SLVAFF0 September   2022 TPS25947 , TPS2597 , TPS25981 , TPS25982 , TPS25985

 

  1.   Abstract
  2.   Trademarks
  3. 1Understanding the FET SOA
  4. 2Ensuring FET SOA in Hot-Swap Design
  5. 3eFuse Ensuring Integrated FET SOA Operation
    1. 3.1 Thermal Shutdown
    2. 3.2 eFuse Response to Events Stressing Integrated FET
  6. 4Plotting eFuse AOA
  7. 5eFuse Application Design Recommendations to Ensure Integrated FET Reliability
  8. 6Summary
  9. 7References

5 eFuse Application Design Recommendations to Ensure Integrated FET Reliability

System designer can consider below recommendations to make sure that unknowingly FET SOA is not violated and FET reliability does not worsen adversely over time.

  1. BVDSS limitation – as shown from the FET SOA curve that there is a limit on voltage which can be applied to FET. Similarly in eFuse data sheet recommended maximum and absolute maximum VIN is mentioned. EFuse can easily operate continuously up to recommended maximum without any long term reliability issues. But transients on VIN pin beyond absolute maximum rating can violate FET SOA and cause electrical overstress damage. To mitigate, it is recommended to use TVS diode close to the eFuse VIN pin. Please refer to this blog for more details. Also, it is recommended to use Schottky diode at output to clamp negative transients below absolute minimum rating of VOUT pin and example part number can be found in eFuse EVM schematic.
  2. During steady state operation junction temperature of integrated FET can be calculated as:
    Equation 1. T J =   T A   +   ( P d i s s   ×   R θ J A )
    Equation 2. P d i s s = I L O A D 2   × R O N

    Board layout has to be optimized to achieve lower RθJA so that during steady state for maximum current the junction temperature is below the absolute maximum junction temperature specification in the data sheet which is typically 125°C. Operating eFuse for long time between 125°C and TSD threshold can cause long term reliability issue to eFuse. Please follow the EVM layout for reference.

  3. Persistent fault with auto retry variant- The auto-retry function helps to improve the system uptime without the need of manual intervention, but there is a concern in a scenario where the fault is real and persistent. Indefinite auto-retries will not cause immediate failure of eFuse but average eFuse junction temperature increases and can be close to the maximum recommended junction temperature thus raising concern on the long-term reliability under persistent fault as depicted in Reducing Power Loss and Overheating During Faults with eFuses application note. To assess reliability of eFuse in system FIT rate available on ti.com can be used. Using mission profile of eFuse, which is percentage of lifetime spent at different temperatures in application, final FIT can be calculated as shown in Calculating FIT for a Mission Profile application note.