Product Overview

The STMicroelectronics LM317K is a versatile, adjustable positive voltage regulator widely used in power supply circuits for providing a stable output voltage over a broad range. This three-terminal device can deliver output currents up to 1.5 A with excellent line and load regulation. It features internal current limiting, thermal shutdown, and safe-area compensation, ensuring reliable operation under various environmental and electrical stresses.

The LM317K supports output voltages adjustable from 1.25 V up to 37 V by using an external resistor divider, making it suitable for diverse applications including industrial equipment, consumer electronics, battery chargers, and general-purpose power supplies. Packaged typically in a robust TO-3 metal can or TO-220 plastic package, the LM317K offers good thermal conductivity and ruggedness.

Key Features

* Adjustable output voltage range: 1.25 V to 37 V
* Output current capability: up to 1.5 A continuous
* Line regulation typically 0.01%/V
* Load regulation typically 0.1%
* Internal short-circuit current limiting
* Thermal overload protection with shutdown
* Safe operating area protection for pass transistor
* Low quiescent current for efficient operation
* Output voltage tolerance typically ±1.5%
* Excellent ripple rejection (> 80 dB) for stable DC output
* Compatible with standard TO-3 or TO-220 packages
* Operating junction temperature up to 125°C
* Wide operating input voltage range (up to 40 V above output voltage)

Electrical Specifications

* Input Voltage (VIN): Up to 40 V above output
* Output Voltage (VOUT): Adjustable 1.25 V to 37 V
* Output Current (IOUT): Up to 1.5 A continuous
* Line Regulation: 0.01% per volt typical
* Load Regulation: 0.1% typical
* Dropout Voltage: Typically 2.5 V at 1.5 A load
* Output Voltage Accuracy: ±1.5% typical
* Quiescent Current: Approximately 5 mA typical
* Temperature Range:

* Junction: –55°C to +150°C
* Operating: 0°C to +125°C
* Thermal Resistance Junction-to-Case (RθJC): Approx. 5°C/W (TO-3)
* Thermal Resistance Junction-to-Ambient (RθJA): Varies by package and heat sinking
* Package Types: TO-3 metal can, TO-220 plastic, others

Functional Description

The LM317K operates as a linear voltage regulator that maintains a constant output voltage by regulating the conduction through an internal pass transistor based on the difference between the output and a fixed internal reference voltage of approximately 1.25 V. By using two external resistors connected between the output and adjustment pin and from adjustment pin to ground, the output voltage can be precisely set to the desired level.

The internal circuitry includes a current limiting mechanism to protect the device and load from excessive current conditions. Thermal shutdown protects against overheating by disabling the output when junction temperatures exceed safe limits, preventing device damage. The device also features safe operating area (SOA) protection to prevent transistor failure under adverse conditions.

The regulator offers low noise and excellent ripple rejection, making it suitable for powering sensitive analog and digital circuits. Its stable operation over a wide temperature range and ability to supply relatively high output current make it a reliable component in power management designs.

Typical Applications

* Adjustable linear power supplies
* Battery chargers and maintenance circuits
* Voltage regulation in instrumentation and control systems
* Post-regulation for switching power supplies
* On-board voltage regulation for microprocessors and digital logic
* Test equipment and laboratory power sources
* Audio amplifiers and other analog circuits requiring clean supply rails

Design Considerations

* External Components: Use a minimum 240 Ω resistor between the output and adjustment pin for proper operation. A small bypass capacitor (e.g., 0.1 µF) on the adjustment pin can improve transient response and reduce output noise.
* Heat Dissipation: Adequate heat sinking is critical for high current and high voltage differential operation to maintain junction temperature within safe limits.
* Input-Output Voltage Differential: Ensure the input voltage is sufficiently higher than the desired output voltage plus the dropout voltage to maintain regulation.
* Output Capacitor: A small output capacitor (1 µF to 10 µF tantalum or aluminum electrolytic) is recommended to improve transient response and stability.
* Thermal Protection: Use thermal interface materials and proper mounting to enhance heat transfer to the heat sink.
* Short-Circuit Protection: The device will limit current in case of short circuits, but long duration short conditions should be avoided to prevent thermal damage.
* Noise Considerations: For sensitive applications, additional LC or RC filters may be implemented to further reduce ripple and noise.
* Package Selection: Choose TO-3 for superior thermal performance; TO-220 provides a cost-effective solution with adequate heat dissipation when combined with an appropriate heat sink.

Summary

The STMicroelectronics LM317K is a reliable, adjustable linear voltage regulator capable of delivering up to 1.5 A with excellent regulation, protection features, and flexibility in output voltage. Its integrated current and thermal protections ensure safe and stable operation, while its ease of use and wide input/output range make it a popular choice for a broad spectrum of power regulation applications. With proper heat sinking and component selection, the LM317K provides a robust and precise voltage regulation solution in industrial, consumer, and instrumentation environments.