LM338T - 5A Adjustable Positive Voltage Regulator

Overview

The LM338T is a member of the LM138 series of adjustable 3-terminal positive voltage regulators capable of delivering more than 5A of output current across an adjustable output voltage range of 1.2V to 32V. This robust regulator is designed for simplicity, requiring only two external resistors to set the output voltage while delivering performance characteristics that rival many commercial power supplies.

Engineered for demanding applications, the LM338T features exceptional line and load regulation characteristics. The device is particularly well-suited for high-current applications where both precision and reliability are critical requirements.

Key Features

• High Output Current: Capable of supplying in excess of 5A continuous current (7A peak)
• Wide Adjustable Output Range: 1.2V to 32V output voltage
• Excellent Regulation: Outstanding line and load regulation comparable to commercial power supplies
• Simple Configuration: Requires only two external resistors to set output voltage
• Thermal Protection: Internal thermal overload protection
• Current Limiting: Internal short-circuit current limiting
• Safe Operating Area Protection: Internal protection against excessive power dissipation

Applications

The LM338T's combination of high current capability and adjustable output makes it suitable for a wide range of power supply applications:

• Bench Power Supplies: Adjustable laboratory and benchtop power sources
• Industrial Power Systems: Factory automation equipment and machinery
• Battery Chargers: High-current charging circuits for lead-acid and other battery types
• Motor Control: Power supply regulation for DC motor controllers
• Audio Amplifiers: Power regulation for high-power audio systems
• Test Equipment: Precision voltage sources for measurement instruments
• Telecommunications: Power regulation in communication infrastructure

Electrical Characteristics

Absolute Maximum Ratings

• Input-Output Voltage Differential: 35V
• Power Dissipation: Internally limited
• Operating Junction Temperature Range: 0°C to +125°C
• Storage Temperature Range: -65°C to +150°C

Electrical Specifications (T_J = 25°C unless otherwise noted)

• Output Voltage Range: 1.2V to 32V
• Output Current: 5A (continuous), 7A (peak)
• Line Regulation: 0.005%/V (typical)
• Load Regulation: 0.1% (typical)
• Reference Voltage: 1.25V (between output and adjust terminal)
• Adjust Terminal Current: 45μA (typical), 100μA (maximum)
• Ripple Rejection: 77dB (typical)
• Thermal Resistance (Junction-to-Case): 3°C/W

Package Information

The LM338T is available in a robust TO-220 package, designed for efficient heat dissipation in high-current applications:

• Package Type: TO-220-3 (also known as TO-220AB)
• Pin Configuration:
  • Pin 1: Adjust
  • Pin 2: Output
  • Pin 3: Input
• Mounting Style: Through-hole
• Package Dimensions: 10.4mm x 4.6mm x 9.15mm (LxWxH)
• Lead Material: Copper alloy
• Lead Finish: Matte Tin
• Weight: Approximately 2.0 grams

Thermal Considerations

Proper heat sinking is critical when operating the LM338T at high current levels or with large input-output differential voltages. The following thermal management guidelines should be observed:

• Always calculate power dissipation: P_D = (V_IN - V_OUT) × I_OUT
• Maintain junction temperature below 125°C for reliable operation
• Use thermal compound between regulator and heat sink
• For continuous full-load operation, select a heat sink with thermal resistance low enough to maintain safe junction temperatures

Typical Application Circuit

The basic configuration for the LM338T requires only two external resistors to set the output voltage according to the formula:

V_OUT = 1.25V × (1 + R2/R1) + I_ADJ × R2

Where:

• V_OUT is the desired output voltage
• R1 is typically 120Ω to 240Ω
• R2 is calculated based on desired output voltage
• I_ADJ is the adjust terminal current (typically 45μA)

Input and output capacitors are recommended for stability, with values typically in the range of 0.1μF to 10μF depending on application requirements.

Reliability and Compliance

The LM338T is designed and manufactured to meet stringent quality and reliability standards:

• Industrial temperature range operation
• ESD protection on all pins
• RoHS compliant version available
• Manufactured under strict quality control processes

Design Considerations

When implementing the LM338T in your design, consider the following:

• Ensure adequate input voltage headroom (typically 3V above desired output)
• Use proper PCB layout techniques with short, wide traces for high-current paths
• Include bypass capacitors near the device pins
• Consider adding protection diodes for applications with large capacitive loads
• Account for power dissipation in component selection and thermal design