79XX Series Negative Voltage Regulators Technical Documentation

Overview

The LM79XX series is a family of fixed-output negative voltage regulators designed to provide reliable power supply regulation in a wide range of applications. These 3-terminal regulators are capable of delivering up to 1.5A of output current with excellent line and load regulation characteristics. The series includes popular output voltage options such as -5V (7905), -12V (7912), and -15V (7915), making them suitable for various circuit requirements.

These regulators feature built-in protection against thermal overload, short circuits, and safe operating area violations, ensuring robust performance in demanding environments. With their high ripple rejection capability, they effectively filter input voltage variations, providing clean, stable output voltages.

Key Features

• Fixed output voltages: -5V, -12V, -15V options
• 1.5A output current capability
• 4% tolerance on preset output voltage
• Thermal overload protection
• Short circuit protection
• Safe operating area protection
• High ripple rejection
• Available in TO-220 and TO-3 packages
• No external components required for basic operation
• Internal current limiting

Applications

The 79XX series regulators find use in a wide variety of electronic systems requiring stable negative voltage rails:

• Power supplies for operational amplifier circuits
• Negative voltage sources for analog circuits
• Industrial control systems
• Test and measurement equipment
• Audio equipment power regulation
• Consumer electronics
• Telecommunications equipment
• Embedded systems
• Laboratory power supplies

Electrical Characteristics

Absolute Maximum Ratings

• Input Voltage: -35V (for 7905), -40V (for 7912/7915)
• Power Dissipation: Internally Limited
• Operating Junction Temperature Range: 0°C to +125°C
• Storage Temperature Range: -65°C to +150°C

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

Package Information

TO-220 Package

The TO-220 package is a through-hole mounting option that provides good thermal performance for moderate power applications. The package dimensions are as follows:

• Total Height: 16.26mm (max)
• Lead Length: 22.86mm (min)
• Lead Spacing: 2.54mm
• Mounting Hole Diameter: 3.81mm
• Package Weight: 2.0g (approx.)

TO-3 Package

The TO-3 metal can package offers superior thermal performance for high-power applications. Key dimensions include:

• Diameter: 22.86mm (max)
• Height: 18.67mm (max)
• Mounting Hole Spacing: 12.7mm
• Lead Diameter: 1.02mm
• Package Weight: 15.0g (approx.)

Typical Application Circuit

The basic configuration for a 79XX series regulator requires only two capacitors for stable operation:

• Input Capacitor (C1): 0.33μF solid tantalum or 25μF aluminum electrolytic (required if regulator is more than 3 inches from filter capacitor)
• Output Capacitor (C2): 0.1μF solid tantalum or 25μF aluminum electrolytic (required for stability)

Thermal Considerations

Proper heat sinking is essential when operating near maximum ratings. The thermal resistance parameters are:

• TO-220 Junction-to-Ambient (no heatsink): 50°C/W
• TO-220 Junction-to-Case: 5°C/W
• TO-3 Junction-to-Case: 3°C/W

The maximum power dissipation can be calculated using: PD = (TJ(MAX) - TA)/θJA

Design Considerations

• Maintain adequate input-output voltage differential (typically 2V minimum)
• Use proper PCB layout techniques to minimize noise and thermal resistance
• For high current applications, consider using a heatsink
• Ensure input voltage does not exceed maximum ratings
• For improved transient response, larger output capacitors may be used

Ordering Information

The 79XX series is available in various package and voltage options:

• LM7905CT: -5V output in TO-220 package
• LM7912CT: -12V output in TO-220 package
• LM7915CT: -15V output in TO-220 package
• TO-3 package variants available with similar part numbers

Reliability and Quality

The 79XX series regulators are manufactured under strict quality control procedures and are designed for long-term reliable operation. Typical MTBF (Mean Time Between Failures) exceeds 100,000 hours when operated within specified parameters.