LM78XX Series Three-Terminal Positive Voltage Regulators

Overview

The LM78XX series is a family of fixed-output positive voltage regulators that provide reliable power supply regulation for a wide range of electronic applications. These three-terminal devices are available in several standard output voltage options, making them versatile solutions for various circuit requirements. The series is particularly valued for its ability to deliver over 1A of output current while maintaining stable voltage regulation.

These regulators are designed to provide local on-card regulation, solving many of the power distribution challenges associated with centralized power supply systems. The LM78XX series finds extensive use in digital logic systems, instrumentation, audio equipment, and other solid-state electronic devices requiring stable DC power.

Key Features

• Output current capability exceeding 1A
• Built-in thermal overload protection
• No external components required for basic operation
• Output transistor safe area compensation
• Internal short-circuit current limiting
• Available in standard voltage options: 5V, 12V, and 15V
• Offered in both TO-3 metal can and TO-220 plastic packages
• Wide operating temperature range

Applications

Power Supply Regulation

The primary application of the LM78XX series is in providing regulated DC power to electronic circuits. Typical uses include:

• Local voltage regulation for digital logic circuits
• Power supply stabilization in instrumentation systems
• Voltage regulation in audio and Hi-Fi equipment
• Microprocessor and microcontroller power supplies
• Peripheral device power regulation

Specialized Configurations

While designed as fixed voltage regulators, the LM78XX can be adapted for various specialized applications:

• Adjustable output voltage when used with external components
• Current boosting configurations for higher output currents
• Battery charger circuits
• Precision voltage references

Electrical Characteristics

Absolute Maximum Ratings

• Input Voltage: 35V (for LM7805), 35V (LM7812), 35V (LM7815)
• Operating Junction Temperature Range: 0°C to +125°C
• Storage Temperature Range: -65°C to +150°C
• Lead Temperature (Soldering, 10 seconds): 300°C

Typical Electrical Specifications (LM7805C)

• Output Voltage: 4.8V to 5.2V (at 25°C)
• Line Regulation: 3mV (Typical), 100mV (Maximum)
• Load Regulation: 5mV (Typical), 100mV (Maximum)
• Quiescent Current: 4.2mA (Typical), 8mA (Maximum)
• Ripple Rejection: 62dB (Typical) at 120Hz
• Dropout Voltage: 2V (Maximum) at 1A output
• Output Noise Voltage: 40μV (Typical) over 10Hz to 100kHz

Package Information

TO-3 Metal Can Package (KC02A)

• Package Type: Hermetically sealed metal can
• Terminals: 3 (Input, Ground, Output)
• Dimensions: 10.67mm height, 22.1mm diameter
• Mounting: Requires insulating hardware for chassis mounting
• Thermal Resistance: 5°C/W (Junction to Case)
• Order Numbers: LM7805CK, LM7812CK, LM7815CK

TO-220 Plastic Package (T03B)

• Package Type: Molded plastic with metal tab
• Terminals: 3 (Input, Ground, Output)
• Dimensions: 10.4mm height, 15.24mm width, 4.7mm thickness
• Mounting: Through-hole or heatsink mounting
• Thermal Resistance: 5°C/W (Junction to Case)
• Order Numbers: LM7805CT, LM7812CT, LM7815CT

Thermal Considerations

Proper thermal management is critical when using the LM78XX series regulators, especially at higher current loads. The internal thermal overload protection will shut down the regulator if excessive junction temperatures are reached, but this should not be relied upon for normal operation.

For continuous operation at maximum current, a heatsink is strongly recommended. The required heatsink thermal resistance can be calculated using the following formula:

θ_SA = [(T_J - T_A)/P_D] - θ_JC - θ_CS

Where:
θ_SA = Heatsink thermal resistance
T_J = Maximum junction temperature (125°C)
T_A = Ambient temperature
P_D = Power dissipation
θ_JC = Junction-to-case thermal resistance (5°C/W)
θ_CS = Case-to-sink thermal resistance (typically 0.5°C/W with thermal compound)

Design Considerations

Input Capacitance

While the LM78XX can operate without external components, a 0.33μF capacitor at the input is recommended when the regulator is located more than a few inches from the power supply filter. This helps improve transient response and stability.

Output Capacitance

An output capacitor of 0.1μF or greater improves transient response and can be required for stability in some applications. For improved ripple rejection, larger values (1μF to 10μF) are recommended.

Protection Diodes

In applications where the output voltage may be held high while the input is pulled low (such as when power is removed), protection diodes should be added between input and output terminals to prevent reverse biasing of the regulator.

Reliability and Quality

The LM78XX series is manufactured using a high-reliability bipolar process with strict quality control measures. Key reliability features include:

• 100% thermal limit and current limit testing
• High temperature reverse bias burn-in
• Extended life testing
• Moisture resistance: MIL-STD-883 Method 1004

Alternative Devices

For applications requiring different specifications, consider these alternatives:

• LM79XX: Negative voltage regulators
• LM317: Adjustable positive voltage regulator
• LM2940: Low dropout regulator
• LM2937: Low quiescent current regulator

Conclusion

The LM78XX series of voltage regulators provides a robust, reliable solution for power supply regulation in a wide variety of electronic systems. With its combination of high current capability, built-in protection features, and simple implementation, this regulator family continues to be a popular choice for designers decades after its introduction. The availability in both metal can and plastic packages allows flexibility in design while maintaining excellent thermal performance.