SNOSD17G October   2017  – December 2020 LM74700-Q1

PRODMIX  

  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 Parameter Measurement Information
  9. 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
  10. 10Application and Implementation
    1. 10.1 Application Information
      1. 10.1.1 Typical Application
        1. 10.1.1.1 Design Requirements
        2. 10.1.1.2 Detailed Design Procedure
          1. 10.1.1.2.1 Design Considerations
          2. 10.1.1.2.2 MOSFET Selection
          3. 10.1.1.2.3 Charge Pump VCAP, input and output capacitance
        3. 10.1.1.3 Selection of TVS Diodes for 12-V Battery Protection Applications
        4. 10.1.1.4 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications
        5. 10.1.1.5 Application Curves
    2. 10.2 OR-ing Application Configuration
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Receiving Notification of Documentation Updates
    2. 13.2 Support Resources
    3. 13.3 Trademarks
    4. 13.4 Electrostatic Discharge Caution
    5. 13.5 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

10.1.1.4 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications

Typical 24-V battery protection application circuit shown in Figure 10-3 uses two uni-directional TVS diodes to protect from positive and negative transient voltages.

GUID-A70FE938-98FA-4820-ADE8-31598820FB68-low.gifFigure 10-3 Typical 24-V Battery Protection with Two Uni-Directional TVS

The breakdown voltage of the TVS+ should be higher than 48-V jump start voltage, less than the absolute maximum ratings of anode and enable pin of LM74700-Q1 (65 V) and should withstand 65-V suppressed load dump. The breakdown voltage of TVS- should be lower than maximum reverse battery voltage –32 V, so that the TVS- is not damaged due to long time exposure to reverse connected battery.

During ISO 7637-2 pulse 1, the input voltage goes up to –600 V with a generator impedance of 50 Ω. This translates to 12 A flowing through the TVS-. The clamping voltage of the TVS- cannot be same as that of 12-V battery protection circuit. Because during the ISO 7637-2 pulse, the Anode to Cathode voltage seen is equal to (-TVS Clamping voltage + Output capacitor voltage). For a 24-V battery application, the maximum battery voltage is 32 V, then the clamping voltage of the TVS- should not exceed, 75 V – 32 V = 43 V.

Single bi-directional TVS cannot be used for 24-V battery protection because breakdown voltage for TVS+ ≥ 65 V, maximum clamping voltage is ≤ 43 V and the clamping voltage cannot be less than the breakdown voltage. Two un-directional TVS connected back-back needs to be used at the input. For positive side TVS+, SMBJ58A with the breakdown voltage of 64.4 V (minimum), 67.8 (typical) is recommended. For the negative side TVS-, SMBJ26A with breakdown voltage close to 32 V (to withstand maximum reverse battery voltage –32 V) and maximum clamping voltage of 42.1 V is recommended.

For 24-V battery protection, a 75-V rated MOSFET is recommended to be used along with SMBJ26A and SMBJ58A connected back-back at the input.