Question
L7983 - Low-side MOSFET failure (LX shorted to GND)
Hello,
We are investigating a recurring failure of an L7983 used in a medical device.
Application:
- VIN ≈ 40 V
- VOUT = 5 V
- L = 68 µH
- Cout = 2.2 µF
- LCM mode
- Very light load conditions possible
Failure mode:
- 5 V output lost.
- After removing the output inductor, LX remains shorted to GND.
- Resistance LX-GND ≈ 2.5 Ω.
- High-side MOSFET appears intact (no abnormal resistance between VIN and LX).
- Failure seems limited to the internal low-side MOSFET.
Field observations:
- The failure is always preceded by a battery-related error.
- Batteries are often found degraded or unable to charge correctly.
- Similar failures have also occurred during production.
Tests performed:
- Startup without battery.
- Startup with low battery conditions.
- Long-duration operation with low battery voltage.
- Heavy battery discharge during operation.
- Various startup sequences.
No test has reproduced the failure so far.
Measurements:
- LX peak ≈ 50 V.
- Undershoot down to approximately -1.7V
- No obvious overcurrent event detected.
Questions:
- Have you seen failures where only the low-side MOSFET becomes shorted?
- Could repeated reverse-current events or output energy fed back into the regulator progressively damage the low-side MOSFET?
- Are there known failure mechanisms associated with LCM operation at very light loads?
- What stress tests would you recommend to reproduce this type of failure?
Thank you for any suggestions or similar experience with the L7983.
