LM317Adjustable linear regulator
Set the voltage. Understand the heat.
LM317 is a three-terminal adjustable linear regulator. Two external resistors set its output, while an internal reference and pass stage maintain approximately 1.25 V between OUTPUT and ADJUST. This reference follows STMicroelectronics LM317T and LM317D2T-TR. Its output can be adjusted over the manufacturer’s 1.2–37 V range when input headroom, minimum load and thermal conditions are met. The metal tab is connected to OUTPUT. A useful design starts with the feedback equation, then checks bypass capacitors, load current and heat. The current rating alone does not determine how much power the package can dissipate.
Adjustment & feedbackRegulated outputInput supply
LM317TTO-220Front view · marking toward you · leads down
LM317D2T-TRD²PAKTop view · thermal pad is pin 2 (OUTPUT)
Click a lead or use the arrow keys. Front view · marking toward you · leads down
PIN 1 · Adjustment & feedback
Feedback referenceADJUST
Function
The control loop holds approximately 1.25 V between OUTPUT and ADJUST.
How to connect
Connect R1 from OUTPUT to ADJUST and R2 from ADJUST to the load return.
Electrical note
Adjustment current flows out of this pin; include IADJ × R2 when estimating output voltage. This is not a ground pin.
PIN 2 · Regulated output
Regulated outputOUTPUT
Function
Supplies the load and the external feedback divider. The metal tab is also OUTPUT.
How to connect
Connect the load and the top of R1 here. Keep the R1 connection close to the output lead.
Electrical note
The exposed TO-220 tab and D²PAK thermal pad are electrically live at output voltage; isolate a grounded heatsink where needed.
PIN 3 · Input supply
Unregulated inputINPUT
Function
Feeds the pass stage from a DC voltage sufficiently above the requested output.
How to connect
Place a 0.1 µF input bypass close to the device when required by the supply wiring.
Electrical note
The 40 V absolute limit is input-to-output differential, not a recommended operating input or an unconditional output range.
At a glance
Adjustable linear regulator
The regulated quantity is the voltage between OUTPUT and ADJUST. The external divider determines the output relative to ground.
1.25 V · OUT to ADJNominal reference
1.2–37 V · with adequate headroomAdjustable output
0–125 °C · LM317 gradeOperating junction
3 ADJ · OUT · INElectrical connections
Operating profile
LM317T
Operating characteristics
Key parameters and their test conditions
- Output adjustment
- 1.2–37 VConditions / notes: Requires input headroom, load and suitable cooling
- Reference voltage
- 1.2 / 1.25 / 1.3 VConditions / notes: Minimum / typical / maximum; Table 4 conditions
- Junction temperature
- 0–125°CConditions / notes: Selected LM317 grade
- Minimum load at 40 V differential
- 3.5 mA typical; 10 mA maximumConditions / notes: Output may rise if the load is insufficient
- Adjustment current
- 50 µA typical; 100 µA maximumConditions / notes: Include its contribution through R2
Package details
Follows the selected physical package
- Manufacturer
- STMicroelectronics
- Part number
- LM317T
- Package
- TO-220
- Connections
- 3 pins
- Mounting
- Through-hole
- Lead pitch
- 2.54 mm
- Package code
- T
- Thermal reference
- Cooling and mounting dependent; see DS0433
Connections by function
Select a pin to open its function, electrical notes and wiring guidance.
Adjustment & feedback1 pins
Regulated output1 pins
Input supply1 pins
Adjustment & feedback1 pins
Regulated output1 pins
Input supply1 pins
Create the reference
The regulator maintains approximately 1.25 V from OUTPUT to ADJUST. Unlike a 7805, the adjustment terminal is not tied directly to ground.
Set output with two resistors
R1 runs from OUTPUT to ADJUST; R2 runs from ADJUST to the return. Their ratio sets the output, with a small additional term from adjustment-pin current.
Keep the loop within its limits
The source must provide headroom and the load must draw sufficient current. Input-to-output voltage and current become heat in the pass stage.
With R1 = 120 Ω and R2 = 360 Ω, the ideal result is 5.00 V. Using 50 µA typical adjustment current gives approximately 5.018 V; tolerances and operating conditions add further variation.
Pin reference
Select a pin to explore its function and connection.
3 pins
| Pin | Signal | Type | Function |
|---|---|---|---|
| ADJUST | Feedback reference | The control loop holds approximately 1.25 V between OUTPUT and ADJUST. | |
| OUTPUT | Regulated output | Supplies the load and the external feedback divider. The metal tab is also OUTPUT. | |
| INPUT | Unregulated input | Feeds the pass stage from a DC voltage sufficiently above the requested output. |
No pins match. Try a signal name or choose All pins.
