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BQ34Z100: Resistor Value Calculation

Bhanu Indu Sekhar Muppalla
Intellectual 400 points
Part Number: BQ34Z100


Hello TI Support Team,

         I am currently designing a BMS board for  a Li-Ion battery pack. The battery pack has 13 cells in series and 3 in parallel . The battery pack Voltage is 60V and 4 amps (MAX). I was designing the circuit for Multi-Cell application as in a battery pack. I could not find the formulas to calculate R2,R4,R3,R30. 

Would very much appreciate the help

Thanks

  • +1
    TI__Genius 11770 points

    Hello Bhanu,

    Did you see section 8.1.2 of the datasheet? It doesn't give exact equations but does offer some guidance. The reason being that these resistors don't have an exact requirement according to an equation.

    • R2 and R4 make a voltage divider that drives the FET controlling another voltage divider that feeds bat pin. This is optional and can save power. Choose the values so that Vbat * (R2/(R2+R4)) is a Vgs that will exceed the Vth by a large margin over the whole Vbat range. In addition the higher R2+R4, the less power used by the circuit. 
    • R3 is a current limiting resistor for the zener in the simple LDO circuit. For example, the Zener in this circuit needs about 5mA to reach the desired 5.6V breakdown voltage, so the resistor is chosen so that (Vbat-Vzener)/R3 = Iz. You may need to solve iteratively(Vzener depends on Iz) or simulate. Make it as large as possible to generate the minimum necessary Iz.
    • R30 is the sense resistor. It should be selected so that your load current generates a range of +-125mV. So if your load is 4A, you will get the best accuracy if the sense is 125mV = R * 4, so R= 31.25mΩ. The downside, is that such a large sense resistor will dissipate a lot of power. So there is a tradeoff involved between heat and accuracy. With 4A through this sense it would dissipate 0.5W, which is likely unacceptable. A 10mΩ may be a better balance here.

    To reiterate, all of these values vary based on the system range and what you are trying to optimize. Therefore, we cannot simply provide an equation to calculate them. Make sure you check all of these values for the min and max cases for both Vbat and current. Let me know if you have any other questions.

    thanks,

    Alex M.