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

8.3.3 Short Circuit Protection (CS+, CS-, ISCP)

LM74912-Q1 offers fast response to output short circuit events with the short circuit protection feature. The internal short circuit comparator is enabled when HGATE-OUT voltage is higher than 6.4-V typical. This is to ensure that external FET is fully enhanced and there are no false short circuit triggers during device start-up. When short circuit condition appears at the output and the voltage across CS+ and ISCP exceeds the default short circuit comparator threshold of 50-mV typical, HGATE is pulled to OUT within 2-µs protecting the HFET. FLT asserts low at the same time. Once short circuit condition is detected then device latches off MOSFET Q2 until either the EN, SLEEP or the VS pin is toggled from low to high.

GUID-20230704-SS0I-RGJC-FRTS-L1K3T1J1TP26-low.svg Figure 8-3 Short Circuit Protection Comparator

Short circuit protection threshold can be increased or decreased from the default 50-mV threshold by using an external series resistor RSET from CS+ pin or RISCP from ISCP pin as shown in Figure 8-4. The RSET resistor shifts the threshold in positive direction while RISCP resistor shifts the threshold in negative direction. The shift in the short circuit protection threshold can be calculated using Equation 3 and Equation 4.

Equation 3. GUID-20230630-SS0I-GWN9-W3HX-MJP0ZH2QJ9WB-low.svg
Equation 4. GUID-20230630-SS0I-TFT2-DM14-GLNMGKQWVMKF-low.svg

An additional de-glitch capacitor CISCP can be added from CS+ pin to ISCP pin as shown in Figure 8-4 to provide blanking on any short spurious current spikes to avoid false short circuit trigger in case of fast automotive transients such as Input Micro cuts (LV124, E-10), AC superimpose (LV124, E-06), ISO7637-2 Pulse 2A.

Also when MOSFET Q2 is turned off due to short circuit condition, there can be voltage oscillations on the supply line due to inductive effect of board parasitic and input wiring harness inductance. To avoid these oscillations from reaching to the device supply pin VS and causing false reset, additional series resistor RVS can be inserted. This series resistor RVS forms a R-C low pass filter with VS side decoupling capacitor CVS and helps with damping the oscillations.

GUID-20230704-SS0I-BW9B-QPDC-KG1RW9K3TBJH-low.svgFigure 8-4 Short Circuit Protection With De-glitch Filter

The LM74912-Q1 is intended for applications where precise current sensing is not required, but some level of fault protection is needed. Examples are applications where inductance or impedance in the power path limits the current rise in a short circuit condition.

The Safe Operating Area (SOA) of the external N-Channel MOSFET must be carefully considered to make sure the peak drain-to-source current and the duration of the short circuit protection response time is within the SOA rating of the MOSFET. Also note that the RDS(ON) variations of the external N-Channel MOSFET over given temperature range affect the accuracy of the overcurrent detection.