Operating characteristics
ST LM317 · not LM217 or LM317B
Manufacturer specifications 5 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Output adjustment | 1.2–37 V | Requires input headroom, load and suitable cooling |
| Reference voltage | 1.2 / 1.25 / 1.3 V | Minimum / typical / maximum; Table 4 conditions |
| Junction temperature | 0–125°C | Selected LM317 grade |
| Minimum load at 40 V differential | 3.5 mA typical; 10 mA maximum | Output may rise if the load is insufficient |
| Adjustment current | 50 µA typical; 100 µA maximum | Include its contribution through R2 |
No specifications match. Try a parameter name or a unit.
Base conditions: VIN − VOUT = 5 V, IOUT = 500 mA, IMAX = 1.5 A, PMAX = 20 W unless stated. These are test conditions, not a package dissipation guarantee.
Regulation and rejection
Manufacturer conditions retained
Manufacturer specifications 5 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Line regulation at 25°C | 0.01%/V typical; 0.04%/V maximum | VIN − VOUT = 3–40 V |
| Line regulation over temperature | 0.02%/V typical; 0.07%/V maximum | VIN − VOUT = 3–40 V |
| Load regulation, output ≤5 V | 5 mV typical; 25 mV maximum | 25°C; IOUT = 10 mA to IMAX |
| Ripple rejection without CADJ | 65 dB typical | 120 Hz; 25°C |
| Ripple rejection with CADJ | 66 dB minimum; 80 dB typical | CADJ = 10 µF; 120 Hz; 25°C |
No specifications match. Try a parameter name or a unit.
The adjustable output and test load affect which row applies; consult Table 4 for complete conditions.
A nominal 5 V supply
9 V input · 120 Ω / 360 Ω divider
Manufacturer specifications 6 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| R1 · OUTPUT to ADJUST | 120 Ω | Reference current ≈ 1.25 / 120 = 10.42 mA |
| R2 · ADJUST to GND | 360 Ω | Nominal resistor ratio 3:1 |
| Ideal output | 5.00 V | 1.25 × (1 + 360 / 120) |
| Including typical IADJ | 5.018 V | Adds 50 µA × 360 Ω = 18 mV |
| Input bypass | 0.1 µF | Place close to the input and return |
| Output capacitor | 1 µF tantalum | Illustrative transient improvement from manufacturer guidance |
No specifications match. Try a parameter name or a unit.
Use resistor and capacitor ratings appropriate to the circuit. The drawing omits optional adjustment bypass and its associated protection diode.
Headroom and thermal design
Current ratings require cooling
Manufacturer specifications 5 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Pass-stage estimate | PD ≈ (VIN − VOUT) × IOUT | Include divider current in IOUT |
| TO-220 RθJC | 5°C/W | Package reference; actual mounting matters |
| D²PAK RθJC | 3°C/W | Package reference; actual PCB copper matters |
| Overload protections | Current, thermal and safe-area protection | Do not use protection as normal regulation |
| High differential voltage | Available current decreases | See manufacturer curves and operating limits |
No specifications match. Try a parameter name or a unit.
The LM317 is a linear regulator: excess voltage is dissipated rather than efficiently converted.
Package and stress limits
LM317T · TO-220
Manufacturer specifications 5 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Connections | 1 ADJUST · 2 OUTPUT · 3 INPUT | Follow the selected package orientation |
| Tab / thermal pad | OUTPUT · pin 2 | Not GND |
| Input-to-output absolute maximum | 40 V | Stress rating; not a normal design target |
| Storage temperature | −65 to +150°C | Unpowered |
| Lead pitch | 2.54 mm | Verify the selected manufacturer drawing |
No specifications match. Try a parameter name or a unit.
The same three numbers have different functions from a 7805. Do not substitute without checking connections.
Adjustable supplies
Set a local rail with a fixed divider or an appropriately designed adjustment network.
Manufacturer application guideCurrent regulation
Use the reference across a current-setting resistor in the manufacturer’s constant-current configuration.
Manufacturer application guideRegulator experiments
Explore resistor ratios, minimum load, dropout and dissipation with a low-voltage supply.
Manufacturer application guideTake it into ProTech
Find LM317 in the Components Library to place its schematic symbol and connect its pins.
Reference page · circuit simulation model not yet availableOpen ProTech