LM317T PCB Design Guide: Footprint, Pinout, and Alternatives
TI's classic 1.5 A adjustable 3-terminal regulator in TO-220, the industry-standard LM317T — NRND, legacy-chip only, and emphatically not an LDO
The LM317T is the TO-220 incarnation of the LM317, the adjustable 3-terminal positive voltage regulator that defined a category when National Semiconductor introduced it in 1976 and that Texas Instruments still ships today. It delivers more than 1.5 A from an output adjustable between 1.25 V and 37 V with two external resistors, and it rides on an NPN Darlington output stage that is inherently stable with no output capacitor — a trait that distinguishes it from nearly every modern LDO and that still makes it the right choice when you need a regulator that does not oscillate regardless of what you hang on its output. The current datasheet is SLVS044Z Rev. Z, revised April 2025.
Pick it when you want a bulletproof adjustable rail on a through-hole board and you are not trying to squeeze the last volt of headroom or the last milliamp of quiescent current out of the design. Pick something else for a new design. The TO-220 package is NRND and is available only with the legacy silicon; TI's SOT-223 (DCY) and TO-263 (KTT) packages carry both the legacy and the improved new chip, and they are fully active. If you are designing a new board and you want the LM317, reach for the DCY or KTT. If you are repairing or cloning an existing design that has TO-220 pads, the LM317T is still stocked and still exactly what the schematic expects.
The recurring failures are all about treating it like a modern LDO: starving it of headroom, forgetting the minimum load, omitting the protection diodes when bypass capacitors are added, and not running the thermal numbers on a package whose tab is the output net and whose junction-to-ambient figure is low enough to lull you into skipping the heat sink math. Each is covered below.
What breaks boards
TO-220 is NRND and legacy-chip only; design new boards around SOT-223 or TO-263
The LM317KCS (TO-220, straight leads) and LM317KCT (TO-220, formed leads) are marked NOT RECOMMENDED FOR NEW DESIGNS on TI's product page. The TO-220 package is only available with the legacy chip — there is no new-chip silicon in TO-220. The new chip improves PSRR from 64 dB to 80 dB (with CADJ = 10 µF), adds output safe-area compensation, and has different thermal characteristics, but you cannot get it in a through-hole power package. For new designs, use the DCY (SOT-223) or KTT (TO-263) packages, both of which are ACTIVE and available in new-chip silicon.
Not an LDO: budget 3 V of headroom, not millivolts
The LM317 uses an NPN Darlington output topology that drops roughly 2 V at full load. The datasheet specifies a minimum input-to-output differential of 3 V in the Recommended Operating Conditions table, and Section 7.3.1 states 'A 3 V headroom is recommended (VI - VO) to support maximum current and lowest temperature.' A 5 V rail needs at least 8 V at the input; a 12 V rail needs at least 15 V. Do not put this part on a battery rail and expect it to behave like a modern LDO with a 200 mV dropout — it will not regulate, and the output will collapse to VIN minus the dropout voltage.
Minimum load current: 3.5 mA typ, 10 mA worst case
The LM317 sources its internal bias current to the OUTPUT pin, and the load or feedback divider must sink at least 3.5 mA typical (10 mA max at VIN − VOUT = 40 V) for the part to stay in regulation. Section 7.4.3 warns: 'Make sure the load or feedback consumes this minimum current for regulation or the output is potentially too high.' The classic fix is to size R1 (the resistor from OUTPUT to ADJUST) at 240 Ω or lower, which sets the minimum programming current at 1.25 V / 240 Ω ≈ 5.2 mA, comfortably above the 3.5 mA typical. Using a larger R1 to save quiescent current is a trap — the output drifts high at light load and the rail measures fine under load, which is exactly when you will not notice the problem.
No output capacitor needed for stability, but protection diodes are mandatory if you add bypass capacitors
The NPN Darlington output stage is inherently stable with no output capacitor — a fundamental difference from modern LDOs. However, if you add an adjustment-pin bypass capacitor (CADJ) to improve ripple rejection, you must add a protection diode (D2) from the ADJUST pin to the OUTPUT pin. If you add an output capacitor (CO) to improve transient response, you must add a protection diode (D1) from the OUTPUT pin to the INPUT pin. Without these diodes, a shorted output or a sudden input short can force the capacitors to discharge backward through the regulator, destroying it. The datasheet shows both diodes in the typical application circuit (Figure 8-1) and lists them as design requirements in Section 8.2.1.
The tab is the OUTPUT net, not ground — route and heat-sink accordingly
On the TO-220 LM317T, pin 2 and the metal tab are the OUTPUT. This is opposite to the far more common convention where the tab is ground. If you bolt the tab to a grounded chassis or heat sink without an insulator, you short the output to ground. The TO-220 θJA of 23.5 °C/W (KCS) is deceptively low — it is measured with the tab soldered to a large copper area, and without a heat sink the junction temperature rises fast. At just 1 A with a 5 V drop, the device dissipates 5 W, and the junction hits 117.5 °C above ambient on the KCS package without additional sinking. The absolute maximum junction temperature is 150 °C (legacy chip), and the recommended operating maximum is 125 °C. Run the thermal math — Equation 9 through Equation 12 in Section 8.5.1.1.1 — or add a heat sink.
