SNOSDF5 September   2023 LM74700D-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Revision History
  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
  8. 7 Typical Characteristics
  9. 8 Parameter Measurement Information
  10. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Input Voltage
      2. 9.3.2 Charge Pump
      3. 9.3.3 Gate Driver
      4. 9.3.4 Enable
    4. 9.4 Device Functional Modes
      1. 9.4.1 Shutdown Mode
      2. 9.4.2 Conduction Mode
        1. 9.4.2.1 Regulated Conduction Mode
        2. 9.4.2.2 Full Conduction Mode
        3. 9.4.2.3 Reverse Current Protection Mode
  11. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Design Considerations
        2. 10.2.2.2 MOSFET Selection
        3. 10.2.2.3 Charge Pump VCAP, Input and Output Capacitance
      3. 10.2.3 Selection of TVS Diodes for 12-V Battery Protection Applications
      4. 10.2.4 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications
      5. 10.2.5 Application Curves
      6. 10.2.6 OR-ing Application Configuration
    3. 10.3 Power Supply Recommendations
    4. 10.4 Layout
      1. 10.4.1 Layout Guidelines
      2. 10.4.2 Layout Example
  12. 11Device and Documentation Support
    1. 11.1 Receiving Notification of Documentation Updates
    2. 11.2 Support Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  13. 12Mechanical, Packaging, and Orderable Information

Package Options

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

10.2.2.2 MOSFET Selection

The important MOSFET electrical parameters are the maximum continuous drain current ID, the maximum drain-to-source voltage VDS(MAX), the maximum 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. This includes any anticipated fault conditions. TI recommends to use MOSFETs with voltage rating up to 60-V maximum with the LM74700D-Q1 because anode-cathode maximum voltage is 65 V. The maximum VGS LM74700D-Q1 can drive is 13 V, so select s MOSFET with 15-V minimum VGS. If a MOSFET with < 15-V VGS rating is selected, a Zener diode can be used to clamp VGS to safe level. During start-up, 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, lowest possible RDS(ON) is preferred, but selecting a MOSFET based on low RDS(ON) cam not be beneficial always. Higher RDS(ON) provides increased voltage information to the LM74700D-Q1 reverse comparator at a lower reverse current. Reverse current detection is better with increased RDS(ON). TI recommends to operate the MOSFET in regulated conduction mode during nominal load conditions and select RDS(ON) such that at nominal operating current, forward voltage drop VDS is close to 20-mV regulation point and not more than 50 mV.

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

MOSFET manufacturers usually 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, TI recommends to operate the MOSFET close to 4.5-V VGS, that is, much higher than MOSFET gate threshold voltage. TI recommends to choose MOSFET gate threshold voltage Vth of 2-V to 2.5-V maximum. Choosing a lower Vth MOSFET also reduces the turn-ON time.

Based on the design requirements, preferred MOSFET ratings are:

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

DMT6007LFG MOSFET from Diodes Inc. is selected to meet this 12-V reverse battery protection design requirements and is rated at:

  • 60-V VDS(MAX) and ±20-V VGS(MAX)
  • RDS(ON) 6.5-mΩ typical and 8.5-mΩ maximum rated at 4.5-V VGS
  • MOSFET Vth: 2-V maximum

Consider thermal resistance of the MOSFET against the expected maximum power dissipation in the MOSFET to make sure that the junction temperature (TJ) is well controlled.