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BQ7718: Vout design

Charles Zhang1
Expert 3060 points
Part Number: BQ7718
Other Parts Discussed in Thread: BQ76952EVM, BQ77216

Hi Team,

According to the datasheet, the relationship between VDD and VOUT is shown below.

Then how to design the resistors below in BQ76952EVM if each BQ7718 has different number of series cells? Is there any design guideline? 

 

Thanks,

Charles

  • +1
    TI__Guru* 79893 points

    Hi Charles,

    In most cases, there should be around 7V for VOUT if there are 3 cells connected to a BQ7718 device. The resistor on VOUT forms a resistor divider with R66 to drive the gate of Q12, so as long as the voltage is high enough to turn on Q12, it should be okay. The EVM is tricky because we are not sure how many cells a user will connect to the board. For your application, you should have a better idea, so this should be easier to calculate.

    The newer BQ77216 may also be an option to consider since it can handle up to 16 cells with one device.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Expert 3060 points

    Hi Matt,

    Thanks for your explanation.

    I am also wondering about the resistance selection of R38, R58 in the picture, why 20k in this case?

    Regards,

    Charles  

  • 0 in reply to Charles Zhang1
    TI__Guru* 79893 points

    Hi Charles,

    These are to set the base current for the PNPs. I don't have the calculation details, but I don't think this needs to change based on the number of cells.

    Matt