Texas Instruments LM117K

Overview

LM117K from STMicroelectronics is a monolithic positive adjustable voltage regulator in TO-3 metal can package, delivering ≥1.5A output current with 1.2V to 37V adjustable output range, 0.1% line/load regulation, and internal current limiting, thermal shutdown, and SOA protection. It operates as a floating regulator for high-voltage applications such as lab power supplies and industrial DC rails.

For engineers reviewing the LM117K datasheet, LM117K pinout, LM117K application, or LM117K equivalent, key selection criteria include input-output differential voltage (≤40V), minimum load current (3.5–5mA), reference voltage (1.25V ±0.05V), junction temperature range (–55°C to +150°C), and thermal resistance (4°C/W junction-to-case).

Technical Context

The LM117K implements a bandgap-referenced error amplifier driving a series pass transistor, with all quiescent current returned to the output to maintain regulation under light loads. Its floating architecture allows operation with input voltages far above ground-only the 40V max Vi–Vo differential matters.

Protection circuitry includes foldback current limiting that reduces output current during overload, thermal shutdown at ~150°C junction temperature, and safe operating area (SOA) control to prevent second breakdown in the pass element. The 1.25V internal reference is stable over temperature and line variations.

Key Specifications

Pinout & Package

LM117K is housed in a hermetically sealed TO-3 metal can package with three terminals: Input (pin 1), Output (pin 2), and Adjustment (pin 3). The case is electrically connected to the output terminal, requiring insulated mounting hardware when 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 LM317K, LM117K offers wider temperature range and marginally better thermal performance; compared with TL783CK, it trades off maximum Vi–Vo for higher output current and tighter regulation-making LM117K optimal for high-reliability 1.5A adjustable supplies within 40V differential limits.

Availability

LM117K is available at Aetrix Electronics and suitable for industrial motor control bias supplies, lab bench power supplies, battery chargers, and precision current sources requiring stable component supply across extended temperature and long-lifecycle programs.

Supply support for LM117K 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, designing and manufacturing microcontrollers, power management ICs, analog chips, and discrete components for industrial, automotive, and consumer markets.

The LM117 series belongs to ST's legacy linear regulator product line, engineered specifically for high-reliability, high-current adjustable voltage regulation in harsh environments-including military, aerospace, and industrial automation systems.

FAQ

What is the minimum load current required for stable regulation with LM117K?

The LM117K requires a minimum load current of 3.5mA to 5mA (depending on junction temperature and Vi–Vo) to maintain regulation. This is due to its design where all quiescent current returns to the output node. Below this threshold, output voltage rises uncontrollably. A bleeder resistor or light-load dummy load is mandatory in no-load or ultra-low-load applications.

Can LM117K be used in parallel for higher current output?

No-LM117K is not designed for direct parallel operation due to mismatch in reference voltage and load regulation. Uneven current sharing will occur, risking thermal runaway. For >1.5A, use a single higher-rated regulator (e.g., LM138K) or implement active current-balancing circuitry with external op-amps and sense resistors.

Is a heat sink mandatory for LM117K in TO-3 package?

A heat sink is mandatory whenever power dissipation exceeds ~2W. With R_th(j-c) = 4°C/W and T_j(max) = 150°C, even moderate Vi–Vo differentials (e.g., 12V dropout at 1A = 12W) require substantial heatsinking. Mounting must use insulating washer and thermal compound since the TO-3 case is electrically connected to the output pin.

How does the adjustment pin current affect output voltage accuracy?

I_ADJ (50–100µA) flows through R2 in the feedback divider, adding an error term I_ADJ·R2 to the nominal V_OUT = 1.25V(1 + R2/R1). For R2 = 2.4kΩ, this introduces up to 0.24V error-so R2 should be kept ≤1kΩ or compensated via calculation. Precision designs often use a buffer amplifier to eliminate I_ADJ loading.