LM1117-3.3 PCB Design Guide: Footprint, Pinout, and Alternatives
Fixed 3.3 V, 800 mA low-dropout regulator in the classic SOT-223 1117 footprint.
The LM1117-3.3 is the fixed 3.3-V member of TI's LM1117 family, an 800-mA low-dropout linear regulator in production since 2000 and still active, with the datasheet last revised January 2023 (SNOS412Q, Rev. Q). At 25 °C it holds the output between 3.267 and 3.333 V, it has current limiting and thermal shutdown built in, and it lives in the SOT-223 outline shared by the whole widely second-sourced 1117 family, which is why the footprint is already in every board house's library and most engineers' muscle memory.
Pick it with clear eyes: this is a low-dropout regulator by the standards of its era. Dropout is 1.2 V typical at 800 mA, quiescent current is 5 mA whether the load needs it or not, the output requires a tantalum capacitor with real ESR to stay stable, and the standard grade is only specified from 0 °C to 125 °C junction (the LM1117I extends to −40 °C). It earns its keep when the input is a solid 5-V rail, the load is modest, and you want a jellybean part with second sources everywhere. If you need battery standby life, ceramic-cap stability, or cold-temperature margin, TI's own datasheet features list points you at the TLV1117 and TLV761 instead.
The board-level failures are predictable and recurring: an all-ceramic output capacitor that violates the ESR window and oscillates, a lithium-ion input that never had enough headroom, thermal math done with the SOT-223's best-case number instead of your actual copper, a quiescent current that flattens batteries in standby, and an output capacitor discharging backward through the part when the input collapses. Each is covered below.
What breaks boards
An all-ceramic output capacitor can make it oscillate
The LM1117 predates ceramic-stable LDOs: the datasheet requires a minimum 10-µF tantalum capacitor on the output and an output-capacitor ESR between 0.3 and 22 Ω. A modern MLCC sits far below that ESR floor, so the all-ceramic layout that works with newer regulators can leave this part unstable, showing up as ripple you can't explain or an MCU that misbehaves. Use a tantalum, or verify your capacitor actually lands inside the ESR window, and place it close to the output pin. More capacitance only improves stability; lower ESR does not.
Dropout is 1.2 V: this is not a modern low-dropout part
Dropout is 1.2 V typical and 1.3 V max at 800 mA, and still 1.1 V typical at 100 mA. TI only specifies the 3.3-V output over temperature with VIN at 4.75 V or above. A regulated 5-V rail is the intended input; a single lithium-ion cell is not, because most of its discharge curve sits below the input this part needs, so the output quietly follows the battery down instead of regulating. Budget real headroom, including any polyfuse, diode, or cable drop between your 5-V source and the VIN pin.
SOT-223 thermal performance is your copper, not the package
Dissipation is (VIN − VOUT) × load current plus VIN × quiescent current, and it all leaves through the tab. The headline 61.6 °C/W junction-to-ambient figure assumes a favorable test board: TI's own measurements show 136 °C/W with minimal topside copper, improving to 66 °C/W with 1 in² of copper attached to the tab. The junction ceiling is 125 °C, and the standard grade is only specified from 0 °C up. Thermal shutdown will save the silicon, but your rail will sag and cycle. Pour copper under the tab, or pre-regulate with a buck when the input is high.
5 mA of quiescent current flows whether the load needs it or not
The LM1117-3.3 draws 5 mA typical, 10 mA max, of quiescent current (specified with VIN up to 15 V), and on the fixed version that current flows to ground continuously; there is no shutdown pin and no way to trim it. On a mains-powered board that is irrelevant. On anything battery-powered it is often the single largest standby drain, dwarfing a sleeping MCU by orders of magnitude. If your product spends its life asleep, use a modern low-IQ regulator and keep the LM1117 for rails that are only up while the system is active.
A shorted input dumps the output capacitor back through the part
If VIN falls faster than VOUT — input shorted to ground, supply hot-unplugged — the output capacitor discharges backward through the regulator. The internal diode between the output and input pins withstands microsecond surge currents of 10 A to 20 A, so ordinary designs survive this. But with an extremely large output capacitor (1000 µF or more) and a hard input short, the regulator can be damaged. For that case TI recommends an external diode from VOUT to VIN, placed close to the regulator's pins so it actually diverts the surge current.
