LM317G-AA3-R — Adjustable 1.3V–37V/1A Linear Voltage Regulator, SOT-223 (UTC Unisonic)

What Are the Technical Specifications of LM317G-AA3-R?

LM317 SOT-223 vs TO-220 — same silicon, radically different thermal envelope. The electrical specs are identical. The thermal specs are not. Here's what the package choice means for your design.

What these bars mean: The SOT-223 can dissipate about 0.6W before hitting the 125°C junction limit at 25°C ambient. With a 10V drop across the regulator (say 15V in, 5V out), that's 60mA max. Push more current and the thermal shutdown kicks in — the output cycles on and off as the die heats and cools. The TO-220 buys you roughly 3× the power handling before you even bolt on a heatsink. Add a heatsink and the TO-220 pulls far ahead.

But the SOT-223's thermal story isn't all bad. Its θ_JC of 23°C/W is reasonable — most of the 165°C/W θ_JA is the junction-to-board path. Maximize the copper pour under the package, add thermal vias to an internal ground plane, and you can push θ_JA down toward 60–80°C/W. A well-designed SOT-223 layout can handle 0.5A at a 5V drop — not 1A, but usable for a lot of real designs.

When Should You Use (and NOT Use) the LM317G-AA3-R?

✅ Use LM317G-AA3-R when:

• You need an adjustable rail on an SMD-only board. If your entire BOM is surface-mount and you don't want a through-hole LM317 forcing a hand-soldering step, the SOT-223 version fits the reflow process. This is the single biggest reason to choose the LM317G over an LM317T.
• Low power dissipation (<0.5W). Dropping 5V at 100mA or 3.3V at 150mA — these are in the SOT-223's comfort zone with decent PCB copper. LED current sources, op-amp bias supplies, low-current battery chargers all fit.
• Cost-sensitive consumer designs. UTC's pricing is aggressive. For products shipping in volume where every cent counts and the thermal math works, the LM317G-AA3-R is the economic choice over TI or onsemi equivalents.
• Prototyping a design that will eventually switch to a fixed LDO. The LM317's adjustability makes it ideal for development — tweak R₂ until the voltage is right, then swap to a fixed regulator for production. The SOT-223 footprint is compatible with many fixed LDOs in the same package.

❌ Don't use LM317G-AA3-R when:

• You're dropping more than 10V at more than 100mA. At 10V × 200mA = 2W, you're well past what the SOT-223 can handle. Use an LM317T (TO-220 with heatsink) or a switching regulator like the LM2596.
• Ambient temperature exceeds 70°C. With only 55°C of rise budget (125−70), you can dissipate just 0.33W. That's a 3.3V drop at 100mA. For hot environments, the TO-220 or a switcher is mandatory.
• Low noise is critical. At 65dB PSRR at 120Hz, the LM317 is fine for general-purpose use, but for RF or precision analog, you'll want a modern LDO with >80dB PSRR and lower output noise. Look at LP5912 or TPS7A series.
• You need sub-1.25V output. The LM317's reference is 1.25V — you cannot regulate below that without a negative rail. For outputs below 1.25V, use an adjustable LDO with a lower reference, like the AMS1117-ADJ (1.25V ref but lower dropout) or a dedicated low-V_OUT regulator.

What Are the Alternatives to LM317G-AA3-R?

The LM317G-AA3-R's real competition is the AMS1117-ADJ. Same SOT-223 package, same 1.25V reference, but the AMS1117 drops out at 1.1V versus the LM317's ~2.5V. In a 5V-to-3.3V application, the AMS1117 runs cooler because less voltage is dropped across the regulator. The LM317G wins when you need the full 37V input rating or when the AMS1117 is on allocation — a situation that happens more often than datasheets would suggest.

What Are the Typical Applications of LM317G-AA3-R?

Low-current adjustable bench supply (0–100mA). The classic LM317 lab supply shrunk to SMD. With a 12V input, you can dial anything from 1.25V to 9V at up to 100mA without heatsinking — perfect for powering sensors, op-amp circuits, and microcontroller prototypes on a breadboard adapter PCB.

LED constant-current driver. A single resistor between OUT and ADJ sets the current: I = 1.25V / R. At 100mA, R = 12.5Ω and the regulator dissipates (V_IN − V_LED − 1.25V) × 0.1A. For a 12V input driving a 3V LED, that's 0.78W — warm but within the SOT-223's capability on a decent copper pour. Common in architectural and automotive LED lighting where SMD assembly is required.

Battery charger for NiMH and Li-Ion (trickle/top-off). The LM317 in constant-current mode delivers a precise charge current set by one resistor. For a 4.2V Li-Ion top-off at 100mA from a 9V supply, power dissipation is (9 − 4.2 − 1.25) × 0.1 = 0.36W — comfortably inside the SOT-223 envelope. Add a voltage-limiting circuit on the ADJ pin and you have a complete CC/CV charger in two square centimeters of board space.

Post-regulation after a switching supply. A switcher knocks 24V down to 7V at 85% efficiency, then the LM317G cleans up the ripple and provides a precise adjustable output. This combo — switcher for efficiency, linear for noise — is common in test equipment and audio circuits. The LM317 does the last 1–2V of regulation where the power loss is small and the PSRR cleans up the switching noise.

Why Buy LM317G-AA3-R from ICMASS?

We stock the UTC LM317G alongside TI and onsemi LM317 variants. If your BOM calls for the TI part but lead times are stretching, we can help you cross-reference to the UTC version. Same reference voltage, same pinout, same external resistor formula — the circuit doesn't know the difference. Thousands of production lines run UTC LM317s without issues.

