Voltage Regulators & LDOsActive

LM317T

Manufacturer: STMicroelectronics

Figures from the datasheet: STMicroelectronics DS0433, rev. Rev 22, December 2021

Adjustable 1.2V to 37V positive linear voltage regulator capable of supplying in excess of 1.5A in a TO-220 package with ADJ-OUT-IN pinout and internal thermal protection.

PackagesTO-220

Official Datasheet (PDF)

Technical Overview

The LM317T is an adjustable 3-terminal positive linear voltage regulator manufactured by STMicroelectronics (Document DS0433, Rev 22, December 2021). Housed in the through-hole TO-220 package with an ADJ-OUT-IN pinout (Pin 1 ADJ, Pin 2 OUT, Pin 3 IN, with the metallic tab internally connected to OUT), it supplies in excess of 1.5A when VI - VO <= 15V and internal dissipation is within rated limits, with a maximum input-to-output differential of 40V. Operating characteristics under Table 4 include a nominal reference voltage VREF of 1.25V (1.20V to 1.30V), typical line regulation of 0.01%/V, typical load regulation of 5.0mV (for VO <= 5V), typical adjustment pin current of 50 µA, and minimum load current requirement up to 10mA at 40V differential. The regulator integrates internal current limiting, thermal overload protection, and safe-operating-area (SOA) protection.

Package Pinout & Alternate Functions

Interactive package pinout and pin mapping for TO-220-3.

Package illustration, not a dimensioned mechanical drawing.

3 / 3 pins

Pin name	Type	5V Tolerant	Alternate functions / details
ADJ	passive	—	Output adjustment terminal (maintains 1.25V nominal reference between OUT and ADJ; adjustment pin current ~50 µA typical)
OUT	power out	—	Regulated output voltage terminal (internally connected to the TO-220 metallic mounting tab)
IN	power in	—	Unregulated DC input voltage terminal (input-to-output differential VI - VO rated up to 40V max)

Key Specifications

Parametric attributes

Parameter	Min	Typical	Max	Unit	Test condition
Reference Voltage(VREF)	1.2	1.25	1.3	V	VI - VO = 2.5 V to 40 V, IO = 10 mA to 1.5 A, PD <= 20 W, TJ = 0°C to 125°C
Line Regulation(Line_reg)	—	0.01	0.04	%/V	VI - VO = 3 V to 40 V, IO = 500 mA, TJ = 25°C
Load Regulation(Load_reg)	—	5.0	25	mV	VI - VO = 5 V, VO <= 5 V, IO = 10 mA to 1.5 A, TJ = 25°C
Adjustment Pin Current(IADJ)	—	50	100	µA	VI - VO = 5 V, IO = 500 mA, TJ = 0°C to 125°C
Adjustment Pin Current Change(delta_IADJ)	—	0.2	5	µA	VI - VO = 2.5 V to 40 V, IO = 10 mA to 1.5 A, TJ = 0°C to 125°C
Minimum Load Current(IO_min)	—	3.5	10	mA	VI - VO = 40 V, TJ = 0°C to 125°C
Maximum Output Current(IO_max)	1.5	2.2	—	A	VI - VO <= 15 V, PD < 20 W, TJ = 0°C to 125°C
Output Noise Voltage(eN)	—	0.003	—	%	VI - VO = 5 V, IO = 500 mA, B = 10 Hz to 100 kHz, TJ = 25°C (percentage of VO)
Supply Voltage Rejection(SVR)	66	80	—	dB	VI - VO = 5 V, IO = 500 mA, CADJ = 10 µF between ADJ and GND, f = 120 Hz, TJ = 25°C
Operating Junction Temperature (Absolute Maximum)(TJ)	0	—	125	°C	LM317, Table 1 absolute maximum junction-temperature range; not an ambient-temperature rating

Absolute Maximum Ratings

Storage temperature	-65 – 150 °C
Thermal resistance θJA (TO-220)	50 °C/W
Thermal resistance θJC (TO-220)	5 °C/W

Hardware Engineering Gotchas

Real-world hardware caveats, PCB layout constraints and silicon errata to know before manufacturing.

Critical

Mounting Tab Electrically Connected to VOUT, NOT Ground

The TO-220 metallic mounting tab is internally connected to Pin 2 (OUT). Mounting the regulator directly to a grounded metal chassis or shared heatsink ties the regulated output to ground or interconnects outputs of multiple regulators on the same heatsink. If grounded, the regulated output collapses and internal protection limits current according to thermal and voltage conditions.

Mitigation

When mounting to a grounded heatsink or shared metallic surface, electrically isolate the mounting tab using an electrically insulating and thermally conductive pad together with an insulating shoulder washer on the mounting screw. Verify electrical isolation with an ohmmeter before applying power.

Critical

Minimum Load Current Requirement (10mA Maximum at 40V Differential)

The internal bias current of the LM317 returns to circuit common through the OUT pin, requiring a minimum load current of 3.5mA typical and up to 10mA (at VI - VO = 40V over operating temperature) to maintain proper regulation. If total output load current is insufficient, the output voltage can rise toward the unregulated input rail.

Mitigation

Size the output voltage divider so that programming resistor R1 (between OUT and ADJ) draws the required minimum load current: check VREF(min) / R1(max) >= 10 mA using the applicable reference-voltage conditions and worst-case resistor tolerance, including temperature. A nominal 120 Ω resistor draws 1.25 V / 120 Ω = 10.4 mA nominally; at VREF = 1.20 V and the +1% resistance extreme (R1 = 121.2 Ω), the calculated current is approximately 9.90 mA. Verify minimum-load requirements for the operating envelope.

Warning

Discharge Protection Diodes for Capacitive Loads (CADJ and COUT)

When using an adjustment pin bypass capacitor CADJ (recommended for improving ripple rejection up to 80dB typical) or an output filter capacitor COUT, sudden loss or shorting of the input supply or output rail can cause capacitors to discharge back into internal regulator nodes. Specifically, an input short discharges COUT back through the regulator toward IN, while an output short discharges CADJ into the ADJ terminal.

Mitigation

ST DS0433 Section 6 describes external protection diodes as good practice with bypass or output capacitors: D1 has anode at OUT and cathode at IN to bypass COUT discharge around the regulator; D2 has anode at ADJ and cathode at OUT to discharge CADJ into the output rail. Select diode voltage, discharge-current and energy ratings for the actual capacitances, voltages and fault conditions.

Reference Circuits

LM317 Constant Current Source

Topology: ST LM317T current regulator with a series sense resistor between OUT and IOUT.

Inspect schematic Open in Web CAD Editor

Official Documentation & Cited Datasheets

Primary semiconductor vendor datasheets, errata, and application notes.

DATASHEETSTMicroelectronics

LM217, LM317: 1.2 V to 37 V adjustable voltage regulators

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Technical Overview

Package Pinout & Alternate Functions

Key Specifications

Parametric attributes

Absolute Maximum Ratings

Hardware Engineering Gotchas

Reference Circuits

LM317 Constant Current Source

Official Documentation & Cited Datasheets

LM217, LM317: 1.2 V to 37 V adjustable voltage regulators

Help keep this page accurate

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