Key specifications
| Parameter | Value | Source |
|---|---|---|
| Output voltage (3.3 V fixed) | 3.267 V min / 3.3 V typ / 3.333 V max (IOUT = 10 mA, VIN = 5 V, TJ = 25 C); 3.235 V min / 3.365 V max over the junction temperature range 0 C to 125 C (0 <= IOUT <= 800 mA, 4.75 V <= VIN <= 10 V) | SNOS412Q Rev Q, Section 7.5 LM1117 Electrical Characteristics, VOUT LM1117-3.3 rows |
| Dropout voltage (VIN - VOUT) | 1.2 V typ (IOUT = 800 mA, TJ = 25 C), 1.3 V max over the junction temperature range 0 C to 125 C; 1.1 V typ / 1.2 V max at IOUT = 100 mA | SNOS412Q Rev Q, Section 7.5 LM1117 Electrical Characteristics, dropout voltage rows |
| Input voltage range | 15 V max recommended (VIN to GND); absolute maximum input voltage 20 V (VIN to GND) | SNOS412Q Rev Q, Section 7.3 Recommended Operating Conditions + Section 7.1 Absolute Maximum Ratings |
| Quiescent current | 5 mA typ (TJ = 25 C), 10 mA max over the junction temperature range 0 C to 125 C (VIN <= 15 V) | SNOS412Q Rev Q, Section 7.5 LM1117 Electrical Characteristics, quiescent current LM1117-3.3 rows |
| Current limit | 800 mA min / 1200 mA typ / 1500 mA max (VIN - VOUT = 5 V, TJ = 25 C) | SNOS412Q Rev Q, Section 7.5 LM1117 Electrical Characteristics, ILIMIT row |
| Output capacitor requirement | A minimum of 10-uF tantalum capacitor is required at the output to improve the transient response and stability | SNOS412Q Rev Q, Section 3 Description |
| Output capacitor ESR range | The ESR of the output capacitor should range between 0.3 ohm to 22 ohm; the minimum output capacitance required by the LM1117 is 10 uF if a tantalum capacitor is used, and any increase of the output capacitance will merely improve the loop stability and transient response | SNOS412Q Rev Q, Section 9.2.2.1.3 Output Capacitor |
| Thermal resistance, SOT-223 | RthetaJA = 61.6 C/W junction-to-ambient (DCY, 4 pins); junction temperature range 0 C to 125 C for LM1117 (-40 C to 125 C for LM1117I) | SNOS412Q Rev Q, Section 7.4 Thermal Information + Section 7.3 Recommended Operating Conditions |
| RthetaJA vs copper area, SOT-223 | 136 C/W with 0.0123 in2 topside copper; 66 C/W with 1 in2 topside copper (1-oz copper, tab of device attached to topside copper, still air) | SNOS412Q Rev Q, Section 9.5.1.1, Table 9-2 RthetaJA Different Heat Sink Area + Figure 9-11 |
| Reverse discharge withstand | The internal diode between the output and input pins can withstand microsecond surge currents of 10 A to 20 A; with an extremely large output capacitor (>= 1000 uF) and the input instantaneously shorted to ground, the regulator could be damaged, and an external diode between the output and input pins is recommended to protect the regulator | SNOS412Q Rev Q, Section 8.4.1 Protection Diodes |
Verified against the manufacturer datasheet on 2026-07-10. Confirm the current revision before production use.
Alternatives
- TLV1117: TI's newer drop-in alternative, named in the LM1117 datasheet's own features list; its recommended operating range extends to −40 to 125 °C where the standard LM1117 starts at 0 °C.
- AMS1117-3.3: the ubiquitous low-cost 1117-family second source in the same SOT-223 pinout; specs are not identical, so verify against the AMS datasheet instead of assuming TI's numbers.
- TLV761: named in the LM1117 datasheet features as the drop-in replacement for fixed-output SOT-223 configurations with improved functionality.
Common questions
- Can the LM1117-3.3 regulate 3.3 V from a single lithium-ion cell?
- No. Dropout is 1.2 V typical and 1.3 V max at 800 mA, and TI specifies the 3.3-V output over temperature only with VIN at 4.75 V or above. A lithium-ion cell spends most of its discharge below that, so the part drops out and the output follows the battery down. Use a genuinely low-dropout regulator or a buck-boost instead.
- Can I use a ceramic output capacitor with the LM1117-3.3?
- Not by itself. The datasheet requires a minimum 10-µF tantalum output capacitor and an output-capacitor ESR between 0.3 and 22 Ω, and typical MLCCs sit well below that window, risking oscillation. Use a capacitor that lands inside the ESR range, or pick a regulator that is specified as stable with ceramic output capacitors.
- What input voltage does the LM1117-3.3 accept?
- Recommended operation is up to 15 V input, with an absolute maximum of 20 V. On the low end, the 3.3-V output is only specified over temperature with 4.75 V ≤ VIN ≤ 10 V, so plan on a 5-V rail as the natural input and check your headroom after any series drops.
- How much current can the LM1117-3.3 supply?
- It is an 800-mA regulator, with internal current limiting specified at 800 mA min / 1200 mA typ / 1500 mA max (measured at VIN − VOUT = 5 V). In practice the SOT-223's thermal limits usually bind first: with 61.6 °C/W junction-to-ambient at best and a 125 °C junction ceiling, sustained current at higher input voltages is capped by heat, not by the current limit.