LM2678T

LM2678T - 5A Step-Down Switching Voltage Regulator

Overview

The LM2678T is a monolithic integrated circuit belonging to the LM2678 series of high-efficiency step-down (buck) switching voltage regulators. Capable of delivering up to 5A of output current, this regulator offers outstanding line and load regulation characteristics while maintaining exceptional efficiency exceeding 90%. The device incorporates a low ON-resistance DMOS power switch that contributes to its high performance.

The series includes fixed output voltage versions (3.3V, 5V, and 12V) as well as an adjustable output variant, providing design flexibility for various applications. The LM2678T simplifies power supply design by requiring minimal external components while delivering robust performance.

Key Features

• High efficiency up to 92%
• 5A output current capability
• Available in fixed output voltages (3.3V, 5V, 12V) and adjustable version
• Wide input voltage range: 8V to 40V
• Low ON-resistance DMOS power switch (typically 130mΩ)
• 150kHz fixed frequency internal oscillator
• Simple design with minimal external components required
• Thermal shutdown and current limit protection
• ±2% output voltage tolerance over line and load conditions
• TTL shutdown capability
• Uses readily available standard inductors

Applications

The LM2678T is suitable for a wide range of applications requiring efficient power conversion:

• Industrial power systems
• Automotive electronics
• Distributed power systems
• Battery chargers
• Telecommunications equipment
• Computer peripherals
• Embedded systems
• LED drivers
• Point-of-load regulation
• Consumer electronics

Electrical Characteristics

All specifications are typical at 25°C unless otherwise noted.

Absolute Maximum Ratings

• Input Voltage (VIN): 45V
• Switch Voltage (VSW): 45V
• Feedback Pin Voltage (VFB): -0.3V to 40V
• Shutdown Pin Voltage: -0.3V to 40V
• Operating Junction Temperature: -40°C to +125°C
• Storage Temperature Range: -65°C to +150°C

Recommended Operating Conditions

• Input Voltage Range: 8V to 40V
• Output Current: 0A to 5A
• Operating Junction Temperature: -40°C to +125°C

Electrical Specifications (5V Output Version)

• Output Voltage: 5V ±2%
• Feedback Voltage (Adjustable Version): 1.21V
• Oscillator Frequency: 150kHz ±10%
• Maximum Duty Cycle: 90%
• Quiescent Current: 5mA typical
• Shutdown Current: 50μA typical
• Current Limit Threshold: 6A typical
• Thermal Shutdown Threshold: 150°C typical

Package Information

The LM2678T is available in a TO-220 package with the following mechanical specifications:

TO-220 Package Dimensions

• Package Type: Through-hole, 5-pin
• Total Height: 9.40mm max
• Body Width: 10.00mm max
• Body Length: 15.40mm max
• Lead Pitch: 2.54mm
• Lead Length: 3.30mm min
• Mounting Hole Diameter: 3.60mm
• Thermal Resistance (Junction-to-Ambient): 40°C/W
• Thermal Resistance (Junction-to-Case): 3°C/W

Pin Configuration

• Pin 1: VIN - Input voltage
• Pin 2: Output - Switch output
• Pin 3: GND - Ground
• Pin 4: Feedback - Output voltage sense (adjustable version) or NC (fixed version)
• Pin 5: Shutdown - TTL compatible shutdown input

Design Considerations

The LM2678T requires careful consideration of several design aspects to ensure optimal performance:

Input Capacitor Selection

A low ESR aluminum or tantalum capacitor with a minimum value of 100μF is recommended near the input pin to minimize input voltage ripple. The capacitor should be rated for at least the maximum input voltage with sufficient margin.

Output Filter Components

The output filter consists of an inductor and capacitor. The inductor value should be selected based on the desired ripple current (typically 20-40% of maximum output current). The output capacitor should have low ESR to minimize output voltage ripple.

Thermal Management

While the LM2678T is highly efficient, proper heat sinking may be required depending on the input-output voltage differential and load current. The TO-220 package allows for easy attachment of heat sinks when necessary.

PCB Layout Guidelines

• Keep high current paths short and wide
• Place input capacitor close to the VIN and GND pins
• Use a ground plane for improved thermal performance and noise immunity
• Route feedback traces away from noisy switching nodes
• Consider using a separate analog ground for sensitive control circuitry

Performance Characteristics

The LM2678T exhibits excellent performance across various operating conditions:

Efficiency vs. Load Current

Efficiency remains above 90% for a wide range of load currents, peaking at approximately 92% under typical operating conditions. This high efficiency reduces thermal dissipation and improves system reliability.

Line Regulation

The output voltage remains stable within ±1% for input voltage variations across the entire operating range, demonstrating excellent line regulation characteristics.

Load Regulation

Output voltage variation is typically less than ±0.5% for load current changes from 10% to 100% of maximum rated current, showcasing superior load regulation performance.

Protection Features

The LM2678T incorporates multiple protection mechanisms to ensure safe operation:

• Current Limiting: Protects against output short circuits and overload conditions
• Thermal Shutdown: Disables the regulator if junction temperature exceeds safe limits
• Undervoltage Lockout: Prevents operation at input voltages below specification

Conclusion

The LM2678T switching regulator provides an excellent combination of high efficiency, robust performance, and design simplicity. Its ability to deliver up to 5A of output current with minimal external components makes it an ideal choice for a wide range of power conversion applications. The comprehensive protection features and wide operating temperature range ensure reliable operation in demanding environments.