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BQ76920: BQ76920 boot switch alternatives

Part Number: BQ76920

Hello everybody,

I am testing a board with the components : BQ7692003 and BQ78350DBT, I made a first version of it, with the boot switch linking VC1 to TS1 and it worked well, but for now I needed to change it and adopt a boot switch alternatives as shown in the "Application Report - SLUA769". I choose the scheme "Figure 5. Pack Boot Switch From REGSRC, I put here a screenshot of it, with the components values :

My schema is the following: 

BQ_TS1 : is connected to the p.6 TS1 of the BQ76920.

RegSRC : is the input of the BQ76920, linked to its p.9 REGSRC and p.10 BAT.

Vdrive : is the output of the BQ76920, linked to its p.8 REGOUT.

Pack-_CHG : is the Source of my Charge MOSFET, so this is the Neg pin of the charging unit placed before the BMS unit.

GND1 : is the GND of the BMS unit, linked to the p.3 VSS of the BQ76920.

Normally I should use 5 x 18650 cells, but for the test I replaced them with a power supply, set at +19.0V (like if each battery had +3.8V). When I switch on the power supply, I can see on the Q408's p.2 (S) +19.4V and +17.0V on the Q408's p.1 (G), so my Vgs=17-19.4= -2.4V but I can't see any tension on the Q408's p.3 (D), the current should pass through the FET isn't it ?

I am a little bit confused with all the differents ground references on the scheme, I think it could be a source of error.

Can you help me with that please ?



  • Hello Lucas,

    Are you attaching a charger on PACK+/PACK-? This boot circuit works by attaching a charger, if the charger voltage is sufficient, Q409 would turn-on, which would turn-on Q408 and provide a voltage/current to the TS2 pin. Once the device boots up, Q406 would turn on to disable the boot circuit by pulling Q407 down.

    What value do you read if you directly measure the gate-source voltage of the FET, as opposed to just the gate and just the source? The voltage of the gate of Q408 should be about the same as its source if Q409 is not enabled.

    The grounds are all okay, Q409 is referenced to PACK-, so that it will turn-on once a large enough charging voltage is attached. Since the gate is referenced to GND, the source voltage would be what determines if it turns on or not. With a large enough charger voltage, the gate-source voltage would be enough to turn this FET on.

    I've simulated this circuit and it seems to work fine! It also was tested by the author of the application note.

    Best Regards,

    Luis Hernandez Salomon

  • Thanks a lot, my circuit board is quite complex and so I wanted to test just the BMS part, I totally forgot that the Charger needed to be llinked to boot it. Of course I have a CC-CV Charger, I plugged a power supply to replace it and the BQ76920 has started.

    Have a good day