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bq25892 | Q2, Q4 Fault Modes

Antenna_Head
Genius 11395 points
Other Parts Discussed in Thread: BQ25892

Team-

We have a customer that is trying to assess the failure mode/rate of the bq25892, specifically the Q2, Q4 FETs.

If these FETs were to both fail short, it could potentially connect the battery to Vbus.

Given that Q2 is driven off the SW node by the high side driver, I believe that it is a completely isolated from Q4, please confirm.

In this case, Vbus = 5V, and the cell is a 3.1A-hr Li-ion.

Additionally, they would like to know the locations of these FETs on the die- a micrograph would be helpful. 

We have an NDA with this customer, so please contact me privately if needed to close out this thread.

Thanks, Best, S. Dunbar, AFA Colorado

  • 0
    TI__Guru*** 143980 points

    Steve,

    Q2 is the high side buck N FET. Its source connects to the SW node which connects to SYS through the inductor.  The output side of the inductor also connects to the drain of Q4.  It is highly improbable that both Q2 and Q4 would fail at the same time.  If Q2 fails to short, BTST will not get boosted high enough to turn on reverse blocking NFET Q1 so you will have protection.  If Q4 fails to short, the battery's internal protector OVP circuit should protect the battery. I will email you separately regarding the chip layout.

  • 0 in reply to Jeff F
    TI__Genius 11395 points

    Thanks; are there any failure rate statistics you can provide or any specifics about how one of these FETs could fail?  I'm assuming that we design these things not to fail and there is a lot of margin, not to mention the fact that a cascading failure is even more unlikely.

    At any rate, trying really hard to get closure here.

    Thanks, Best, Steve