Answer: “LM317T” is the common name for an LM317 in a 3-pin TO-220. TI does not use that order code: TI’s TO-220 orderable is LM317KCS (status NRND, tube of 50). The SOT-223 (LM317DCY/DCYR) and TO-263 (LM317KTTR) versions are Active. Pinout: 1 ADJUST, 2 OUTPUT (and tab), 3 INPUT. VOUT = 1.25 V × (1 + R2/R1) + IADJ × R2, from 1.25 V to 37 V, up to 1.5 A. (TI SLVS044Z pp.1, 5, 6, 16; Package Option Addendum p.1.)
Key specifications
| Parameter | Value | Condition | Source |
|---|---|---|---|
| Reference voltage VREF | 1.2 / 1.25 / 1.3 V | 3 V ≤ VIN − VOUT ≤ 40 V, 10 mA ≤ IOUT ≤ 1.5 A, PD ≤ 20 W; 1.25 V typ at TJ = 25 °C | p.8 §6.6 Electrical Characteristics |
| Adjustment pin current IADJ | 50 typ / 100 max µA | Full operating temperature range | p.8 §6.6 |
| IADJ change | 0.2 typ / 5 max µA | 10 mA ≤ IOUT ≤ IMAX, 3 V ≤ VIN − VOUT ≤ 40 V | p.8 §6.6 |
| Minimum load current | 3.5 typ / 10 max mA | VIN − VOUT = 40 V, full temperature range | p.8 §6.6 |
| Current limit | 1.5 min / 2.2 typ A (VIN − VOUT ≤ 15 V); 0.15 min / 0.4 typ A (40 V, TJ = 25 °C) | PD < PMAX | p.8 §6.6 |
| Line regulation | 0.01 typ / 0.04 max %/V (TJ = 25 °C); 0.02 / 0.07 %/V full range | 3 V ≤ VIN − VOUT ≤ 40 V | p.8 §6.6 |
| Load regulation (legacy chip, VO ≥ 5 V) | 0.1 typ / 0.5 max %VO at 25 °C; 0.3 / 1.5 %VO at 0–125 °C | IO = 10 mA to 1.5 A | p.8 §6.6 |
| Ripple rejection (legacy chip) | 57 dB typ (CADJ = 0); 62 min / 64 typ dB (CADJ = 10 µF) | VOUT = 10 V, f = 120 Hz | p.8 §6.6 |
| Input-to-output differential | 3 to 40 V recommended; −0.3 to 40 V absolute maximum | — | p.6 §6.1, §6.3 |
| Output voltage / current | 1.25 to 37 V; 0.01 to 1.5 A | Recommended operating conditions | p.6 §6.3 |
| Junction temperature | 0 to 125 °C recommended; legacy chip absolute max −65 to 150 °C | — | p.6 §6.1, §6.3 |
| Thermal resistance TO-220 (legacy chip) | RθJA 23.5 °C/W (KCS) / 37.9 °C/W (KCT); RθJC(bot) 0.1 / 4.2 °C/W | JEDEC metrics | p.7 §6.4 |
| TI TO-220 orderable | LM317KCS (also LM317KCS.A, LM317KCSE3): NRND, tube of 50, marking LM317, 0 to 125 °C | No KCT orderable in the addendum | Package Option Addendum p.1 (PDF p.30) |
What “LM317T” means and what TI actually sells
The suffix “T” for TO-220 comes from other makers’ naming. In TI’s April 2025 datasheet the TO-220 options are package codes KCS and KCT, both “legacy chip” (p.1). The Package Option Addendum lists only:
| Orderable (TI) | Package | Status | Pack | Marking |
|---|---|---|---|---|
| LM317KCS, LM317KCS.A, LM317KCSE3 | TO-220 (KCS), 3 pins | NRND | Tube, 50 | LM317 |
| LM317DCY / LM317DCYR (+ .A/.B/G3) | SOT-223 (DCY), 4 pins | Active | Tube 80 / reel 2500 | L3 |
| LM317KTTR (+ .A/.B/G3) | TO-263 (KTT), 3 pins | Active | Reel, 500 | LM317 |
(Addendum p.1, PDF p.30.) There is no KCT orderable in the addendum, even though the datasheet gives KCT thermal data. That is why this is a family page rather than a single-part page. For a TO-220 from other makers (ST, onsemi and others), see LM317 alternatives.
Pinout
| Pin | TO-220 (KCS/KCT) | TO-263 (KTT) | SOT-223 (DCY) |
|---|---|---|---|
| 1 | ADJUST | ADJUST | ADJUST |
| 2 | OUTPUT | OUTPUT | OUTPUT |
| 3 | INPUT | INPUT | INPUT |
| Tab | OUTPUT | OUTPUT | OUTPUT (pin 4) |
(p.5, Pin Functions.) The tab is the output, so it is live: insulate a TO-220 from a grounded heat sink.
