and Pin Functions

Pin	Name	Type	Description
1	GND (fixed) / ADJ (adjustable)	Reference	Fixed versions (AMS1117-3.3, -5.0): tie straight to the ground plane. Adjustable version: feedback node — Vout = 1.25 × (1 + R2/R1), with R1 from VOUT to ADJ and R2 from ADJ to ground.
2	VOUT	Output	Regulated output. The output capacitor lives here — 10–22µF tantalum, whose ESR the control loop needs for stability.
3	VIN	Input	Unregulated input, 4.5V to 12V. A 10µF input capacitor sits within millimetres of this pin.
Tab	VOUT	Output / heatsink	The metal tab is electrically VOUT and thermally the heatsink. Solder it to copper pour — never to ground.

Fixed vs adjustable is the whole confusion. Buy an AMS1117-ADJ and treat pin 1 as ground, and the output hangs at the 1.25V reference. Buy a fixed 3.3V part and hook a resistor divider to pin 1, and you fight the internal divider.

Read the suffix, then read pin 1.

Typical Application Circuit

The whole circuit in one picture: input cap at pin 3, output tantalum at pin 2, pin 1 to ground (fixed) or to a divider (adjustable). Note R1 ≤ 120Ω on the ADJ version — the reference needs about 10mA of divider current.

The output capacitor is not optional. The 1117 family is an older bipolar design: the control loop expects the output capacitor’s ESR between 0.1Ω and 10Ω. Ceramics measure milliohms — no ESR, no compensation — and the rail oscillates. Use 10–22µF tantalum; with ceramic, add 0.5–2Ω in series.

The adjustable version adds one rule: the divider sets the output and doubles as the minimum load — around 10mA. With R1 at 120Ω the reference’s 1.25V pulls 10.4mA through the divider, which satisfies it. Skip that load? The unloaded output drifts up.

PCB Layout Tips

Give the tab real copper. The SOT-223 sheds heat through the tab, which is VOUT — tie the pour to the output net, never ground. How much is enough? At least a square inch of 1oz copper for light loads; near 1A, pour both sides and stitch with vias.

Input cap within millimetres of pin 3. The AMS1117’s PSRR assumes the input rail stays stiff at the pin. A long trace adds inductance, and load transients turn into input dips that look exactly like a failing regulator.

Output cap next to pin 2, with the ESR in mind. The loop stability relies on that capacitor being close. And since the tab is VOUT, the output pour does double duty: power plane and heatsink. Don’t let an unconnected tab pad become a cold solder joint.

Dissipation at 500mA Load — Pd = (VIN − 3.3) × 0.5

5V input

0.85W

9V input

2.85W

12V input

4.35W

Red line ≈ 1.5W — the SOT-223’s practical heat budget (≈1.2W bare, ~2W with generous copper pour). Past it, thermal shutdown (typ 165°C) answers for you.

Frequently Asked Questions

Q1: What is the pinout of the AMS1117?

A: SOT-223: pin 1 = GND (fixed) or ADJ (adjustable), pin 2 = VOUT, pin 3 = VIN, and the tab = VOUT. Pins count left to right with the tab on the wide end. TO-252 keeps the same 1-2-3 order.

Q2: Is the AMS1117 tab connected to ground?

A: No — the tab is VOUT. The datasheet prints "TAB IS OUT" precisely because designers assume ground. Tying the tab to the ground plane shorts the output. Solder it to the output copper pour, where it also sheds heat.

Q3: Can I use an AMS1117-ADJ where a fixed version was?

A: Only with a divider. On the adjustable part, pin 1 is the feedback node: Vout = 1.25 × (1 + R2/R1), R1 from VOUT to ADJ, R2 from ADJ to ground. Keep R1 at 120Ω or less — the divider doubles as the ~10mA minimum load.

Q4: Why does my AMS1117 oscillate with a ceramic output capacitor?

A: The control loop expects 0.1–10Ω of capacitor ESR. Ceramics measure milliohms — effectively no ESR — so the loop loses its compensation and the rail becomes a 100–500kHz sawtooth. Use 10–22µF tantalum or electrolytic, or add 0.5–2Ω in series with a ceramic.

Q5: Why does my AMS1117 get hot on a 12V rail?

A: Linear math: Pd = (VIN − VOUT) × Iload. At 12V in, 3.3V out, 500mA, that’s 4.35W — the SOT-223 handles roughly 1.2W bare and ~2W with generous copper. Either drop the input with a pre-regulator or lighten the load.

Q6: What’s the minimum input voltage for AMS1117-3.3?

A: About 4.5V at light load, 4.6V at full 1A. Regulation needs VOUT plus dropout — 1.3V max at 1A per the datasheet, less at lower current. A 3.7V LiPo is already below that; a sub-300mV-dropout LDO is the right tool there.

Q7: Can I swap AMS1117 into a TO-252 footprint from SOT-223?

A: Not without a layout change. Same pinout order and same tab = VOUT, but the packages differ physically. The TO-252’s larger tab is the upgrade when thermals push the SOT-223 out — it is a board change, not a drop-in.

Q8: Which fixed voltages does the AMS1117 come in?

A: 1.5V, 1.8V, 2.5V, 2.85V, 3.3V, and 5.0V in the standard set per the datasheet, plus an adjustable version covering 1.25V to 13.8V with a divider. The pinout never changes between them — only pin 1’s meaning flips on the ADJ part.