AMS1117 regulator: pinout, dropout, the output capacitor and heat

The number after the dash is the output: AMS1117-1.2, -1.5, -1.8, -2.5, -3.3 and -5.0 are fixed, and AMS1117-ADJ sets its voltage with two resistors. UMW prints the full name on the SOT-223, for example AMS1117-3.3 over a UMW date code; on the smaller SOT-89 it shortens to 1117-3.3, and the TO-252 version is the AMS1117CD.

Many makers sell an AMS1117, and their sheets are not word for word the same. The two read for this page agree on the pins and the output limits and differ on input voltage, thermal resistance and the capacitor advice. Where they differ, this page designs to the less generous figure.

Both sheets put the absolute maximum input at 18 V. UMW recommends no more than 12 V in normal use, SLKOR allows 18 V; take 12 V as the ceiling. SLKOR also limits the SOT-223 to 600 mA and 750 mW, which is a better guide to the real part than the 1 A in either title.

The thermal resistance is the number that matters most. UMW gives 120 °C/W from junction to air, SLKOR 135 °C/W. Design to 135 °C/W: with a 125 °C junction limit and 35 °C air, the package can lose 670 mW and no more.

Dropout is the smallest input-to-output difference that still holds the output. UMW gives at most 1.20 V at 100 mA, 1.25 V at 500 mA and 1.30 V at 1 A; SLKOR quotes only 1 A, at 1.40 V. So a 3.3 V part needs its input at 4.55 V or more for a few hundred milliamps, and 4.70 V at a full amp.

The output capacitor is where the two sheets differ most in wording. UMW says the part needs an output capacitor for stability and that 22 µF tantalum "covers all cases"; its application circuit shows 10 µF in and 22 µF out. SLKOR asks for at least 10 µF tantalum or 50 µF aluminium electrolytic, with an ESR of no more than 0.5 Ω. Neither sheet sets a lower ESR limit or says anything about ceramic capacitors.

A 22 µF tantalum on the output and 10 µF on the input satisfies both. If a board uses only ceramics, which the sheets neither allow nor forbid, check the output on a scope while switching the load on and off before you rely on it. The tantalum capacitors and ESR lessons explain what that ESR figure means.

The 3.3 V version is the one on ESP32 boards. Its output is held between 3.234 V and 3.366 V in both sheets, within the 3.0 to 3.6 V an ESP32 accepts. It needs at least 4.55 V in at a few hundred milliamps (4.7 V on SLKOR's 1 A figure). USB's 5 V is enough; a lithium cell at 4.2 V is not, and the ESP32 power guide covers what to use instead.

The fixed versions take their ground current, up to 10 mA, from the input and return it through pin 1. That current is small, but at 12 V it adds 120 mW of heat on its own.

• 3.3 V from a 5 V USB or module rail, up to a few hundred milliamps.
• 1.8 V or 2.5 V core rails from 3.3 V or 5 V, where the current is small.
• Odd voltages with the ADJ version and two resistors, as with an LM317.

It is the wrong part for anything with a large drop. From 7 V, 9 V or 12 V at a few hundred milliamps the SOT-223 cannot lose the heat. A buck module such as the LM2596 does that job; for a bigger linear regulator, see the LM317.

An ESP32 draws about 240 mA at its Wi-Fi transmit peak (the Espressif figure our power guide uses). Model it as a 13.7 Ω load on 3.3 V, the nearest E96 value: 241 mA. The regulator has 10 µF on its input and 22 µF on its output, as in UMW's circuit.

The fix for a 12 V supply is a buck converter down to 5 V ahead of the board, not a bigger linear regulator. The regulator thermal design lesson works the same sums for other packages.

• Fit 10 µF on the input and 22 µF tantalum on the output, both close to the pins. That meets both datasheets at once.
• Feed an ESP32 or similar board from 5 V. Anything higher into the board's 5 V or VIN pin goes straight into the AMS1117 as heat.
• Solder the tab to as much copper as the board allows. The tab is VOUT, so that copper belongs to the 3.3 V net, never to ground.
• Budget 670 mW of heat at 35 °C. Multiply the drop by the current and add the input voltage times 10 mA before you choose it.
• If a board resets under Wi-Fi load, measure the regulator's input while it transmits. Below 4.55 V it is in dropout, not faulty.

TI's LM1117 has the same pins and the same capacitor rule and is the safest swap, as long as the load stays under 800 mA. An AMS1117 from another maker also drops in, but read its input-voltage line: the two sheets here allow 12 V and 18 V.

1. Power off, meter on continuity: the tab should read 0 Ω to pin 2.
2. Feed 5 V from a current-limited supply with no load. An AMS1117-3.3 should read 3.234 to 3.366 V at the output.
3. Add a 13.7 Ω resistor rated 2 W or more as the load (241 mA, about 790 mW in the resistor). The output should barely move.
4. Turn the supply down slowly and note where the output starts to fall. The difference between input and output there is the dropout, and it should be no more than about 1.25 V at this current.

The common mistakes: a 12 V adapter on the 5 V pin, a 3.3 V board run from a single lithium cell straight into the regulator, the tab soldered to a ground pour, and a tiny output capacitor left out to save space.

Use the AMS1117-3.3 from a 5 V rail, with 10 µF in and 22 µF tantalum out, and keep the load under about 360 mA in warm air. From 7 V or more, put a buck converter in front of it or use one instead. When it gets hot, work out the heat before you blame the part: it is almost always the input voltage.

The AMS1117-3.3 and -5.0 are sold loose in SOT-223 and on small regulator modules with the capacitors already fitted. Many makers' parts carry the same number, so ask whose it is and check the tab and the output with the bench test above. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.