SNOSDE8A July   2023  – September 2023 LM74912-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
    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 Charge Pump
      2. 8.3.2 Dual Gate Control (DGATE, HGATE)
        1. 8.3.2.1 Reverse Battery Protection (A, C, DGATE)
        2. 8.3.2.2 Load Disconnect Switch Control (HGATE, OUT)
      3. 8.3.3 Short Circuit Protection (CS+, CS-, ISCP)
      4. 8.3.4 Overvoltage Protection and Battery Voltage Sensing (SW, OV, UVLO)
      5. 8.3.5 Low IQ SLEEP Mode (SLEEP, SLEEP_OV)
    4. 8.4 Device Functional Modes
  10. 9 Applications and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical 12-V Reverse Battery Protection Application
      1. 9.2.1 Design Requirements for 12-V Battery Protection
      2. 9.2.2 Automotive Reverse Battery Protection
        1. 9.2.2.1 Input Transient Protection: ISO 7637-2 Pulse 1
        2. 9.2.2.2 AC Super Imposed Input Rectification: ISO 16750-2 and LV124 E-06
        3. 9.2.2.3 Input Micro-Short Protection: LV124 E-10
      3. 9.2.3 Detailed Design Procedure
        1. 9.2.3.1 Design Considerations
        2. 9.2.3.2 Charge Pump Capacitance VCAP
        3. 9.2.3.3 Input , Supply and Output Capacitance
        4. 9.2.3.4 Hold-Up Capacitance
        5. 9.2.3.5 Overvoltage Protection and Battery Monitor
        6. 9.2.3.6 Selecting Short Circuit Current Threshold
          1. 9.2.3.6.1 Selection of Scaling Resistor RSET and RISCP for Short Circuit Protection
      4. 9.2.4 MOSFET Selection: Blocking MOSFET Q1
      5. 9.2.5 MOSFET Selection: Hot-Swap MOSFET Q2
      6. 9.2.6 TVS Selection
      7. 9.2.7 Application Curves
    3. 9.3 Best Design Practices
    4. 9.4 Power Supply Recommendations
      1. 9.4.1 Transient Protection
      2. 9.4.2 TVS Selection for 12-V Battery Systems
      3. 9.4.3 TVS Selection for 24-V Battery Systems
    5. 9.5 Layout
      1. 9.5.1 Layout Guidelines
      2. 9.5.2 Layout Example
  11. 10Device and Documentation Support
    1. 10.1 Receiving Notification of Documentation Updates
    2. 10.2 Support Resources
    3. 10.3 Trademarks
    4. 10.4 Electrostatic Discharge Caution
    5. 10.5 Glossary
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.4 MOSFET Selection: Blocking MOSFET Q1

For selecting the blocking MOSFET Q1, important electrical parameters are the maximum continuous drain current ID, the maximum drain-to-source voltage VDS(MAX), the maximum drain-to-source voltage VGS(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 would include all the automotive transient events and any anticipated fault conditions. It is recommended to use MOSFETs with VDS voltage rating of 60 V along with a single bidirectional TVS or a VDS rating 40-V maximum rating along with two unidirectional TVS connected back to back at the input.

The maximum VGS LM74912-Q1 can drive is 14 V, so a MOSFET with 15-V minimum VGS rating should be selected. If a MOSFET with < 15-V VGS rating is selected, a Zener diode can be used to clamp VGS to safe level, but this would result in increased IQ current.

To reduce the MOSFET conduction losses, lowest possible RDS(ON) is preferred, but selecting a MOSFET based on low RDS(ON) may not be beneficial always. Higher RDS(ON) will provide increased voltage information to LM74912-Q1 reverse comparator at a lower reverse current. Reverse current detection is better with increased RDS(ON). Choosing a MOSFET with < 50-mV forward voltage drop at maximum current is a good starting point.

For active rectification of AC super imposed ripple on the battery supply voltage, gate-source charge QGS of Q1 must be selected to meet the required AC ripple frequency. Maximum gate-source charge QGS (at 4.5-V VGS) for active rectification every cycle is:

Equation 11. GUID-20230908-SS0I-XDQG-BQKT-Z8HXJKBHQSGG-low.svg

Where 2.5 mA is minimum charge pump current at 7 V (VDGATE – VA). FAC_RIPPLE is frequency of the AC ripple superimposed on the battery and QGS_MAX is the QGS value specified in manufacturer data sheet at 6-V VGS.

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