SNOSDB7 December   2020 LM74500-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 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. 7 Typical Characteristics
  8. 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
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Conduction Mode
  9. 9 Application and Implementation
    1. 9.1 Reverse Battery Protection for Automotive Body Control Module Applications
    2. 9.2 Reverse Polarity Protection
    3. 9.3 Application Information
      1. 9.3.1 Typical Application
        1. 9.3.1.1 Design Requirements
        2. 9.3.1.2 Detailed Design Procedure
          1. 9.3.1.2.1 Design Considerations
          2. 9.3.1.2.2 MOSFET Selection
          3. 9.3.1.2.3 Charge Pump VCAP, Input and Output Capacitance
        3. 9.3.1.3 Selection of TVS Diodes for 12-V Battery Protection Applications
        4. 9.3.1.4 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications
        5. 9.3.1.5 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Receiving Notification of Documentation Updates
    2. 12.2 Support Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

9.2 Reverse Polarity Protection

P-FET based reverse polarity protection is a very commonly used scheme in industrial and automotive applications to achieve low insertion loss protection solution. A low loss reverse polarity protection solution can be realised using LM74500-Q1 with an external N-FET to replace P-FET based solution. LM74500-Q1 based reverse polarity protection solution offers better cold crank performance (low VIN operation) and smaller solution size compared to P-FET based solution. Figure 9-3 compares the performance benefits of LM74500-Q1 +N-FET over traditional P-FET based reverse polarity protection solution. As shown in Figure 9-3, for a given power level LM74500-Q1+N-FET solution can be three times smaller than a similar power rated P-FET solution. Also as P-FET is self biased by simply pulling it's gate pin low and thus P-FET shows poorer cold crank performance (low VIN operation) compared to LM74500-Q1. During severe cold crank where battery voltage falls below 4 V, P-FET series resistance increase drastically as shown in Figure 9-3. This leads to higher voltage drop across the P-FET. Also with higher gate to source threshold (VT) this can sometimes lead to system reset due to turning off of the P-FET. On the other side LM74500-Q1 has excellent severe cold crank performance. LM74500-Q1 keeps external FET completely enhanced even when input voltage falls to 3.2 V during severe cold crank operation.

GUID-20201215-CA0I-XFSC-WKLQ-4HNMXF7QXFC3-low.gif Figure 9-3 Performance Comparison of P-FET and LM74500-Q1 Based Reverse Polarity Protection Solution.