LM137/LM337 Adjustable Negative Voltage Regulator

Overview

The LM137/LM337 are adjustable three-terminal negative voltage regulators designed to deliver over 1.5A of output current across an adjustable voltage range of -1.2V to -37V. These regulators are highly versatile and easy to implement, requiring only two external resistors to set the output voltage and a single output capacitor for frequency compensation. Engineered for superior regulation performance and minimal thermal transients, the LM137/LM337 series is well-suited for a broad spectrum of applications, from on-card voltage regulation to precision current control. These devices serve as ideal counterparts to the LM117/LM317 positive voltage regulators.

Key Features

• Adjustable Output Voltage: -1.2V to -37V range for flexible design applications.
• High Output Current: Capable of supplying over 1.5A of continuous current.
• Internal Protection Mechanisms: Includes current limiting, thermal shutdown, and safe-area compensation to prevent damage from overloads.
• Excellent Line and Load Regulation: Optimized for stable performance under varying input and load conditions.
• Minimal External Components: Requires only two resistors and one output capacitor for operation.
• Thermal Stability: Designed to minimize thermal transients for reliable operation.
• Complementary to LM117/LM317: Ideal for dual-supply systems requiring both positive and negative regulation.

Applications

The LM137/LM337 regulators are widely used in various electronic systems due to their robustness and flexibility. Some common applications include:

• Local On-Card Regulation: Provides stable negative voltage supply for individual circuit boards.
• Programmable Voltage Supplies: Adjustable output allows for custom voltage settings in test equipment and prototyping.
• Precision Current Regulation: Suitable for constant-current applications such as LED drivers and sensor biasing.
• Dual-Supply Systems: Pairs with LM117/LM317 for symmetrical positive and negative voltage rails in audio amplifiers and data acquisition systems.
• Industrial Power Supplies: Used in machinery and control systems requiring reliable negative voltage regulation.
• Consumer Electronics: Found in devices like power adapters and home entertainment systems.

Electrical Characteristics

The following table summarizes the key electrical specifications of the LM137/LM337 under typical operating conditions (unless otherwise noted):

Absolute Maximum Ratings

• Input-Output Voltage Differential (V_IN - V_OUT): 40V
• Output Current (I_OUT): 1.5A (continuous)
• Power Dissipation (P_D): Internally limited by thermal shutdown
• Operating Junction Temperature (T_J): -40°C to +125°C
• Storage Temperature (T_STG): -65°C to +150°C

Electrical Specifications (T_J = 25°C)

• Output Voltage Range: -1.2V to -37V
• Line Regulation (ΔV_OUT/ΔV_IN): 0.01%/V (typical)
• Load Regulation (ΔV_OUT/ΔI_OUT): 0.1%/A (typical)
• Reference Voltage (V_REF): -1.25V (typical)
• Adjust Pin Current (I_ADJ): 50µA (typical)
• Ripple Rejection: 80dB (typical at 120Hz)
• Thermal Resistance (Junction-to-Case, θ_JC): 3°C/W (TO-220 package)

Package Information

The LM137/LM337 are available in multiple package options to accommodate different thermal and space requirements. The following are the most common packages:

TO-220 Package

• Package Type: Through-hole, 3-pin
• Pin Configuration:
  • Pin 1: Adjust
  • Pin 2: Input (V_IN)
  • Pin 3: Output (V_OUT)
• Thermal Characteristics: Low thermal resistance for efficient heat dissipation.
• Mounting: Suitable for heatsink attachment for high-power applications.

TO-263 (D2PAK) Package

• Package Type: Surface-mount, 3-pin
• Pin Configuration: Same as TO-220.
• Thermal Characteristics: Designed for PCB thermal relief and improved power dissipation.
• Applications: Ideal for space-constrained designs requiring surface-mount components.

Design Considerations

When implementing the LM137/LM337 in a circuit, the following design guidelines should be observed:

• Output Voltage Setting: The output voltage is set using a resistor divider between the output and adjust pins. The formula for the output voltage is:

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

  where R1 is typically 240Ω for stability.
• Input Capacitor: A 0.1µF to 1µF ceramic capacitor near the input pin is recommended to reduce input impedance and improve transient response.
• Output Capacitor: A 10µF or larger capacitor is required for frequency compensation and stability. Low-ESR capacitors are preferred.
• Thermal Management: For high-current applications, ensure proper heatsinking to maintain junction temperature within safe limits.
• Protection Diodes: In applications where the input may be shorted to ground, protection diodes are advised to prevent reverse biasing the regulator.

Conclusion

The LM137/LM337 adjustable negative voltage regulators offer a robust and flexible solution for a wide range of power supply applications. With their high current capability, adjustable output, and comprehensive protection features, these regulators are well-suited for both industrial and consumer electronics. By following the recommended design practices, engineers can achieve reliable and efficient performance in their circuits.