STMicroelectronics LM317P

Overview

LM317P from STMicroelectronics is a monolithic adjustable positive voltage regulator in TO-220 single-gauge package, delivering 1.5 A output current with 1.2–37 V adjustable output range, 0.1% line/load regulation, and integrated thermal shutdown, current limiting, and safe operating area (SOA) protection-used in lab power supplies, battery chargers, and industrial DC power rails.

For engineers reviewing the LM317P datasheet, LM317P pinout, LM317P application, or LM317P equivalent, key selection considerations include input-output differential voltage limit (40 V), reference voltage tolerance (1.20–1.30 V), minimum load current requirement (3.5–10 mA), and thermal resistance (R_thJA = 50 °C/W for TO-220).

Technical Context

The LM317P operates as a floating three-terminal regulator with an internal 1.25 V bandgap reference between output and adjust pins, enabling precise voltage setting via external resistor divider (V_O = 1.25 × (1 + R₂/R₁) + I_ADJR₂). Its quiescent current returns to the output terminal, imposing a minimum load current to maintain regulation.

It features SOA-controlled pass transistor operation, thermal foldback during overload, and supply voltage rejection of 65–80 dB (120 Hz, with 10 µF CADJ). The device supports high-voltage floating configurations since it only senses VI–VO differential-not absolute input voltage-enabling use in >100 V systems with proper isolation.

Key Specifications

Pinout & Package

LM317P is supplied in TO-220 single-gauge package (ECOPACK® compliant), with metal tab electrically connected to the output pin and suitable for direct heatsinking.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LM317T, the LM317P offers identical regulation performance and thermal behavior in the same TO-220 package; compared with TLV1117-33CDCY, it provides full adjustability and higher current but requires greater input-output headroom and dissipates more quiescent power.

Availability

LM317P is available at Aetrix Electronics and suitable for lab power supplies, battery charging circuits, industrial sensor power rails, and programmable reference generators requiring stable component supply, long-term obsolescence management, and traceable sourcing.

Supply support for LM317P 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, designing and manufacturing analog, power, microcontroller, and sensor solutions for industrial, automotive, and consumer markets.

The LM317 series belongs to ST's legacy linear regulator product line, engineered for robustness, wide operating range, and ease of use in non-switching power applications where low noise and simplicity outweigh efficiency concerns.

FAQ

What is the minimum load current required for stable regulation with LM317P?

The LM317P requires a minimum load current of 3.5–10 mA (depending on VI–VO and temperature) to maintain regulation. This is due to its internal design that routes quiescent current to the output pin. If load current falls below this threshold-such as in light-load or standby conditions-the output voltage may rise uncontrollably. A bleeder resistor from OUT to GND is commonly used to guarantee compliance.

Can LM317P be used in a negative voltage configuration?

No, the LM317P is a positive adjustable regulator only and cannot regulate negative voltages. It lacks the internal architecture for inverted operation. For negative adjustable regulation, ST's LM337P (complementary negative counterpart) must be used instead, with matching pinout and specifications.

How does the adjustment pin current affect output voltage accuracy?

The adjustment pin draws 50–100 µA, which flows through R2 in the feedback divider and introduces an error term (I_ADJ × R2) in the output equation V_O = 1.25 × (1 + R₂/R₁) + I_ADJR₂. To keep error <1%, R2 should be ≤1 kΩ for 1.25 V reference. Higher R2 values require correction or use of a buffer amplifier.

Is a heat sink always required for LM317P operation?

A heat sink is required whenever power dissipation exceeds the package's natural convection capability. With R_thJA = 50 °C/W and max junction temperature of 125 °C, even at 1 A load and 10 V dropout (10 W dissipation), junction temperature rises ~500 °C above ambient-far exceeding limits. Derating curves in DS0433 show heatsinking is mandatory above ~0.5 A at typical dropout voltages.