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BQ25731: Failure with low side mosfet drain-gate shorted

Part Number: BQ25731


I've some problem with mosfets in my design using BQ25731.

I'm testing with no battery connected, for now I just try to adjust charge voltage, but Q14 fail in tow board, it is in drain-gate shorted. After some time on, SW1 turn shorted with gnd.

I tried to use a mosfet already used in another charger, but with no succeed. 

I intent to charge a 7A lead acid battery with 14.4V and 1A.

I appreciate your help suggesting me changes to solve that. 

Thank you.

  • Hi Leandro,

    I see that comp2 compensation is not exactly what is recommended by the datasheet section 9.3.12. Also, according to the schematic checklist here, VBUS power should be drawn from the drain of Q11.

    Other than this, please share the register map in order to find other additional issues.

    Also, what do you mean by Q14 fail? 

    Best Regards,


  • Hi Munir,

    when I was setting ChargeVoltage with no battery connected

    - Q14 short between gate and drain. SW1 pin is short to GND.

    "VBUS power should be drawn from the drain of Q11". How may I prevent that, because I use Input Voltage to turn on/off the charger output?

    SMAAI Power Management.pdf

  • Hi Leandro,

    If SW1 is grounded that means the converter is not switching due to some sort of fault. Please capture the register map (values of all the registers) and attach here. If it does not show any fault we need to look at scope captures in order to find the issue. I am guessing that the charger is in SYSOVP fault because CELL_BATPRESZ is ~24% but you may be setting the ChargeVoltage higher.

    Can you elaborate on what is the function of the circuit containing Q7,Q8,Q9 and Q10?

    VBUS power should be drawn from the drain of Q11, by that I simply meant this connection as recommended by the checklist. However, the way you have it is not incorrect. 

    Best Regards,


  • Hi Munir, I've used TIDMAR6 as reference.

    Q7 and Q8 is oring circuit , BQ25731 is used only to charge battery.

    Q9 is used to drain 400mA from battery to capacity test.

    Thank you

  • Hi Leandro,

    Please ignore my comment about CELL_BATPRESZ being 24%. I was looking at ILIM_HIZ pin instead of CELL_BATPRESZ pin. The charger does not seem to be any kind of fault based on charge status register 21/20h. Just making sure did you capture this map when the problem occurred i.e. SW1 was shorted to GND?

    Looking at the schematic more closely, the mosfet you are using is SOT package. It does not have any exposed thermal pad. It also has a higher on-resistance. Hence, a small of power dissipation can damage it. The FET is probably damaged. You can use the mosfets from the TIDMAR6 reference design.

    Best Regards,


  • Hi Munir, I sent to you the dump of register the another board.

    The board with SW1 is shorted with GND, does not respond to i2c commands anymore, and VDDA is in 1.8V.

    I'd like to know how may I prevent MOSFET burn like that, because in two board the low side mosfet failure, and one board sw1 pin goes to GND and bq25731 does not work.

    May be my tests without battery can be a problem. The main power is 15V, I set CELL_BATPRESZ to almost 80% of VDDA, I was expect to have 16.8V  on VBAT, but I is only 15.9V.

    I'll change CELL_BATPRESZ to start in 0% without battery, and when battery is present to 50% of VDDA. And by software I'll set ChargeVoltage to 14.4V to charge in Buck mode only.

    I think choosing better MOSFET and do that changes I could fix my design problem.

    I appreciate some comments.

    Thank yout



  • Hi Leandro,

    When you have no battery CELL_BATPRESZ  should be pulled to ground. However, leaving it to 50% of VDDA and testing without battery should not burn the mosfets. 

    And Yes choosing appropriate mosfets will fix the issue. You can choose the mosfets in TIDMAR6 reference design.

    Best Regards,