LM7912 - Negative Voltage Regulator Technical Summary

Overview

The LM7912 is a member of the LM79XX series of three-terminal negative voltage regulators. This component provides a fixed -12V output with a maximum current capability exceeding 1A. Designed for ease of use and reliability, the LM7912 incorporates several protection features including thermal overload protection, short circuit protection, and safe operating area compensation for the output transistor.

As part of the widely-used 79XX series, the LM7912 is available in a standard TO-220 package, making it suitable for both prototyping and production applications. The device requires minimal external components for operation, typically just input and output capacitors, simplifying circuit design.

Features

• Fixed -12V output voltage
• Output current capability exceeding 1A
• Internal thermal overload protection
• Short circuit protection
• Output transistor safe operating area compensation
• Available in TO-220 package
• Wide operating temperature range
• Low cost and high reliability
• Minimal external components required

Applications

The LM7912 is commonly used in various electronic systems requiring stable negative voltage rails:

• Power supplies for operational amplifier circuits
• Negative voltage rails in audio equipment
• Industrial control systems
• Test and measurement equipment
• Consumer electronics
• Telecommunications equipment
• Automotive electronics (non-critical systems)
• Educational and hobbyist projects

The regulator is particularly useful in dual-supply systems where both positive and negative voltages are required, often paired with its positive counterpart, the LM7812.

Electrical Characteristics

Absolute Maximum Ratings

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

Recommended Operating Conditions

• Input Voltage Range: -14.5V to -27V
• Output Current Range: 0mA to 1A
• Operating Temperature Range: 0°C to +125°C

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

• Output Voltage: -12V ±4%
• Line Regulation: 120mV (VI = -14.5V to -27V, IO = 500mA)
• Load Regulation: 240mV (VI = -18V, IO = 5mA to 1.5A)
• Quiescent Current: 4.3mA (VI = -18V, IO = 0)
• Ripple Rejection: 55dB (f = 120Hz, VI = -18V, IO = 500mA)
• Dropout Voltage: 2V (IO = 1A, TJ = 25°C)

Package Information

TO-220 Package Details

The LM7912 is available in a standard TO-220 package with the following mechanical characteristics:

• Package Type: Through-hole, three-terminal
• Terminal Configuration:
  • Pin 1: Ground
  • Pin 2: Input
  • Pin 3: Output
• Package Dimensions: 10.4mm x 4.6mm x 9.15mm (LxWxH)
• Lead Spacing: 2.54mm (standard pitch)
• Mounting Style: Through-hole or with heatsink
• Material: Molded plastic

Thermal Characteristics

• Thermal Resistance, Junction-to-Ambient: 50°C/W
• Thermal Resistance, Junction-to-Case: 5°C/W
• Maximum Junction Temperature: 125°C

Typical Application Circuit

The basic application circuit for the LM7912 requires only two external capacitors:

• Input capacitor (0.33μF) placed close to the input pin
• Output capacitor (0.1μF) placed close to the output pin

For improved transient response and stability, larger capacitor values may be used. The input capacitor helps smooth the input voltage and provides local energy storage, while the output capacitor improves the regulator's transient response and stability.

Design Considerations

Thermal Management

When operating at high currents or with large input-output voltage differentials, the LM7912 may require heatsinking to maintain safe operating temperatures. The power dissipation can be calculated as:

P_D = (V_IN - V_OUT) × I_OUT

Proper heatsink selection should ensure the junction temperature remains below the maximum rating of 125°C.

Input Voltage Selection

The input voltage should be at least 2V greater than the output voltage (14V minimum for -12V output) to ensure proper regulation. However, excessively high input voltages will increase power dissipation and reduce efficiency.

Protection Features

The LM7912 includes several built-in protection mechanisms:

• Thermal Shutdown: Automatically disables the output if the junction temperature exceeds safe limits
• Short Circuit Protection: Limits current during output short conditions
• Safe Operating Area Compensation: Protects the output transistor under all operating conditions

Comparison with Similar Devices

The LM7912 is part of a family of negative voltage regulators with different output voltages:

• LM7905: -5V output
• LM7906: -6V output
• LM7908: -8V output
• LM7909: -9V output
• LM7910: -10V output
• LM7915: -15V output
• LM7918: -18V output
• LM7924: -24V output

All devices in the series share similar electrical characteristics and package options, differing only in their output voltages.

Reliability and Quality

The LM7912 is designed for high reliability and consistent performance:

• Manufactured using high-quality silicon processing
• Rigorous production testing
• Extended temperature range operation
• Robust protection features enhance long-term reliability

When properly used within its specified limits, the LM7912 provides years of maintenance-free operation in electronic systems.