LM337T

LM337T: Adjustable 3-Terminal Negative Voltage Regulator

Overview

The LM337T is a highly reliable, adjustable 3-terminal negative voltage regulator designed to deliver an output current of up to -1.5A. It operates over an output voltage range of -1.2V to -37V, making it suitable for a wide variety of applications requiring stable negative voltage supplies. The device is exceptionally easy to implement, requiring only two external resistors to set the output voltage and a single output capacitor for frequency compensation. The LM337T is optimized for excellent line and load regulation while minimizing thermal transients, ensuring consistent performance under varying conditions.

Features

• Adjustable Output Voltage: Supports an output range of -1.2V to -37V.
• High Output Current: Capable of delivering up to -1.5A of continuous output current.
• Excellent Regulation: Optimized for superior line and load regulation.
• Internal Current Limiting: Built-in protection against excessive current draw.
• Thermal Overload Protection: Safeguards the device from overheating.
• Wide Operating Temperature Range: Suitable for industrial and commercial applications.
• Minimal External Components: Requires only two resistors and one capacitor for operation.
• Low Thermal Transients: Ensures stable performance during temperature fluctuations.

Applications

The LM337T is widely used in applications requiring a stable and adjustable negative voltage supply. Common use cases include:

• Power Supplies: Used in both linear and switching power supply designs.
• Industrial Equipment: Provides regulated voltage for control systems and instrumentation.
• Audio Amplifiers: Delivers negative rail voltage for audio circuits.
• Test and Measurement Devices: Ensures precise voltage regulation in lab equipment.
• Consumer Electronics: Found in devices requiring negative voltage rails, such as displays and communication modules.
• Automotive Systems: Used in vehicle electronics where stable negative voltage is needed.

Electrical Characteristics

The following table summarizes the key electrical specifications of the LM337T under typical operating conditions (unless otherwise noted):

• Output Voltage Range: -1.2V to -37V
• Output Current: Up to -1.5A
• Line Regulation: 0.01%/V (Typical)
• Load Regulation: 0.1% (Typical)
• Dropout Voltage: 1.5V (Typical at -1A)
• Operating Junction Temperature: -40°C to +125°C
• Thermal Resistance (Junction-to-Case): 3°C/W
• Reference Voltage: -1.25V (Typical)

Package Information

The LM337T is available in a TO-220 package, which is widely used for power applications due to its excellent thermal performance and ease of mounting. Key package details include:

• Package Type: TO-220 (Through-Hole)
• Pin Configuration:
  • Pin 1: Adjust (Connects to the voltage-setting resistor divider)
  • Pin 2: Output (Provides the regulated negative voltage)
  • Pin 3: Input (Connected to the unregulated negative input voltage)
• Mounting Style: Through-Hole
• Thermal Pad: The package includes a metal tab for heat dissipation, which can be attached to a heatsink for improved thermal performance.
• Dimensions: 10.4mm x 4.6mm x 9.15mm (L x W x H)

Detailed Operation

The LM337T operates by comparing the reference voltage (-1.25V) to a fraction of the output voltage, set by an external resistor divider network. The regulator adjusts its internal pass transistor to maintain the desired output voltage, compensating for variations in input voltage and load current. The device includes internal current limiting to protect against short circuits and thermal shutdown to prevent damage from excessive power dissipation.

Design Considerations

When designing with the LM337T, the following factors should be considered to ensure optimal performance:

• Input Voltage: Must be at least 1.5V higher in magnitude than the desired output voltage to account for dropout.
• Output Capacitor: A minimum of 10µF tantalum or 50µF aluminum electrolytic capacitor is recommended for stability.
• Resistor Selection: The voltage-setting resistors should be chosen to provide sufficient bias current (typically 5-10mA) for the adjust pin.
• Thermal Management: For high-current applications, a heatsink may be required to keep the junction temperature within safe limits.

Conclusion

The LM337T is a versatile and robust negative voltage regulator, offering adjustable output, high current capability, and reliable performance. Its ease of use, combined with comprehensive protection features, makes it an excellent choice for a wide range of applications. Whether used in power supplies, industrial systems, or consumer electronics, the LM337T delivers consistent and stable negative voltage regulation.