onsemi LM317T

Overview

LM317T from onsemi is a 3-terminal adjustable positive linear voltage regulator delivering up to 1.5 A across an output range of 1.2 V to 37 V, featuring internal thermal shutdown, current limiting, and safe-area compensation. It operates within 0°C to +125°C junction temperature and uses only two external resistors for output programming - widely deployed in local on-card regulation and lab power supplies.

For engineers reviewing the LM317T datasheet, pinout, applications, or equivalent options, this page delivers verified technical context, package-specific pin roles, real-world design meaning of key specs, and validated alternative options for industrial power management, programmable current sources, and precision analog supply design.

Technical Context

The LM317T implements a floating regulator architecture where a stable 1.25 V reference (V_ref) is maintained between the output and adjustment terminals. This reference drives a constant programming current through R1, enabling precise output voltage calculation via V_out = 1.25 V × (1 + R2/R1) + I_Adj × R2.

It integrates protection circuitry including thermal overload shutdown at ~180°C, foldback current limiting, and safe-area compensation to prevent second breakdown during high-voltage/low-current operation. Its TO-220 package provides direct heatsink mounting with θ_JC = 5.0°C/W, supporting continuous operation under thermally demanding conditions.

Key Specifications

Pinout & Package

LM317T is housed in a TO-220 package (Case 221AB), with the metal tab electrically connected to Pin 2 (V_out) and serving as the primary thermal path to an external heatsink.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LM317K, the LM317T offers lower cost and standard TO-220 heatsink compatibility but less thermal margin in sealed enclosures; versus TLV1117-ADJ, it delivers higher current and wider voltage range but requires greater input headroom and lacks LDO efficiency.

Availability

LM317T is available at Aetrix Electronics and suitable for industrial power supplies, test equipment design, and analog signal chain regulation requiring stable component supply and long-lifecycle support.

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

onsemi (formerly ON Semiconductor) is a global semiconductor supplier focused on energy-efficient power management, analog, sensing, and connectivity solutions for automotive, industrial, and cloud infrastructure markets.

The LM317T belongs to onsemi's legacy linear regulator product line, engineered for robustness in harsh environments and designed to simplify discrete power system design with integrated protection and wide operating margins.

FAQ

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

The LM317T requires a minimum load current of 3.5 mA (typical) to 10 mA (max) to maintain regulation, especially when the input-output differential exceeds 15 V. This is due to internal quiescent current being returned to the output pin; insufficient load causes output voltage to rise uncontrollably. For reliable operation, design with ≥10 mA minimum load or add a bleeder resistor across the output.

Can the LM317T be used as a constant-current source, and how is it configured?

Yes, the LM317T can function as a precision constant-current source. Connect a single resistor R between the Adjust and Output pins; output current is I_out ≈ 1.25 V / R. For example, a 125 Ω resistor sets 10 mA. The LM317T maintains this current regardless of load voltage (within its 1.2–37 V compliance range), making it ideal for LED drivers and sensor excitation circuits.

What is the maximum allowable input-to-output voltage differential for the LM317T?

The LM317T supports a maximum input-to-output voltage differential (V_in – V_out) of 40 V DC, with a minimum of −0.3 V. Exceeding 40 V risks internal junction breakdown. In practice, thermal limits often constrain usable differential more than absolute rating - e.g., at 1.5 A output, even 10 V differential dissipates 15 W, requiring substantial heatsinking to stay within 0°C to +125°C operating range.

Does the LM317T require an output capacitor for stability, and what value is recommended?

No, the LM317T is unconditionally stable without an output capacitor. However, adding a 1.0 µF tantalum or 25 µF aluminum electrolytic capacitor improves transient response and suppresses ringing caused by long PCB traces or inductive loads. Avoid ceramic capacitors below 10 µF unless series resistance ≥1 Ω is added, as low-ESR ceramics may induce instability in some layouts.

How does the LM317T handle thermal overload, and is external thermal protection needed?

The LM317T incorporates fully integrated thermal shutdown that activates at approximately 180°C junction temperature, halting output until die cools sufficiently. This protection is self-resetting and requires no external components. External thermal protection is unnecessary for standard operation - but proper heatsink sizing per θ_JA = 65°C/W (TO-220) remains critical to avoid repeated cycling or premature aging.