STMicroelectronics LM123K

Overview

LM123K from STMicroelectronics is a three-terminal, fixed 5V positive voltage regulator delivering up to 3A output current with internal current and thermal limiting, 0.01Ω typical output impedance, and minimum input voltage of 7.5V. It operates over -55°C to +150°C junction temperature and is housed in a TO-3 metal can package for high-power industrial power supplies.

For engineers reviewing the LM123K datasheet, LM123K pinout, LM123K application, or LM123K equivalent, this device is selected for high-reliability 5V/3A regulation where thermal robustness, short-circuit immunity, and long-term stability (≤35mV) are critical in aerospace, test equipment, and legacy industrial control systems.

Technical Context

The LM123K implements a monolithic bipolar linear regulator architecture with integrated foldback current limiting, thermal shutdown, and power dissipation limiting-ensuring safe operation up to 30W. Its design maintains tight output voltage tolerance (4.6V–5.4V across full temp/load range) without external compensation.

It requires no output capacitor for stability into resistive loads and achieves 40µV_RMS output noise (10Hz–100kHz) and ≥50dB ripple rejection at 120Hz. Dropout voltage is not specified as a standalone parameter but is implied by the 7.5V minimum input requirement at 3A load.

Key Specifications

Pinout & Package

LM123K is supplied in a hermetically sealed TO-3 metal can package with three terminals: Input (collector), Output (emitter), and Ground (case). The case serves as the thermal and electrical ground path, requiring insulated mounting hardware when chassis ground isolation is needed.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar fixed-output linear regulator applications.

Compared with LM323K and LM223K, the LM123K uniquely supports operation down to -55°C while retaining full 3A output and 30W dissipation capability-making it the sole choice for space, defense, and deep-cold industrial deployments requiring guaranteed startup and regulation at extreme low temperatures.

Availability

LM123K is available at Aetrix Electronics and suitable for aerospace power distribution, industrial test equipment, military communication radios, and legacy industrial control panels requiring stable component supply across extended temperature ranges and long product lifecycles.

Supply support for LM123K 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, MCU, and sensor solutions for industrial, automotive, and consumer markets.

The LM123 series belongs to ST's legacy linear regulator product line, engineered specifically for high-reliability, high-current fixed-voltage regulation in mission-critical analog and mixed-signal systems where robustness outweighs switching efficiency.

FAQ

Is LM123K pin-compatible with LM323K and LM223K?

Yes-LM123K, LM223K, and LM323K share identical TO-3 mechanical footprint, terminal arrangement (Input–Output–Case), and basic electrical connections. All three require the case to be electrically grounded and thermally coupled to a heatsink. No PCB layout change is needed when substituting within their respective temperature-rated ranges.

Does LM123K require an output capacitor for stability?

No-LM123K is internally compensated and stable into purely resistive loads with no output capacitor required. An output capacitor may be added to improve transient response or reduce residual ripple, but it is not necessary for loop stability per the datasheet's Figure 12 and text on page 6.

What is the maximum allowable power dissipation for LM123K at 25°C ambient?

At 25°C ambient, using a heatsink with ≤25°C/W total thermal resistance (junction-to-ambient), LM123K can safely dissipate up to 30W-matching its absolute maximum rating. This assumes proper mounting torque, thermal interface material, and unrestricted airflow. Derating is required above 25°C ambient per the R_thj-amb = 35°C/W spec for TO-3.

Can LM123K be used in adjustable configurations?

No-LM123K is a fixed 5V output device with no adjustment pin or feedback node. It cannot be configured for other voltages. For adjustable 3A regulation, ST recommends pairing an LM101A op-amp with external pass transistors (see Figure 15), or selecting modern alternatives like the L200 or LR8N series with dedicated adjust pins.