SOT-223 tape & reel, ready for your pick-and-place. The AA3-R suffix means SOT-223 in tape & reel — exactly what your assembly house needs. No cutting, no re-packing, no hand-placement surcharge. Tell us your monthly run rate and we'll keep a buffer reel allocated so your line never stops.

We know the thermal math. The most common reason an LM317G-AA3-R gets returned is thermal shutdown in a design that should have used a TO-220 or a switcher. Tell us your input voltage, output voltage, and load current before you order — we'll help you confirm the SOT-223 is the right package for your design before the boards go to assembly.

Frequently Asked Questions

Q1: What is the difference between LM317G-AA3-R and LM317T?

A: Package only. The silicon is functionally identical. LM317G-AA3-R is SOT-223 (SMD, θ_JA=165°C/W, max ~0.6W dissipation). LM317T is TO-220 (through-hole, θ_JA=50°C/W, heatsink-ready). Same reference voltage, same output formula. Choose based on your power dissipation and assembly process.

Q2: How much current can the LM317G-AA3-R really deliver?

A: It depends entirely on the input-output voltage difference. At 3V drop (e.g., 8V in, 5V out), you can pull about 200mA before hitting the 125°C junction limit at room temperature. At 10V drop, that falls to ~60mA. The 1A datasheet rating assumes the package is kept at 25°C — practically impossible in SOT-223 without active cooling. Maximize your PCB copper pour under the tab and use thermal vias to internal planes to get the most out of the package.

Q3: Is UTC Unisonic a reliable manufacturer?

A: Yes, for general-purpose and consumer applications. UTC is a Taiwanese semiconductor company founded in 1990, IATF 16949 and ISO 9001 certified. Their LM317 is a second-source clone of the original National Semiconductor design. For commercial and industrial use, UTC parts are widely deployed and considered reliable. For mil-spec, aerospace, or medical where full traceability matters, you may want the TI or onsemi version with full lot-level documentation — but for the other 95% of designs, the UTC part works.

Q4: What capacitors do I need with the LM317G-AA3-R?

A: Input: 0.1µF ceramic close to the V_IN pin. Output: 1µF (or larger) electrolytic or tantalum. The input capacitor is not optional if the regulator is more than a few inches from the input filter cap — without it, the LM317 can oscillate. The output capacitor improves transient response but is technically optional for stability. If you use an output capacitor larger than 25µF, add a protection diode from V_OUT to V_IN to prevent reverse current during power-down.

Q5: Why does my LM317G-AA3-R keep shutting down and restarting?

A: Thermal overload. The junction is hitting ~150°C, the thermal protection kicks in and shuts off the output, the die cools to ~125°C, the output comes back, and the cycle repeats. This is the single most common issue with SOT-223 LM317s. Solutions: reduce the input voltage, reduce the load current, or increase PCB copper area under the tab. If you can't change any of those, switch to an LM317T with a heatsink.

Q6: Can I use the LM317G-AA3-R to generate voltages below 1.25V?

A: Not without a negative supply. The internal reference is 1.25V, which sets the minimum output. To go lower, you need to pull the ADJ pin below ground with a negative voltage — typically -1.25V from a charge pump or an extra winding on the transformer. For sub-1.25V outputs without the hassle, use a modern adjustable LDO with a 0.6V or 0.8V reference.

Q7: What is the dropout voltage of the LM317G-AA3-R?

A: About 2.5V at 1A, decreasing at lower currents. At 100mA, dropout is roughly 1.5V. This is higher than modern LDOs like the AMS1117 (1.1V) because the LM317 uses a Darlington pass transistor rather than a PNP or P-channel FET. In a 5V-to-3.3V application, the LM317G needs at least 5.8V input to regulate properly — if your 5V rail is actually 4.9V, you're in trouble.

Q8: How do I set the output voltage?

A: V_OUT = 1.25 × (1 + R₂/R₁). R₁ goes between OUT and ADJ. R₂ goes between ADJ and ground. Standard practice: set R₁ = 240Ω (gives ~5.2mA through the divider, satisfying the 10mA minimum load when combined with the output load). Then R₂ = (V_OUT/1.25 − 1) × R₁. For 5V output: R₂ = (5/1.25 − 1) × 240 = 720Ω. Use 1% resistors — the ADJ pin current (50µA typ) adds a small error term with high-value R₂.

Q9: Does the LM317G-AA3-R need a heatsink?

A: Not in the traditional sense — the PCB is the heatsink. The SOT-223 package has no mounting hole. The tab is soldered directly to a copper pad on the PCB, and heat flows down through the board. Maximize the copper area connected to the tab (at least 1 square inch on both top and bottom layers, stitched with thermal vias) to improve θ_JA. At very low power (<0.3W), the package body alone is sufficient.

Q10: What is the difference between LM317G, LM317, and LM317M?

A: LM317G is UTC's halogen-free ("Green") version. Standard LM317 from any manufacturer follows the same 1.25V reference design. LM317M is a medium-current variant rated for 500mA instead of 1A, with slightly better line/load regulation specs. The "M" version is available in smaller packages (SOT-89, TO-252) and is a good choice when you know you'll never exceed 500mA.

Related Products

• LM317T — Adjustable Regulator, TO-220 (1.5A, Heatsink-Ready)
• AMS1117-ADJ — Adjustable LDO, SOT-223 (1A, 1.1V Dropout)
• LM1117IMPX-3.3 — Fixed 3.3V LDO, SOT-223 (800mA)
• LM2596S-ADJ — Adjustable Buck Converter, TO-263 (3A, >80% Eff)
• LM317 vs AMS1117 vs LM2596 — Linear vs LDO vs Switching Comparison
• LM7805L-TA3-T — Fixed 5V Regulator, TO-220 (1.5A)