LM317 3-Terminal Adjustable Positive Voltage Regulator

Overview

The LM317 is a versatile, adjustable 3-terminal positive voltage regulator capable of supplying more than 1.5A of output current over an adjustable output voltage range of 1.2V to 37V. Designed for ease of use, it requires only two external resistors to set the output voltage. The LM317 offers superior line and load regulation compared to standard fixed regulators, making it ideal for a wide range of applications.

This regulator is available in multiple package options, including TO-3, TO-220, TO-263 (D²PAK), SOT-223, and TO-39, providing flexibility for different design requirements. The LM317 series includes variants such as the LM317A, which offers tighter tolerances and improved performance specifications.

Features

• Adjustable Output Voltage: Ranges from 1.2V to 37V, providing flexibility for various applications.
• High Output Current: Guaranteed 1.5A output current (depending on package and thermal conditions).
• Precision Voltage Tolerance: LM317A variant guarantees 1% output voltage tolerance.
• Excellent Line Regulation: LM317A offers a maximum of 0.01%/V line regulation.
• Superior Load Regulation: LM117 variant guarantees a maximum of 0.3% load regulation.
• Thermal Stability: Current limit remains constant with temperature variations.
• Ripple Rejection: 80 dB ripple rejection ensures clean output voltage.
• Protection Features: Output is short-circuit protected, enhancing reliability.
• P+ Product Enhancement: Tested for improved performance and reliability.

Applications

The LM317 is widely used in both industrial and consumer electronics due to its flexibility and robust performance. Common applications include:

• Adjustable Voltage Supplies: Used in benchtop power supplies and variable voltage applications.
• Battery Chargers: Provides regulated voltage for charging circuits.
• LED Drivers: Adjustable output makes it suitable for driving LED arrays.
• Audio Amplifiers: Clean voltage regulation ensures stable performance in audio circuits.
• Industrial Control Systems: Reliable voltage regulation for control and automation systems.
• Embedded Systems: Used in microcontroller and FPGA power supplies.

Electrical Characteristics

Absolute Maximum Ratings

• Input-Output Voltage Differential: 40V (maximum)
• Power Dissipation: Internally limited by thermal shutdown
• Operating Junction Temperature: -55°C to +150°C
• Storage Temperature Range: -65°C to +150°C

Typical Performance Characteristics

• Output Voltage Range: 1.2V to 37V
• Line Regulation: 0.01%/V (LM317A)
• Load Regulation: 0.3% (LM117)
• Output Current: Up to 1.5A (depending on package and heat dissipation)
• Dropout Voltage: Typically 2V at full load
• Ripple Rejection: 80 dB (typical)

Package Information

The LM317 is available in multiple package options to suit different design requirements. Below is a summary of available packages and their respective output current capabilities:

• TO-3 (K): Metal can package, 1.5A output current
• TO-220 (T): Plastic package, 1.5A output current
• TO-263 (S): Surface-mount package (D²PAK), 1.5A output current
• SOT-223 (EMP): Surface-mount package, 1.0A output current
• TO-39 (H): Metal can package, 0.5A output current
• LCC (E): Ceramic leadless chip carrier, 0.5A output current
• TO-252 (MDT): Surface-mount package (D-Pak), 0.5A output current

Package Dimensions and Pinouts

The pin configuration varies depending on the package. Below are the key pinouts for common packages:

• TO-3 (K): Case is connected to the output. Pins: Adjust (1), Output (2), Input (3).
• TO-220 (T): Pins (front view): Adjust (1), Output (2), Input (3).
• TO-263 (S): Pins (top view): Adjust (1), Output (2), Input (3).
• SOT-223: Pins (top view): Input (1), Ground (2), Output (3).

Design Considerations

When designing with the LM317, consider the following:

• Heat Dissipation: Ensure adequate heat sinking for high current applications to prevent thermal shutdown.
• Input Capacitor: A 0.1μF ceramic capacitor near the input pin improves transient response.
• Output Capacitor: Use 1μF to 1000μF aluminum or tantalum capacitors to enhance output stability and transient rejection.
• Adjust Pin Stability: A small capacitor (10μF) on the adjust pin can improve ripple rejection.
• PCB Layout: Keep traces short and wide to minimize voltage drops and thermal resistance.

Conclusion

The LM317 is a highly reliable and flexible voltage regulator suitable for a wide range of applications. Its adjustable output, high current capability, and robust protection features make it a preferred choice for engineers designing power supplies and voltage regulation circuits. With multiple package options and excellent performance characteristics, the LM317 continues to be a cornerstone component in electronic design.