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Original question:

BQ76952: High side MOSFET switch on

BQ76952: External Cell-Balancing Circuit Design using N-channel FETs

Raymond Tan
Prodigy 170 points
Part Number: BQ76952

From the manual, BQ76952 cell balancing, I would like to try out using external N-MOSFET.

Want to clarify on the following: 

1. the cell input resistors can be increased to the maximum recommended value of 100 Ω.

Which is cell input resistor? Rn?

2. I am using Li-FePO4 cells, would like to know whether can I lower my balance resistor value to ~10ohms? 

Where the balance current can increase to 0.36A

3. The external FET should have an RDSON defined at or below 3.9V x 100 / (100 + 100 + 25) = 1.73 V

What does the above statement mean? Why can't I get a MOSFET whose defined Rds(ON) is greater than the voltage, the only downside is the Rds resistor is higher isn't it? 
I just need to make sure that the Vgs (ON) voltage is less than (16mA*100ohm =1.6V)? 

  • +1
    TI__Genius 11315 points

    Hey Raymond,

    1. Rn is cell input resistor.

    2. With external cell balancing, there is no minimum requirement for cell balancing resistors. It is simply a matter of how much current you need for balancing and how much heat your FET and balance resistor can dissipate.

    3. Rds(ON) is the resistance between the drain and source when the FET is on, Vgs is the maximum voltage allowed between the gate and source. The equation you mentioned is to ensure that the MOSTFET can turn on even if the cell has a lower than nominal voltage, since the gate voltage is dependent on the battery.

    Thanks,
    Caleb