SNOSDF7A May   2023  – December 2023 LM74703-Q1 , LM74704-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 Typical Characteristics
  8. 7 Parameter Measurement Information
  9. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Input Voltage
      2. 8.3.2 Charge Pump
      3. 8.3.3 Gate Driver
      4. 8.3.4 Enable
      5. 8.3.5 FET Status Indication (FETGOOD)
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Conduction Mode
        1. 8.4.2.1 Regulated Conduction Mode
        2. 8.4.2.2 Full Conduction Mode
        3. 8.4.2.3 Reverse Current Protection Mode
  10. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Design Considerations
        2. 9.2.2.2 MOSFET Selection
        3. 9.2.2.3 Charge Pump VCAP, Input and Output Capacitance
        4. 9.2.2.4 Selection of TVS Diodes for 12-V Battery Protection Applications
        5. 9.2.2.5 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications
      3. 9.2.3 Application Curves
    3. 9.3 Power Supply Recommendations
    4. 9.4 Layout
      1. 9.4.1 Layout Guidelines
      2. 9.4.2 Layout Example
  11. 10Device and Documentation Support
    1. 10.1 Documentation Support
      1. 10.1.1 Related Documentation
    2. 10.2 Receiving Notification of Documentation Updates
    3. 10.3 Support Resources
    4. 10.4 Trademarks
    5. 10.5 Electrostatic Discharge Caution
    6. 10.6 Glossary
  12. 11Revision History
  13. 12Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.2.2 MOSFET Selection

Important MOSFET electrical parameters are the maximum continuous drain current ID, the maximum drain-to-source voltage VDS(MAX), the maximum source current through body diode, and the drain-to-source On resistance RDSON.

The maximum continuous drain current, ID, rating must exceed the maximum continuous load current. The maximum drain-to-source voltage, VDS(MAX), must be high enough to withstand the highest differential voltage seen in the application, including any anticipated fault conditions. Using MOSFETs with voltage rating up to 60-V maximum with the LM74704-Q1 is recommended because the ANODE and CATHODE pin maximum voltage rating is 65 V. The maximum VGS that the LM74704-Q1 can drive is 13 V, so a MOSFET with 15-V minimum VGS must be selected. If a MOSFET with < 15-V VGS rating is selected, a Zener diode can be used to clamp VGS to safe level. During startup, inrush current flows through the body diode to charge the bulk hold-up capacitors at the output. The maximum source current through the body diode must be higher than the inrush current that can be seen in the application.

To reduce the MOSFET conduction losses, the lowest possible RDS(ON) is preferred, but selecting a MOSFET based on low RDS(ON) is not always beneficial. Higher RDS(ON) provides increased voltage information to the reverse comparator of the LM74704-Q1 at a lower reverse current. Reverse current detection is better with increased RDS(ON). Operating the MOSFET in regulated conduction mode during nominal load conditions is recommended as well as selecting RDS(ON) such that at nominal operating current the forward voltage drop VDS is close to 20-mV regulation point and not more than 50 mV.

As a guideline, choose (20 mV / ILoad(Nominal)) ≤ RDS(ON) ≤ (50 mV / ILoad(Nominal)).

MOSFET manufacturers typically specify RDS(ON) at 4.5-V VGS and 10-V VGS. RDS(ON) increases drastically below 4.5-V VGS and RDS(ON) is highest when VGS is close to MOSFET Vth. For stable regulation at light load conditions, operating the MOSFET close to 4.5-V VGS is recommended, which is much higher than the MOSFET gate threshold voltage. Choosing a MOSFET with a typical gate threshold voltage Vth of 2 V to 2.5 V is recommended. Choosing a lower Vth MOSFET also reduces the turn ON time.

Based on the design requirements, the preferred MOSFET ratings are:

  • 60-V VDS(MAX) and ±20-V VGS(MAX)
  • RDS(ON) at 3-A nominal current: (20 mV / 3 A ) ≤ RDS(ON) ≤ (50 mV / 3 A ) = 6.67 mΩ ≤ RDS(ON) ≤ 16.67 mΩ
  • MOSFET gate threshold voltage Vth: 2 V typical

Thermal resistance of the MOSFET must be considered against the expected maximum power dissipation in the MOSFET to make sure that the junction temperature (TJ) is well controlled.