LM317T

LM317T Adjustable Positive Voltage Regulator

Overview

The LM317T is a member of the LM117 series of adjustable 3-terminal positive voltage regulators. Designed for simplicity and reliability, this component can supply over 1.5A of output current across an adjustable output voltage range of 1.2V to 37V. Its ease of use makes it a popular choice in both prototyping and production environments, requiring only two external resistors to set the desired output voltage. The LM317T offers superior line and load regulation compared to standard fixed regulators, ensuring stable performance under varying conditions.

Packaged in a standard transistor form factor, the LM317T is easy to mount and integrate into various circuit designs. Its robust design includes built-in protection features such as current limiting, thermal overload protection, and safe operating area (SOA) protection, making it suitable for demanding applications.

Features

• Adjustable Output Voltage: 1.2V to 37V range, providing flexibility for diverse applications.
• High Output Current: Capable of delivering over 1.5A continuously.
• Precision Voltage Reference: Guaranteed 1% output voltage tolerance (LM317A variant).
• Superior Regulation: Excellent line and load regulation performance compared to fixed regulators.
• Thermal Overload Protection: Built-in safeguards prevent damage due to excessive temperature.
• Current Limiting: Protects the regulator and connected components from overcurrent conditions.
• Safe Operating Area (SOA) Protection: Ensures reliable operation under varying load conditions.
• Standard Transistor Package: TO-220 package for easy mounting and heat dissipation.

Applications

The LM317T is widely used in both commercial and industrial applications due to its versatility and reliability. Common applications include:

• Power Supplies: Adjustable bench power supplies, fixed-voltage supply replacements.
• Battery Chargers: Customizable charging circuits for various battery types.
• Voltage Regulation: Stabilizing voltage for sensitive analog and digital circuits.
• Motor Control: Providing regulated voltage to small DC motors.
• LED Drivers: Constant current or voltage supply for LED arrays.
• Audio Amplifiers: Voltage regulation in preamplifier and amplifier stages.
• Test and Measurement Equipment: Precision voltage references and calibration circuits.

Electrical Characteristics

The following table summarizes the key electrical specifications of the LM317T under typical operating conditions (unless otherwise noted):

• Output Voltage Range: 1.2V to 37V
• Output Current: 1.5A (maximum continuous)
• Line Regulation: 0.01%/V (typical)
• Load Regulation: 0.1% (typical)
• Reference Voltage (Vref): 1.25V (between output and adjust terminal)
• Dropout Voltage: 2V to 2.5V (depending on load current)
• Operating Junction Temperature: -40°C to +125°C
• Thermal Resistance (Junction-to-Ambient): 50°C/W (TO-220 package, no heatsink)

Package Information

The LM317T is available in a TO-220 package, which is a through-hole, 3-pin configuration designed for easy mounting and efficient heat dissipation. Key package details include:

• Package Type: TO-220 (Transistor Outline)
• Pin Configuration:
  • Pin 1 (Adjust): Connected to the voltage divider network to set the output voltage.
  • Pin 2 (Output): Provides the regulated output voltage.
  • Pin 3 (Input): Connected to the unregulated input voltage source.
• Mounting: Through-hole mounting with standard 0.1" pin spacing.
• Thermal Considerations: A heatsink is recommended for high-current applications or when operating near maximum power dissipation limits.
• Dimensions: Approximately 10.4mm x 4.6mm x 9.15mm (L x W x H).

Design Considerations

When designing with the LM317T, the following factors should be considered to ensure optimal performance:

• Input Voltage: Must be at least 2V to 2.5V higher than the desired output voltage to account for dropout voltage.
• Output Voltage Setting: The output voltage is set using a voltage divider between the output and adjust pins. The formula for calculating the output voltage is: Vout = Vref (1 + R2/R1) + Iadj * R2, where Vref is typically 1.25V, and Iadj is the adjust pin current (usually negligible).
• Heat Dissipation: Power dissipation (Pd) is calculated as (Vin - Vout) * Iout. Ensure adequate heatsinking if Pd exceeds a few watts.
• Stability: A small capacitor (typically 10µF) at the output improves transient response and stability, especially with long wires or inductive loads.
• Protection Diodes: Adding diodes between input-to-output and adjust-to-output can protect the regulator from reverse voltages during shutdown or fault conditions.

Conclusion

The LM317T is a versatile and robust adjustable voltage regulator suitable for a wide range of applications. Its combination of high current capability, precise voltage regulation, and built-in protection features make it an excellent choice for designers seeking a reliable solution for variable power supply needs. By following recommended design practices, the LM317T can deliver stable and efficient performance in both simple and complex circuits.