LM79XX Series 3-Terminal Negative Voltage Regulators

Overview

The LM79XX series is a family of fixed-output, negative voltage regulators designed to provide -5V, -12V, and -15V output options. These 3-terminal regulators are capable of delivering up to 1.5A of output current with excellent line and load regulation characteristics. The series features built-in protection against thermal overload, short circuits, and safe operating area violations, making them robust solutions for a wide range of power supply applications.

These regulators are particularly noted for their high ripple rejection capability, which makes them suitable for use in systems where clean, stable negative voltage rails are required. With a 4% tolerance on the preset output voltage, the LM79XX series offers reliable performance across industrial temperature ranges.

Key Features

• Fixed output voltages: -5V (LM7905), -12V (LM7912), -15V (LM7915)
• 1.5A output current capability
• Built-in thermal overload protection
• Short circuit protection
• Safe operating area protection
• High ripple rejection ratio
• 4% output voltage tolerance
• Available in TO-220 and TO-3 packages
• Internal current limiting
• No external components required for fixed operation

Applications

The LM79XX series is widely used in various electronic systems requiring stable negative voltage supplies:

• Power supplies for operational amplifier circuits
• Negative voltage rails in analog systems
• Industrial control systems
• Test and measurement equipment
• Audio equipment
• Consumer electronics
• Telecommunications equipment
• Computer peripherals

Electrical Characteristics

Absolute Maximum Ratings

• Input Voltage (LM7905): -35V
• Input Voltage (LM7912): -35V
• Input Voltage (LM7915): -35V
• 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)

• Output Voltage Tolerance: ±4%
• Line Regulation: 60mV (typical)
• Load Regulation: 120mV (typical)
• Ripple Rejection: 62dB (typical)
• Dropout Voltage: 2V (maximum)
• Output Noise Voltage: 40µV (typical)
• Quiescent Current: 4.3mA (typical)

Package Information

TO-220 Package (LM79XXCT)

• Package Type: TO-220-3
• Mounting: Through-hole
• Pin Configuration:
  • Pin 1: Ground
  • Pin 2: Input
  • Pin 3: Output
• Thermal Resistance (Junction-to-Ambient): 50°C/W
• Thermal Resistance (Junction-to-Case): 5°C/W

TO-3 Package

• Package Type: TO-3 metal can
• Mounting: Through-hole with mounting tab
• Pin Configuration:
  • Case: Ground
  • Pin 1: Input
  • Pin 2: Output
• Thermal Resistance (Junction-to-Ambient): 35°C/W
• Thermal Resistance (Junction-to-Case): 3°C/W

Typical Application Circuit

The basic application circuit for the LM79XX series requires minimal external components:

• Input capacitor (0.33µF) recommended when regulator is more than 3 inches from power supply filter
• Output capacitor (0.1µF) required for stability
• For improved transient response, larger output capacitors may be used (up to 25µF aluminum electrolytic or solid tantalum)

Design Considerations

Thermal Management

For continuous operation at full load current, proper heat sinking is essential. The TO-220 package requires a heatsink for currents above 500mA, while the TO-3 package can handle higher currents before requiring additional cooling.

Input Voltage Requirements

The input voltage must be at least 2V more negative than the output voltage (dropout voltage) and should not exceed -35V absolute maximum rating. For optimal performance, maintain at least 3V difference between input and output.

Stability Considerations

The output capacitor is critical for maintaining stability. Solid tantalum capacitors are recommended, though aluminum electrolytic capacitors may be substituted with proper consideration of ESR characteristics.

Reliability and Compliance

The LM79XX series is designed to meet stringent reliability requirements for industrial applications. The devices are robust against:

• Input voltage transients
• Output short circuits
• Thermal overload conditions
• Reverse polarity situations

Ordering Information

• 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 same part numbers (different suffix)