Setting the output
Equation 2 (p.16): VO = VREF × (1 + R2/R1) + IADJ × R2, with VREF = 1.25 V typ and IADJ = 50 µA typ.
Worked example for 12 V with TI’s R1 = 240 Ω: R2 = 2.05 kΩ (E96) gives 12.03 V typical. At the datasheet limits (VREF 1.2–1.3 V, IADJ 0–100 µA) the same pair spans 11.45–12.61 V. Trim R2 or use 1 % resistors if you need better.
For a current source, put a single resistor between OUTPUT and ADJUST: R = 1.25 V / IO, rated above (1.25 V)² / R (§7.3.3, p.13).
Minimum load: the 240 Ω trap
The minimum load is 3.5 mA typ and 10 mA max (p.8). With R1 = 240 Ω the divider draws only 1.25 / 240 = 5.2 mA, enough for a typical part but not the guaranteed maximum. If the real load can drop to zero, either use R1 ≤ 125 Ω (10 mA) or make sure the load always draws the difference (§7.4.3, p.14).
Headroom and dropout
TI recommends 3 V of headroom (VIN − VOUT) for full current at the lowest temperature (§7.3.1, p.13; §7.4.2, p.14). The datasheet gives dropout only as typical curves; there is no guaranteed dropout value in the Electrical Characteristics, so this page quotes none. Design to the 3 V figure.
Capacitors and protection diodes (p.15)
- Input: 0.1 µF or 1 µF ceramic or tantalum. It is required if the regulator is more than 6 in (about 15 cm) from the filter capacitors (p.1, §8.4 p.23).
- CADJ (10 µF from ADJUST to GND) raises ripple rejection from 57 dB to 64 dB typical at 120 Hz (p.8). If you fit it, add diode D2 so CADJ can’t discharge into the regulator.
- CO improves transient response but isn’t needed for stability. If you fit it, add D1 from OUTPUT to INPUT. TI’s figures use a 1N4002.
Heat
PD = (VIN − VOUT) × IL + VIN × IG (Equation 9, p.23). Required RθJA = (TJ(max) − TA(max)) / PD (Equations 10–11), and a heat sink needs θHA ≤ RθJA − (θCH + RθJC) (Equation 12, p.24).
Example: 15 V in, 5 V out, 1 A is about 10 W. With TJ 125 °C and TA 40 °C the whole path must be ≤ 8.5 °C/W, far below even the 23.5 °C/W JEDEC figure for KCS. A heat sink is mandatory.
Pitfalls specific to the LM317 in TO-220
- Ordering “LM317T” from TI. That code doesn’t exist at TI; the TO-220 is LM317KCS, which is NRND (Addendum p.1).
- Mounting the tab directly on a grounded chassis. The tab is OUTPUT (p.5).
- Copying the 240 Ω divider for a circuit with no load. 5.2 mA is below the 10 mA guaranteed minimum (p.8).
- Running with 1–2 V of headroom and expecting 1.5 A. TI specifies everything from 3 V up (pp.6, 8, 13).
- Assuming 1.5 A at high input-output voltage. The guaranteed limit falls to 0.15 A at 40 V (p.8).
- Swapping in a “new chip” thermal number. TO-220 exists only as the legacy chip, so use the §6.4 values, not §6.5 (p.7).
Related pages
- Linear voltage regulators
- LM317 alternatives
- AMS1117-ADJ calculator: same divider idea for 1117-family regulators
- Power calculator for the PD sum
FAQ
- What is the LM317T pinout? Per TI Figure 5-2 (TO-220, top view): 1 ADJUST, 2 OUTPUT, 3 INPUT; the tab is OUTPUT (TI p.5).
- Is LM317T a TI part number? No. TI’s TO-220 orderable is LM317KCS, currently NRND. SOT-223 (LM317DCYR) and TO-263 (LM317KTTR) are Active.
- What resistors give 12 V from an LM317? R1 = 240 Ω and R2 = 2.05 kΩ give about 12.03 V typical; tolerance spans roughly 11.45–12.61 V.
- What is the minimum load for an LM317? 3.5 mA typical, 10 mA maximum (at 40 V differential), so size R1 or the load accordingly.
- What dropout does an LM317 have? TI gives no guaranteed dropout figure, only typical curves, and recommends 3 V of headroom.
- Does an LM317 need capacitors? Not for stability; TI recommends 0.1–1 µF on the input if far from the filter caps, and optional CADJ/CO with protection diodes.
Related (internal links)
Sources (retrieved 2026-10-26)
- Texas Instruments, LM317 3-Pin Adjustable Regulator, SLVS044Z (revised April 2025): https://www.ti.com/lit/ds/symlink/lm317.pdf
Disclaimer
Values are copied from TI SLVS044Z (revised April 2025) and its Package Option Addendum, read on 26 Oct 2026. TO-220 data are for the TI legacy chip; other makers’ “LM317T” parts have their own datasheets. Lifecycle status (NRND) can change, so check TI before ordering.
Reviewed by: Jasper · Updated: 2026-10-26