Key specifications
| Parameter | Value | Source |
|---|---|---|
| Output voltage range | Adjustable: 1.25V to 37V | Datasheet SLVS044Z Rev. Z, Section 1 Features |
| Output current | 1.5A | Datasheet SLVS044Z Rev. Z, Section 1 Features |
| Line regulation | 0.01%/V (typ) | Datasheet SLVS044Z Rev. Z, Section 1 Features |
| Load regulation | 0.1% (typ) | Datasheet SLVS044Z Rev. Z, Section 1 Features |
| Reference voltage | 1.25 V typ (TJ = 25°C); 1.2 V min / 1.3 V max (3V ≤ VIN-VOUT ≤ 40V, 10mA ≤ IOUT ≤ 1500mA, PD ≤ 20W) | Datasheet SLVS044Z Rev. Z, Section 6.6 Electrical Characteristics, VREF row |
| Input-output differential | 3 V min / 40 V max (recommended operating); -0.3 V / 40 V (absolute max) | Datasheet SLVS044Z Rev. Z, Section 6.3 Recommended Operating Conditions + Section 6.1 Absolute Maximum Ratings |
| Minimum load current | 3.5 mA typ / 10 mA max (VIN - VOUT = 40V, over full operating temperature range) | Datasheet SLVS044Z Rev. Z, Section 6.6 Electrical Characteristics, IMIN(LOAD) row |
| Current limit | 1.5 A typ / 2.2 A max (VIN - VOUT ≤ 15V, PD < PMAX); 0.15 A typ / 0.4 A max (VIN - VOUT = 40V) | Datasheet SLVS044Z Rev. Z, Section 6.6 Electrical Characteristics, ICL rows |
| Ripple rejection (legacy chip) | 64 dB typ (VOUT = 10V, f = 120Hz, CADJ = 0 µF); 57 dB min over full temp | Datasheet SLVS044Z Rev. Z, Section 6.6 Electrical Characteristics, Ripple Rejection Ratio row (legacy chip) |
| Operating junction temperature | 0°C to 125°C (recommended); -65°C to 150°C (absolute max, legacy chip) | Datasheet SLVS044Z Rev. Z, Section 6.3 Recommended Operating Conditions + Section 6.1 Absolute Maximum Ratings |
| Thermal resistance (TO-220) | θJA = 23.5°C/W (KCS, legacy chip); θJA = 37.9°C/W (KCT, legacy chip) | Datasheet SLVS044Z Rev. Z, Section 6.4 Thermal Information (Legacy Chip) |
| Dropout voltage | ~2 V typ (from TI product page parameter table: Vdo typ 2000 mV); NPN Darlington topology requires 3 V headroom per §7.3.1 | TI product page parameters + Datasheet SLVS044Z Rev. Z, Section 7.3.1 NPN Darlington Output Drive |
Verified against the manufacturer datasheet on 2026-07-10. Confirm the current revision before production use.
Alternatives
- LM317DCY: TI's SOT-223 packaged LM317, ACTIVE, available in both legacy and new-chip silicon. Same 1.5 A, same pinout (ADJ-OUT-IN), surface-mount. Recommended for new designs.
- LM317KTTR: TI's TO-263 (D2PAK) packaged LM317, ACTIVE, available in both legacy and new-chip silicon. Surface-mount power package with θJA = 38.0°C/W (legacy) / 41.0°C/W (new).
- LM317A: TI's higher-accuracy LM317: ±1% output tolerance vs ±5%, -40°C to +125°C operating range, ACTIVE. Available in TO-220 (NDE, 3-pin), SOT-223, and TO-252. Pin-compatible upgrade.
- LM317HV: TI's high-voltage LM317: 60 V max input-output differential vs 40 V, same 1.5 A, ACTIVE. For applications needing >40 V headroom.
- TLV1117: TI's 1117-family LDO: lower dropout but only 800 mA, different pinout (tab is OUTPUT on both, but check ADJ/GND pin). SOT-223. Not a drop-in for the LM317T footprint.
Common questions
- Is the LM317T still in production?
- The LM317KCS (TO-220) is NRND — not recommended for new designs — but it is still in production and available for purchase as of July 2026. The DCY (SOT-223) and KTT (TO-263) packages are fully ACTIVE. For new designs, TI recommends the SOT-223 or TO-263 packages.
- What is the dropout voltage of the LM317?
- The LM317 uses an NPN Darlington output stage with a dropout voltage of approximately 2 V typical at 1.5 A. The datasheet specifies a minimum input-to-output differential of 3 V for guaranteed regulation. This is not a low-dropout regulator — budget 3 V of headroom.
- Does the LM317 need an output capacitor?
- No. The NPN Darlington output stage is inherently stable without an output capacitor. This is a fundamental difference from modern LDOs. An output capacitor (CO) is optional and only improves transient response. However, if you add CO, you must add a protection diode (D1) from OUTPUT to INPUT to prevent capacitor discharge damage during a short circuit.
- Why does my LM317 output voltage drift high at light load?
- The LM317 requires a minimum load current of 3.5 mA typical (10 mA max) to stay in regulation. If the feedback divider plus the actual load sinks less than this, the output climbs. Size R1 (the output-to-adjust resistor) at 240 Ω or lower — this guarantees 1.25 V / 240 Ω ≈ 5.2 mA of programming current, which meets the minimum load requirement on its own.
- Can I use the LM317 to regulate a 3.3 V rail from a 5 V supply?
- No. The minimum input-to-output differential is 3 V, so 5 V in to 3.3 V out gives only 1.7 V of headroom, which is below the spec. The output will drop out of regulation. For a 5 V to 3.3 V conversion, use a modern LDO or a buck converter.
- Is the LM317 tab at ground?
- No. On the LM317, the metal tab is the OUTPUT net (pin 2). This is opposite to the common convention where the tab is ground. If you bolt the tab to a grounded chassis without an insulator, you short the output to ground. Use an insulating mounting kit if you need to attach the tab to a grounded heat sink.