VIN Short Failure

Further testing shows that we have had situations where the input voltage briefly (100ms) dips below 10 V.

This causes the RUN pin to drop below 0.9V. 

This leads to the SS capacitor being discharged. 

Once the input voltage recovers, then the LTM8071 starts up again in the soft start mode. 

As the VOUT is higher than the setpoint of the soft start capacitor, the LTM8071 tries to discharge the output VOUT to the level of the soft start set point. 

We suspect that this kills the lower switching FET

Here is a screenshot of the LTM8071 short circuit failure:

• We have to following situation:
  • System is powered up
    • No load on the 13V Rail
  • Motor cranking switch is enabled
  • A short circuit occurs on the Stater
    • System voltage collapses from 28V to 12V 
  • Motor cranking switch reaches short circuit current limit and switches off
    • System voltage rises rapidly and overshoots to 36V - TVS diodes engage to prevent overvoltage
    • LTM8071 output voltage also rises and overshoots the 13V setpoint - maximum 15V 
  • System voltage collapses again due to oscillations
    • LTM8071 output voltage starts to drop
  • System voltage rises again due to oscillations
    • LTM8071 undershoots 13V setpoint and starts to regulate up. 
    • Spike seen on AUX-BIAS 
  • System voltage collapses rapidly due to LTM8071 internal short circuit
  • System current reaches 200A short circuit limit 
    • System input protection switches input off.

• Is the LTM8071 robust to rapid dv/dt changes on VIN? 
  • DCDC Input voltage was measured at the input of the LTM8071
  • No overvoltage or negative voltage observed