LM2679T - 5A Step-Down Switching Voltage Regulator

Overview

The LM2679T is a monolithic integrated circuit belonging to the LM2679 series of high-efficiency step-down (buck) switching voltage regulators. Designed to deliver up to 5A of output current, this regulator provides excellent line and load regulation characteristics while maintaining high efficiency (>90%) through the use of a low ON-resistance DMOS power switch.

The device is available in fixed output voltage versions (3.3V, 5V, and 12V) as well as an adjustable output variant, making it versatile for various power supply designs. The LM2679T incorporates several protection features including thermal shutdown and programmable current limiting, ensuring robust operation under fault conditions.

Key Features

• High Efficiency: Greater than 90% efficiency due to low ON-resistance DMOS power switch
• High Output Current: Capable of delivering up to 5A continuous output current
• Output Voltage Options: Fixed 3.3V, 5V, 12V versions and adjustable output variant
• Precision Output: ±2% output voltage tolerance over line and load conditions
• Soft-Start Capability: Reduces input surge current during power-on
• Programmable Current Limit: Resistor-programmable MOSFET switch current limit
• Thermal Protection: Built-in thermal shutdown protection
• Wide Input Voltage Range: Operates from 8V to 40V input
• Fixed Frequency Operation: 260kHz nominal switching frequency

Applications

The LM2679T is suitable for a wide range of power conversion applications including:

• Industrial power systems
• Automotive electronics
• Distributed power systems
• Battery chargers
• Telecommunications equipment
• Computer peripherals
• LCD displays and monitors
• Embedded systems
• Point-of-load regulation

Electrical Characteristics

Absolute Maximum Ratings

• Input Voltage (VIN): 40V
• Switch Current (ISW): 5A (continuous), 7A (peak)
• Feedback Pin Voltage: -0.3V to +40V
• Operating Junction Temperature: +150°C
• Storage Temperature Range: -65°C to +150°C

Recommended Operating Conditions

• Input Voltage Range: 8V to 40V
• Output Current: 0A to 5A
• Operating Temperature Range: -40°C to +125°C
• Soft-Start Capacitor Range: 0.01µF to 0.1µF

Electrical Specifications (TA = 25°C, VIN = 12V unless otherwise specified)

• Output Voltage Accuracy: ±2% (fixed voltage versions)
• Feedback Voltage: 1.21V (adjustable version)
• Oscillator Frequency: 260kHz (typical)
• Standby Current: 2.5mA (typical)
• Switch ON Resistance: 150mΩ (typical)
• Current Limit Threshold: Programmable from 5A to 7A
• Thermal Shutdown Threshold: 150°C (typical)
• Soft-Start Time: Adjustable via external capacitor

Package Information

The LM2679T is available in a TO-220 package with the following mechanical characteristics:

• Package Type: TO-220-7 (7-lead)
• Mounting Style: Through-hole
• Package Dimensions: 10.16mm x 8.89mm x 4.45mm (body)
• Lead Pitch: 2.54mm
• Lead Material: Copper alloy
• Weight: Approximately 2.0 grams
• Thermal Resistance (Junction-to-Case): 3°C/W
• Thermal Resistance (Junction-to-Ambient): 35°C/W

Functional Description

Internal Architecture

The LM2679T integrates all the necessary components for a complete step-down switching regulator including:

• High-current DMOS power switch
• Fixed-frequency oscillator (260kHz)
• Error amplifier
• PWM comparator
• Soft-start circuit
• Current limit comparator
• Thermal shutdown circuit
• Bandgap reference

Operation Principle

The device operates as a conventional buck converter using pulse-width modulation (PWM) control. The internal power switch turns on at the beginning of each clock cycle, allowing current to flow from the input to the output through an external inductor. When the switch turns off, the inductor current continues to flow through an external Schottky diode.

The output voltage is regulated by adjusting the duty cycle of the power switch based on the feedback signal. The fixed 260kHz switching frequency provides a good balance between efficiency and component size.

Design Considerations

External Component Selection

Proper selection of external components is critical for optimal performance:

• Inductor: 33µH to 100µH, rated for at least 5A with low DCR
• Output Capacitor: Low-ESR electrolytic or ceramic, typically 100µF to 470µF
• Schottky Diode: 5A rating with low forward voltage
• Input Capacitor: Low-ESR type, typically 100µF located close to the IC

Thermal Management

For continuous operation at maximum load current, proper heat sinking is required. The TO-220 package allows for easy attachment of a heatsink. The following thermal considerations apply:

• Maximum power dissipation depends on thermal resistance and ambient temperature
• Use thermal compound between package and heatsink
• Ensure adequate airflow in the application environment

Conclusion

The LM2679T provides a complete, high-efficiency step-down switching regulator solution in a single package. With its high output current capability, precision regulation, and comprehensive protection features, it is well-suited for demanding power conversion applications across various industries. The device's fixed-frequency operation simplifies EMI filter design while its high efficiency minimizes thermal management challenges.