STMicroelectronics LM138K

Overview

LM138K from STMicroelectronics is a three-terminal adjustable positive voltage regulator delivering guaranteed 5 A continuous and 7 A peak output current, with 1.2 V to 32 V adjustable output range, ±1% reference voltage accuracy (1.19–1.29 V), and thermal/overcurrent/safe-area protection. It operates from –55 °C to 150 °C and is used in high-reliability industrial power supplies requiring stable, high-current regulation under wide temperature extremes.

For engineers reviewing the LM138K datasheet, LM138K pinout, LM138K application, or LM138K equivalent, key selection criteria include guaranteed 5 A output at –55 °C, time-dependent current limiting (12 A peak), TO-3 package thermal resistance (1.4 °C/W junction-to-case), ripple rejection up to 75 dB with 10 µF adjustment bypass, and minimum load current of 3.5 mA.

Technical Context

The LM138K implements a floating architecture where only the input-to-output differential voltage (max 35 V) is sensed-enabling regulation in high-voltage systems up to several hundred volts when referenced appropriately. Its internal bandgap reference (1.24 V typ.) drives an error amplifier controlling a series pass transistor with integrated thermal shutdown and safe-area protection.

Time-dependent current limiting uses internal timing circuitry to allow 12 A for ≤0.5 ms transients while reducing to 5–8 A under sustained load, preventing thermal runaway. Adjustment pin current remains tightly controlled (45–100 µA, ΔIADJ ≤5 µA), minimizing output error across line/load variations.

Key Specifications

Pinout & Package

LM138K is housed in a hermetically sealed steel TO-3 package with electrically isolated metal case (pin 2), optimized for high-power thermal management and EMI shielding. The case serves as the output terminal and provides low-inductance ground return path.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LM338K and LM238K, the LM138K uniquely delivers full 5 A performance at –55 °C and matches their thermal robustness-making it the sole choice for cryogenic or space-grade linear regulation where no derating can be tolerated.

Availability

LM138K is available at Aetrix Electronics and suitable for industrial power supplies, test equipment power stages, aerospace avionics, and downhole instrumentation requiring stable component supply across extended temperature ranges and long product lifecycles.

Supply support for LM138K 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 microcontrollers, power ICs, sensors, and discrete components for industrial, automotive, and consumer markets.

The LM138K belongs to ST's legacy high-reliability linear regulator product line, engineered specifically for mission-critical analog power systems demanding guaranteed performance across extreme environmental conditions.

FAQ

What is the maximum input-to-output voltage differential for LM138K?

The absolute maximum input-output differential voltage is 35 V, as specified in Table 2 of the datasheet. Exceeding this risks permanent damage to the internal pass transistor. This limit applies regardless of total input voltage-e.g., a 300 V input can be regulated only if output is set to ≥265 V. Designers must ensure VI – VO stays within 35 V under all operating conditions including transients.

Does LM138K require an input bypass capacitor?

A 0.1 µF ceramic or 1 µF solid tantalum input bypass capacitor is recommended when the device is located more than 6 inches from bulk filter capacitors. Without it, high-frequency noise or switching transients on the input rail may cause instability or degraded line regulation. The TO-3 package's low inductance helps, but bypassing remains best practice for robust EMI immunity in noisy industrial environments.

How does the adjustment pin current affect output accuracy?

The adjustment pin draws 45–100 µA, which flows through R2 in the feedback divider and introduces an error term: VO = VREF × (1 + R2/R1) + IADJ × R2. For R1 = 240 Ω and R2 = 720 Ω, this adds ~72 mV error at 100 µA. Using lower R2 values or compensating in design mitigates this-ST specifies ΔIADJ ≤5 µA across line/load, ensuring predictable, bounded error.

Can LM138K be used in parallel for higher current?

No-LM138K lacks built-in current-sharing features like ballast resistors or master-slave control. Attempting parallel operation causes thermal runaway due to mismatched reference voltages and thermal coefficients. For >5 A, ST recommends using a single LM138K with adequate heatsinking or selecting a switching solution; the datasheet explicitly warns against paralleling without external balancing circuitry.