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BQ76940: Strange behaviour of REGOUT Pin.

Nikesh Bisht
Prodigy 10 points
Part Number: BQ76940

Hello Everyone,

I am working on a 13S Li-ion BMS for the last 12 months, using a BQ76940 AFE IC. I've designed a decent prototype, thanks to detailed application notes and datasheet. A strange issue has come up which i am unable to resolve. Whenever I populate a new board and plug it to the cells header, REGOUT pin gives 9-11V output, The MOSFET which is connected to REGSRC pin blows up, and within a few seconds The IC blows up as well. A blister is formed on the IC near REGOUT pin. This has never happened before. There are prototypes employing same IC with same components for that purpose and are working alright. Here are the things I've tried already:

-Populated a different board with utmost care, double checked for soldering imperfections, solder bridges, etc to no avail.

-Added a 200 ohm resistor in series with REGSRC, it caused the IC to blow up immedieately.

-Replaced the REGSRC MOSFET (Si2302) with 120V Vds MOSFET (TK72E12N1), MOSFET didn't blow up this time, but IC did. Previous boards are working smoothly even with Si2302.

-Cried in a corner for days. Didn't help either.

The Schematic and Layout is in Eagle, which I'm willing to share with TI discreetly.

Thanks in Advance

  • 0
    TI__Guru 54715 points
    Hi Nikesh,
    The part can work properly as you have seen with the evaluation board (if you have one) and your previous prototypes.
    The blister on the IC sounds like excessive voltage has come into the IC causing excess current and excess power dissipation in the device. Your task of course is to find out why and correct the problem. Since your designs have been successful before it is likely not in the schematic but the board or assembly. Some possible causes would seem to be:
    Excessive voltage on the gate of the REGSRC dropping FET. This could put too much voltage and thus power into the IC damaging both the IC and FET. Check that the gate is connected and that the VC5X net segments are all connected (no missing filter resistors, open vias, etc).
    FET backward: Check the FET footprint and be sure gate and drain were not somehow swapped so it is operating backward as a diode.
    The other possible cause would seem some leakage or short from a high voltage net onto REGOUT. Certainly check for solder bridges or resistance in the PCB. This does not immediately explain the damaged FET however.
    A useful technique may be to remove the damaged bq76940, install a good dropping FET and measure voltages.