STMicroelectronics LM317T

Overview

LM317T from STMicroelectronics is a monolithic adjustable positive-voltage linear regulator in TO-220 package, designed for floating operation with input-to-output differential up to 40 V and output current exceeding 1.5 A. It delivers 1.2–37 V output via external resistor divider, features 1.25 V internal reference voltage (pin 3 to pin 1), 0.04 %/V typical line regulation, and integrated thermal shutdown, current limiting, and safe operating area (SOA) protection - widely used in lab power supplies, battery chargers, and precision current sources.

For engineers reviewing the LM317T datasheet, LM317T pinout, LM317T application, or LM317T equivalent, key selection considerations include its minimum load current requirement (3.5–10 mA), adjustment pin current (≤100 µA), thermal resistance junction-to-ambient (50 °C/W in TO-220), supply voltage rejection (65–80 dB), and compatibility with standard 0.1 µF input / 10 µF adjust / 1 µF output capacitor configurations.

Technical Context

The LM317T operates as a three-terminal floating regulator: it regulates voltage by maintaining a precise 1.25 V reference between output and adjustment terminals, enabling programmable output via external resistors. Its internal bandgap reference, error amplifier, and pass transistor are co-integrated on a single silicon die with SOA control circuitry.

Unlike fixed regulators, it requires no external compensation and supports remote sensing when R1 is placed near the device and R2's ground tied to load ground. Protection functions - including foldback current limiting, thermal shutdown at ~150 °C junction temperature, and SOA monitoring - operate independently of external components.

Key Specifications

Pinout & Package

LM317T is supplied in TO-220 (single-gauge) package with integral heat sink tab (electrically connected to output pin). Mounting hole allows mechanical attachment to heatsink; tab must be insulated if output is not at chassis ground.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar adjustable linear regulator applications.

Compared with LM317LZ and TLV1117-ADJ, the LM317T offers superior thermal robustness (50 °C/W R_thJA, SOA control), higher current capacity (1.5 A), and wider differential voltage range (40 V), making it the preferred choice for industrial power rails, test equipment, and reliability-critical analog subsystems.

Availability

LM317T is available at Aetrix Electronics and suitable for lab power supplies, battery chargers, precision current sources, and industrial sensor excitation circuits requiring stable component supply and long-term manufacturability.

Supply support for LM317T includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, specializing in power management, analog ICs, microcontrollers, and automotive-grade components.

The LM317 series belongs to ST's legacy linear regulator product line, engineered for industrial-grade reliability, wide operating temperature ranges (0 to 125 °C), and robust fault protection - targeting applications where simplicity, ruggedness, and field-proven performance outweigh ultra-low quiescent current or ultra-small footprint requirements.

FAQ

What is the minimum load current required for stable regulation with LM317T?

The LM317T requires a minimum load current of 3.5 mA (typical) to 10 mA (maximum) to maintain regulation, as specified in Table 4 of DS0433 Rev 22. This is due to internal quiescent current being returned to the output pin; insufficient load causes output voltage to rise uncontrollably. A 120 Ω resistor from output to ground satisfies this requirement for 10 V outputs.

Can LM317T be used in high-input-voltage applications like 60 V DC input?

Yes - but only if the input-to-output differential voltage stays within the absolute maximum rating of 40 V. For a 60 V input, the output must be set ≥20 V (e.g., 24 V) to comply. The device's floating architecture allows it to regulate high-voltage rails safely, provided thermal design accommodates power dissipation (P_D = (V_IN − V_OUT) × I_OUT).

Why does the LM317T require a capacitor on the adjust pin?

A 10 µF capacitor from adjust pin to ground (CADJ) improves ripple rejection by ~15 dB at 120 Hz, raising SVR from 65 dB to 80 dB. This capacitor stabilizes the reference node against AC perturbations and suppresses noise coupling into the feedback path - critical in low-noise applications like sensor excitation or audio preamplifier supplies.

How does thermal shutdown behave during sustained overload?

Thermal shutdown activates when junction temperature reaches ~150 °C, turning off the pass transistor until temperature drops by ~15–20 °C. During sustained overload, this results in duty-cycled operation - intermittent output - rather than permanent failure. Combined with current limiting and SOA control, this protects the device during fault conditions without requiring external circuitry.