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BQ2970: Removable Lipo cell - reverse polarity protection/behaviour

James Turner
Prodigy 110 points
Part Number: BQ2970
Other Parts Discussed in Thread: BQ24073,

Hi team,

Looking at coupling the BQ2970 with a  BQ24073 charger for a mobile application. The battery is likely to be a removable AAA lithium rechargeable cell. Because of this form factor (ie the battery CAN be inserted backwards) we want to protect against reverse polarity of the battery. Will the BQ2970 cope with such a situation natively  or will we need to add additional protection (ie low side mosfets) which will isolate the cell from the BQ2970?

 

Thanks in advance!

 

James

  • 0
    TI__Genius 10370 points
    James,

    Thanks for the inquiry. When you say protect against reverse polarity do you mean the FETS will open to protect the circuit. Or are you asking this our part can withstand reverse voltage on the bat pins?

    Thanks,
    Eric Vos
  • 0 in reply to Vos
    Prodigy 110 points
    Hi Eric,

    thanks for your reply. Im asking if I have a setup as in the application (page 21 0f data sheet: www.ti.com/.../slusbu9d.pdf ) and reverse the battery polarity will the part handle this or will it be damaged because of VBAT will be approx -4V. If it could be damaged I will need to implement additional isolation like HIGH/LOW side MOSFETS to prevent this. Appreciate your thoughts.

    Kind regards,

    James,
  • 0 in reply to James Turner
    TI__Genius 10370 points
    James,

    after talking with the designers the response i got it. BAT is allowed to be 0.3V below VSS . With a 4V cell connected in reverse, we can expect the battery voltage to be -4V below VSS. However, there is a 300 Ohm series resistor on BAT. The internal ESD diode can carry up to about 20 mA of DC current, so with the battery at -4V, the resistor will probably limit the current to about 10 mA and the ESD diode will reach around -1V by my estimate. The device should have no issue surviving this condition in my opinion.

    Asking what the part might be damaged at I would say about 50 mA DC might cause an issue and certainly above 100 mA. At 10 mA DC, I’m confident the device will not have an issue.

    Does this information help? I know it is not definitive, but i can say the part was not designed to have reverse voltage applied.

    Thanks,
    Eric Vos