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

7.3.3 Gate Driver

The gate drivers are used to control the external N-Channel MOSFETs by setting the GATE to SOURCE voltage to the corresponding mode of operation. The FETs on the top side Q1 and Q2 are driven by gates TG1 and TG2 and the FETs on the bottom side Q3 and Q4 are driven by gates BG1 and BG2 respectively.

The internal charge pump powers the top side gate drivers and depending on the DRAIN to SOURCE voltage of each MOSFET, LM74681 has three defined modes of operation the gate driver operates under which are forward regulation, full conduction mode and reverse current protection. These modes are described in more detail in Section 7.4.1.1, Section 7.4.1.2, and Section 7.4.2. Figure 7-1 depicts how the modes of operation vary according to the DRAIN to SOURCE voltage. The threshold between forward regulation and conduction modes is when the DRAIN to SOURCE voltage is VTG_REG. The threshold between forward regulation mode and reverse current protection mode is when the DRAIN to SOURCE voltage is VREV.

LM74681 Gate Driver Mode TransitionsFigure 7-1 Gate Driver Mode Transitions

The bottom-side gate drivers of the LM74681 are powered directly from the IN1 or IN2 voltage and operate in two distinct modes which are forward full conduction and reverse current blocking. These gate drivers are controlled by below logic to ensure efficient power flow and protection against reverse current.

  • BG1 is enabled and in full conduction when the voltage at IN1 is greater than that GND+2V and is disabled when IN1 falls below IN2 to block reverse current flow.
  • Similarly, BG2 is enabled when IN2 exceeds GND+2V and is disabled when IN2 is lower than IN1.