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

8.2.2.2 MOSFET Selection

The important MOSFET electrical parameters are the maximum continuous drain current ID, the maximum drain-to-source voltage VDS(MAX), the maximum gate-to-source voltage VGS(MAX), and the drain-to-source On resistance RDS(ON).

The VDS(MAX) rating of the MOSFET must be high enough to withstand the highest differential voltage seen in the application, including any anticipated transients during fault conditions. For a 48V PoE application, a MOSFET with a voltage rating of 100V is recommended. The LM74681 can drive a maximum gate-to-source voltage of 13.8V. A MOSFET with a minimum VGS(MAX) rating of 15V should be selected. For MOSFETs with lower VGS ratings, a Zener diode can be used to clamp the voltage to a safe level.

The MOSFET ID rating must exceed the maximum continuous load current to ensure reliable operation under full load conditions. Additionally, the MOSFET thermal resistance should be considered to ensure the junction temperature (TJ) remains within safe limits under the expected maximum power dissipation including the initial inrush phase when the output capacitors are charged through the MOSFETs body diode.

To reduce the MOSFET conduction losses, the lowest possible RDS(ON) is preferred, but selecting a MOSFET based on low RDS(ON) may not always be beneficial. Higher RDS(ON) will provide increased voltage information to the LM74681 reverse comparator at a lower reverse current. Reverse current detection is better with increased RDS(ON). Choosing a MOSFET with RDS(ON) that develops <30mV forward voltage drop at maximum current is a good starting point. Usually, RDS(ON) increases drastically below 4.5V VGS and RDS(ON) is highest when VGS is close to MOSFET Vth. For stable regulation at light load conditions, it is recommended to operate the MOSFET close to 4.5V VGS, that is, much higher than the MOSFET gate threshold voltage. It is recommended to choose MOSFET gate threshold voltage Vth of 2.5V to 3.5V maximum. Choosing a lower Vth MOSFET also reduces the turn ON time.

PSMN040-100MSE N-channel MOSFET is selected to meet this 48V PoE PD bridge rectifier design and it is rated at:

  • VDS(MAX): 100V
  • VGS(MAX): ±20V
  • RDS(ON): 29.4mΩ (typical) and 36.6mΩ (maximum) at 10V VGS