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BQ2970: Battery Protection on PCB side, not battery pack

Part Number: BQ2970
Other Parts Discussed in Thread: BQ2980,

Hi,

I designed a circuit with charging and battery supply where i put the Battery Protection circuit on the PCB and disconnect the bare battery.

It didn't work in the beginning (connecting a battery to the circuit didn't open the discharge FET) and I started debugging by measuring the gate voltages with an oscilloscope. Suddently the circuit worked as expected. A little further investigation i found the hint in the datasheet to add 5MOhms to the GS of both mosfet. This didn't help. But by accident I discovered, that when i short the gates with my finger the circuit works again.

So i soldered a 4.7MOhm resistor between the Gates (Cout and Dout) and everything works as expected. UVL and OVL is in spec, currents i didn't measure.

Can someone please explain whats going on here?

  • Would the BQ2980 be a more suitable device? At least I can't find a note that initial connection may cause DSG not to switch on like in BQ2970.

  • Hello David,

    Did you try shorting Vss and V-? When the battery is connected, the device will act as if it was in an UV condition. You would either have to short Vss and V- is or attach a charger to boot-up the IC.

    The BQ2980 also needs to boot-up, by shorting it's Vdd to PACK+ or attaching a charger, it will also be in a UV-like state when the battery is initially connected.

    I am not sure why placing a resistor between the gates would enable the DSG FET. If adding a charger or shorting Vss/V- does not work, I can inquiry further about this. It is recommended to have the ~5-MOhm resistors between gate and source.

    Best Regards,

    Luis Hernandez Salomon

  • Ok then I need an alternative solution. The device is going to work with of the shelf 18650 LiPo batteries. So a customer can swap batteries while on the move and another button for booting the BP is not an option.

  • Hello David,

    Our protectors are usually placed inside the battery pack, they were not made with swappable batteries in mind. Either a button to shortly short V- and Vss or a charger would be required to initialize the IC.

    We also have a few single-cells secondary protectors that are able to protect against OV by blowing a fuse.

    I apologize for the issue.

    Best Regards,

    Luis Hernandez Salomon