BQ77915: Confirmation about calculation of BQ77915 & BQ76920

Hello Po,

Fist page (Internal 25-mA cell balancing) calculation looks okay!

Second page (External balancing with BQ77915):

• "CSD13202Q2" This MOSFET may be okay, but the drain-source abs max is a bit small. This may make it a bit more susceptible to transients/ESD strikes. We've used 20-V MOSFETs in the past.
• Power rating sounds good! Remember, you can also use multiple resistors in series/parallel to distribute the power dissipation.
• The calculation seems a bit wrong however. To get 500-mA of balancing current, this should just be 4.2V/0.5-A = ~8.4-Ohms.

Third page (External balancing with BQ76920):

• For Rin/Cin when using external balancing MOSFETs, these can be 1-kOhm and 0.22-uF (or 0.1-uF). 1-kOhm would ensure the best voltage drop is provided for the MOSFET turn-on. Power rating for the resistance would be pretty small, as the current flow would not be large. Capacitor voltage rating would depend more on cell voltage, the EVM uses 16-V capacitors.
• I'd suggest to see the TIDA-00449 reference design schematic for an example of the external cell balancing.

Hope this helped .

Best Regards,

Luis Hernandez Salomon