SNOSDG3 December   2024 LM74681

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Switching Characteristics
    7. 5.7 Typical Characteristics
  7. 6 Parameter Measurement Information
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Input and Output Voltage
      2. 7.3.2 Charge Pump
      3. 7.3.3 Gate Driver
      4. 7.3.4 Enable
    4. 7.4 Device Functional Modes
      1. 7.4.1 Conduction Mode
        1. 7.4.1.1 Regulated Conduction Mode
        2. 7.4.1.2 Full Conduction Mode
      2. 7.4.2 Reverse Current Protection Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Design Considerations
        2. 8.2.2.2 MOSFET Selection
        3. 8.2.2.3 Output capacitance
      3. 8.2.3 Application Curves
    3. 8.3 Powered Device for IEEE 802.3bt Class 5-8 (45W-90W) Systems
    4. 8.4 Power Supply Recommendations
      1. 8.4.1 Transient Protection
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
      2. 8.5.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Receiving Notification of Documentation Updates
    2. 9.2 Support Resources
    3. 9.3 Trademarks
    4. 9.4 Electrostatic Discharge Caution
    5. 9.5 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.3 Powered Device for IEEE 802.3bt Class 5-8 (45W-90W) Systems

In PoE PD applications requiring greater than 30W power, the IEEE 802.3bt standard recommends the use of a 4-pair cable for efficient power delivery. To support this, two LM74681 controllers can be used to drive two independent N-channel MOSFET based full-bridge rectifiers connected to a common output. Each LM74681 drives one full-bridge rectifier, enabling power rectification from the two separate 2-pair power paths provided by the 4-pair cable. This approach ensures compliance with the higher power delivery capabilities of the IEEE 802.3bt standard while maintaining high efficiency.

The linear ORing gate control mechanism of LM74681 actively drives the MOSFET gates, allowing forward conduction and reverse current blocking. This prevents backflow between the two power paths and avoids current flow back into the Ethernet cable. By replacing traditional diodes with low RDS(ON) MOSFETs, the LM74681 reduces conduction losses and heat generation, which is critical for high-power PoE systems. The rectified outputs of the two full bridges are combined at the PD input, providing seamless power delivery. This architecture enables efficient, reliable operation for PoE PD systems requiring high power levels in compliance with IEEE 802.3bt standards.

LM74681 Typical PoE PD High Power ApplicationFigure 8-8 Typical PoE PD High Power Application