STMicroelectronics LM138K Overview

The LM138K is a high-performance voltage regulator from STMicroelectronics, designed to provide adjustable output voltages with high current capacity. This linear voltage regulator is capable of delivering up to 5A of output current, making it suitable for a wide variety of applications requiring regulated power, especially in systems with high current demands. The LM138K offers excellent thermal stability and low dropout voltage, making it ideal for use in power supplies, battery chargers, and industrial equipment where precision and reliability are critical.

## Key Features

* Adjustable Output Voltage: The LM138K provides an adjustable output voltage range of 1.25V to 37V, making it versatile and adaptable for various applications requiring different output voltages.
* High Output Current: The device is capable of providing a maximum output current of 5A, making it suitable for powering high-current loads in both commercial and industrial systems.
* Low Dropout Voltage: The LM138K features a low dropout voltage (typically 1.3V) at 5A, ensuring efficient performance even when the input voltage is just slightly higher than the output voltage.
* Thermal Protection: The LM138K is equipped with thermal shutdown and current limiting features, providing protection against thermal runaway and excessive current that could damage the device or the load.
* Easy Adjustment: The LM138K uses a simple two-resistor adjustment to set the output voltage, offering flexibility in design and ease of integration into various applications.
* Good Ripple Rejection: With its excellent ripple rejection of typically 80dB at 120Hz, the LM138K ensures a stable and clean output voltage, which is essential for powering sensitive analog and digital circuits.

## Electrical Specifications

* Input Voltage Range: The LM138K operates with an input voltage of up to 40V. This wide input voltage range makes it compatible with a variety of power sources, including both regulated and unregulated supplies.
* Output Voltage: The output voltage is adjustable between 1.25V and 37V by selecting appropriate external resistors, giving the designer flexibility in choosing the desired output voltage for the application.
* Output Current: The LM138K can supply up to 5A of continuous output current, which is suitable for applications requiring significant power, such as motor controllers, power amplifiers, and high-power sensors.
* Dropout Voltage: The typical dropout voltage is 1.3V at 5A, which is low enough to ensure that the regulator operates efficiently, even when the input voltage is close to the output voltage.
* Quiescent Current: The LM138K has a typical quiescent current of 10mA, which is the current consumed by the regulator itself when there is no load connected, ensuring efficient operation in low-power applications.
* Output Voltage Tolerance: The LM138K provides an output voltage tolerance of ±2% at 25°C, ensuring reliable and precise voltage regulation within the specified temperature range.
* Ripple Rejection: The ripple rejection ratio is typically 80dB at 120Hz, which ensures that any noise or ripple from the input voltage does not adversely affect the output voltage.

## Thermal Characteristics

* Thermal Shutdown: The LM138K includes a thermal shutdown feature, which automatically disables the regulator if the junction temperature exceeds a safe limit, preventing overheating and potential damage to the device.
* Thermal Resistance: The junction-to-case thermal resistance (R\_thJC) is typically 3°C/W, while the junction-to-ambient thermal resistance (R\_thJA) is typically 50°C/W in a typical application. These low thermal resistance values ensure efficient heat dissipation and the ability to operate in demanding environments.
* Operating Junction Temperature: The LM138K is designed to operate within a wide temperature range of -40°C to +125°C, ensuring reliable operation in both industrial and automotive applications, even in harsh environments.

## Package Information

* Package Type: The LM138K comes in a TO-3 metal can package, which provides excellent thermal dissipation. The metal can package is designed to handle high current and power dissipation, making it ideal for high-power applications.
* Package Dimensions: The TO-3 package has typical dimensions of 22.5mm x 10mm x 10mm, making it relatively compact for the amount of power it can handle. The package is designed for easy mounting on heatsinks to improve heat dissipation.
* Weight: The TO-3 package weighs approximately 30g, which is relatively heavy for a voltage regulator but necessary for its ability to handle high currents and power dissipation.

## Applications

* Power Supplies: The LM138K is commonly used in regulated power supplies where high current is required. Its ability to provide stable output voltages at up to 5A makes it ideal for powering complex circuits in applications such as industrial control systems, communication equipment, and test instruments.
* Battery Chargers: The adjustable output voltage feature of the LM138K makes it well-suited for battery charging applications, where precise control over the output voltage is necessary to ensure proper battery charging without overcharging.
* Motor Control Circuits: The LM138K can be used to power motor control circuits where high current and stable voltage are necessary to drive motors efficiently.
* High-Power Amplifiers: With its ability to supply 5A of output current, the LM138K is suitable for use in audio amplifiers, RF power amplifiers, and other high-power applications where a stable voltage supply is essential for performance.
* Medical Equipment: The stability and low dropout voltage of the LM138K make it ideal for powering sensitive medical equipment that requires reliable and clean power, such as patient monitors and diagnostic tools.
* Automotive Systems: In automotive applications, where the voltage supplied by the vehicle’s electrical system can fluctuate, the LM138K can provide a stable voltage output to power sensors, controllers, and other electronic systems in the vehicle.

## Performance Characteristics

* High Efficiency: The LM138K offers low dropout voltage, allowing it to operate efficiently even when the input voltage is close to the desired output. This efficiency helps reduce power losses, especially in battery-operated systems.
* Good Load Regulation: The LM138K features excellent load regulation, ensuring that the output voltage remains stable even as the load current varies. This is particularly important for circuits where the load can change dynamically, such as servo motors or power amplifiers.
* High Current Capability: With a maximum output current of 5A, the LM138K can supply power to a wide range of high-current loads, making it versatile for use in many industrial, automotive, and consumer applications.
* Low Noise: The LM138K’s good ripple rejection and low quiescent current ensure that it operates with minimal noise, which is important for sensitive electronics such as audio equipment, measurement devices, and high-precision circuits.

## Environmental Considerations

* RoHS Compliant: The LM138K is RoHS compliant, ensuring that it meets the international standards for environmental safety by being free of hazardous substances like lead, mercury, and cadmium.
* Energy Efficiency: The device’s low dropout voltage and low quiescent current contribute to its energy efficiency, making it a good choice for battery-operated systems where power consumption is a concern.
* Reliability in Harsh Environments: With its wide operating temperature range and rugged TO-3 metal can package, the LM138K can be used in demanding environments where temperature fluctuations and high current loads are common.

## Conclusion

The STMicroelectronics LM138K is a versatile and robust adjustable voltage regulator designed to provide high current output with low dropout voltage. With its ability to supply up to 5A of output current, low power consumption, and excellent thermal performance, the LM138K is ideal for applications requiring stable and efficient power delivery, such as power supplies, battery chargers, motor controllers, and high-power amplifiers. Its thermal shutdown, current limiting, and adjustable output features make it a reliable choice for both commercial and industrial systems.