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BQ2972: Can I use a series resistance to adjust the RDS value of the MOSFET

Adarsh Chengot Poonthottathil
Prodigy 140 points
Part Number: BQ2972
Other Parts Discussed in Thread: CSD16406Q3

Dear team,

I have used BQ29728DESR IC in my sensor circuit. It is used to protect a 3.7V 500mAh LiPo battery.  My charge current is less than 250 mA. I am using two CSD16406Q3 charge MOSFETs.. Unfortunately, I cannot change the MOSFETs in the circuit. However there is a series 0 ohm resistor is the circuit (R12). Please see the screenshot below. 

These are my requirements. I need over current discharge protection and over current charge protection --> 1A. Is there a way to achieve this requirement using my current setup? Can I adjust the value of R12 to meet my requirements. I am a little bit confused with the calculations here. So can you please suggest the value of R12 for my circuit?

Your help is highly appreciated.

Thank you. 

  • +1
    TI__Mastermind 32350 points

    Hello Adarsh,

    Since the part detects over-current protection from differential measurements between V- and Vss, I believe this could work but we have not tested this. Take into account that the Rdson of the FET changes with the gate-source voltage. See the datasheet for your FET.

    The calculation may be different depending on the part chosen. If you chose an overcurrent during discharge (OCD) threshold of 100-mV. You would require the total resistance between  V- and Vss to be (100-mV)/(1-A) = ~100-mOhm, this would the the Rdson of both FETs + your series resistance R12.

    Best Regards,

    Luis Hernandez Salomon