LM337 Negative Voltage Regulator

Overview

The LM337 is a versatile, adjustable 3-terminal negative voltage regulator capable of supplying in excess of 1.5A over an output voltage range of -1.2V to -37V. This component is the negative complement to the popular LM317 positive voltage regulator, making it ideal for dual-rail power supply designs. The LM337 features excellent line and load regulation, requiring only two external resistors to set the output voltage.

Designed with internal current limiting, thermal shutdown, and safe-area compensation, the LM337 provides robust protection against overload conditions. Its ability to adjust down to 1.2V makes it suitable for a wide variety of applications requiring precise negative voltage regulation.

Features

• Adjustable output voltage from -1.2V to -37V
• Output current capability of 1.5A (minimum)
• Line regulation typically 0.01%/V
• Load regulation typically 0.1%
• Internal thermal overload protection
• Internal short-circuit current limiting
• Output transistor safe-area compensation
• Direct replacement for standard negative voltage regulators
• Available in TO-220, TO-263, and TO-252 packages

Applications

• Negative voltage regulation in dual-rail power supplies
• Adjustable bench power supplies
• Battery chargers with negative voltage requirements
• Audio amplifier power supplies
• Instrumentation and test equipment
• Industrial control systems
• Post-regulation for switching power supplies
• Voltage reference circuits

Electrical Characteristics

Absolute Maximum Ratings

• Input-Output Voltage Differential: 40V
• Power Dissipation: Internally Limited
• Operating Junction Temperature Range: 0°C to +125°C
• Storage Temperature Range: -65°C to +150°C
• Lead Temperature (Soldering, 10 sec): 300°C

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

• Reference Voltage (V_REF): -1.25V (typical)
• Line Regulation: 0.01%/V (typical)
• Load Regulation: 0.1% (typical)
• Adjustment Pin Current: 45μA (typical)
• Minimum Load Current: 3.5mA (typical)
• Ripple Rejection: 77dB (typical at 120Hz)
• Output Noise Voltage: 0.003%/V_O (10Hz to 10kHz)
• Thermal Resistance, Junction-to-Ambient: 50°C/W (TO-220 package)

Package Information

TO-220 Package (LM337T)

• Package Type: Through-hole, 3-pin
• Dimensions: 10.16mm x 4.58mm x 8.89mm
• Pin Configuration:
  • Pin 1: Adjust
  • Pin 2: Input
  • Pin 3: Output
• Mounting Style: Vertical or horizontal
• Thermal Characteristics: Requires heatsink for high current applications

TO-263 Package (LM337S)

• Package Type: Surface Mount, D2PAK
• Dimensions: 10.28mm x 9.65mm x 4.57mm
• Pin Configuration:
  • Pin 1: Adjust
  • Pin 2: Input
  • Pin 3: Output
  • Pins 4-5: Thermal pad (connected to case)
• Mounting Style: Surface mount
• Thermal Characteristics: Improved thermal performance due to large tab

Typical Application Circuit

The basic LM337 configuration 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 I_ADJ is the adjustment pin current (typically 45μA). For most applications, the effect of I_ADJ can be neglected, simplifying the equation to:

V_OUT ≈ -1.25V × (1 + R2/R1)

A typical circuit includes input and output capacitors for stability. A 10μF tantalum or 25μF aluminum electrolytic capacitor at the output improves transient response, while a 0.1μF ceramic capacitor at the input helps with high-frequency noise rejection.

Design Considerations

Thermal Management

Proper heatsinking is critical when operating the LM337 at higher currents or large input-output differential voltages. The power dissipation can be calculated as:

P_D = (V_IN - V_OUT) × I_OUT

Ensure the junction temperature does not exceed 125°C by selecting an appropriate heatsink based on the thermal resistance (θ_JA) of the package and the maximum ambient temperature.

Stability Considerations

While the LM337 is stable with standard capacitor values, certain conditions may require additional compensation:

• When using large output capacitors (>25μF), add a 1μF tantalum or 25μF aluminum electrolytic capacitor at the input
• For applications with rapidly changing loads, increase the output capacitor value
• Keep the adjust pin bypass capacitor (if used) less than 10μF to prevent startup problems

Reliability and Compliance

The LM337 is designed to meet stringent reliability standards and is typically characterized for operation from 0°C to 125°C junction temperature. The device is available in lead-free and RoHS compliant versions to meet modern environmental requirements.

When used within specified operating conditions and with proper thermal management, the LM337 demonstrates excellent long-term reliability with typical MTBF (Mean Time Between Failures) exceeding 100,000 hours.