LM7905 - 3-Terminal Negative Voltage Regulator

Overview

The LM7905 is a member of the LM79XX series of fixed-output negative voltage regulators. This integrated circuit provides a stable -5V DC output voltage with a maximum output current of 1.5A. Designed as a complement to the popular LM7805 positive voltage regulator, the LM7905 offers reliable voltage regulation for negative voltage applications.

As a three-terminal device, the LM7905 requires minimal external components for operation, making it an ideal choice for simple power supply designs. The regulator features built-in protection against thermal overload, short circuits, and safe operating area violations, ensuring robust operation in demanding environments.

Features

• Fixed -5V output voltage with 4% tolerance
• 1.5A output current capability
• High ripple rejection (typically 62dB at 120Hz)
• Built-in thermal overload protection
• Short circuit current limiting
• Safe operating area protection
• TO-220 package for easy mounting and heat dissipation
• Internal current limiting
• No external components required for basic operation
• Wide operating temperature range

Applications

The LM7905 negative voltage regulator finds use in a wide variety of electronic applications including:

• Negative voltage supplies for operational amplifiers
• Power supplies for analog circuits requiring negative voltage rails
• Complementary power supplies (used with LM7805 for ±5V systems)
• Industrial control systems
• Test and measurement equipment
• Consumer electronics
• Audio equipment
• Voltage regulation in embedded systems

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
• Lead Temperature (Soldering, 10 seconds): 260°C

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

• Output Voltage: -5V ±4%
• Line Regulation: 10mV (VI = -7V to -20V, IO = 500mA)
• Load Regulation: 15mV (VI = -8V, IO = 5mA to 1.5A)
• Quiescent Current: 4.3mA (VI = -8V, IO = 0)
• Ripple Rejection: 62dB (f = 120Hz)
• Dropout Voltage: 2V (IO = 1A)
• Output Noise Voltage: 40μV (10Hz to 100kHz)
• Short Circuit Output Current: 2.1A (VI = -12V)
• Peak Output Current: 2.2A (VI = -12V)

Package Information

TO-220 Package Details

The LM7905 is available in the industry-standard TO-220 package, which provides excellent thermal performance and mechanical strength. The package dimensions are as follows:

• Package Type: TO-220-3
• Mounting Style: Through Hole
• Number of Pins: 3
• Pin Spacing: 2.54mm (0.1 inch)
• Package Length: 10.4mm
• Package Width: 4.6mm
• Package Height: 9.15mm
• Lead Length: 3.3mm
• Lead Width: 0.51mm
• Weight: 2.0 grams (approx.)

Pin Configuration

• Pin 1 (Input): Unregulated negative input voltage
• Pin 2 (Ground): Common/ground terminal
• Pin 3 (Output): Regulated -5V output

Typical Application Circuit

The basic application circuit for the LM7905 requires only two capacitors for stable operation:

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

Thermal Considerations

For reliable operation, proper thermal management is essential when using the LM7905:

• Maximum junction temperature: 125°C
• Thermal resistance, junction-to-air: 50°C/W (without heatsink)
• Thermal resistance, junction-to-case: 5°C/W

For high current applications or high ambient temperatures, a heatsink is recommended to maintain safe operating temperatures. The power dissipation can be calculated using the formula:

PD = (VIN - VOUT) × IOUT + VIN × IQ

Where:
PD = Power dissipation
VIN = Input voltage
VOUT = Output voltage (-5V)
IOUT = Output current
IQ = Quiescent current

Design Considerations

Input Voltage Range

The LM7905 requires an input voltage that is at least 2V more negative than the output voltage (-5V). The recommended input voltage range is -7V to -20V for optimal performance.

Stability Requirements

While the LM7905 is stable without output capacitors, adding an output capacitor improves transient response. The output capacitor should be placed as close as possible to the regulator for best performance.

Protection Features

The LM7905 incorporates three key protection mechanisms:

• Thermal Shutdown: Automatically disables the regulator if the junction temperature exceeds safe limits
• Current Limiting: Protects against excessive output currents
• Safe Operating Area Protection: Ensures the regulator operates within its specified power limits

Alternative Components

For different voltage requirements, consider these alternatives from the same family:

• LM7905: -5V output
• LM7912: -12V output
• LM7915: -15V output
• LM7906: -6V output
• LM7908: -8V output
• LM7909: -9V output
• LM7924: -24V output

Conclusion

The LM7905 negative voltage regulator provides a simple, reliable solution for -5V power supply requirements. With its robust protection features, high ripple rejection, and stable output characteristics, it remains a popular choice for designers needing negative voltage regulation. The TO-220 package offers excellent thermal performance while maintaining a compact footprint suitable for a wide range of applications.