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BQ40Z80: BQ40Z80 DSG MOSFETs selection

Lucien Thiriet
Prodigy 40 points
Part Number: BQ40Z80

Dear TI experts,

In the BQ40Z80 datasheet, it is stated that a 40V CHG MOSFET is sufficient, but that for DSG a higher votage MOSFET is needed.

60V MOSFETs have quite a higher Rds than 40V ones.

In which cases is it needed to have such a high voltage? And can I get away with 40V ones if the battery pack is 25.2V max?

Thanks in advance for your help

Regards

  • 0
    Prodigy 40 points
    Another question:

    On "9.2.2.4.2 External Cell Balancing" of datasheet it is said that "Internal cell balancing for cells 1 through 6 can only support up to 10 mA"
    However, with the 100 Ohm resistors on the EVM, that would give 42 mA at 4.2 V.
    Am I missing something? Do we need to duty cycle to meet the max 10 mA spec with internal balancing?

    Thank you
    Regards
  • 0 in reply to Lucien Thiriet
    TI__Mastermind 42535 points
    We spec it such that it meets transient requirements. Therefore a 60V threshold is recommended. There is one change in the ref spec, please remove C6 from the design. It slows down the AFE response to transients. The max current that can be sinked by this device is 50mA. Therefore since you will not be balancing more than 3 cells at the same time and as there's an internal resistance of 200 Ohm in series, you will meet spec even with df writes. However, if you are using a high power application we do recommend that you increase the R to 300 Ohm for using internal cell balancing.
  • 0 in reply to Batt
    Prodigy 40 points

    Batt,

    Thank you very much for your help. I indeed removed C6.
    Ok, I overlooked the 200 Ohm FET resistance. So current on each channel is 14 mA max, that sounds more reasonable.

    I am not sure I understand the part about increasing to 300 Ohm.
    Do I need to increase the external resistors (R38-R43 on EVM) to 300 Ohm in order to be able to balance on all channels simultaneously?
    Is that what you mean by high power application?

    On a lighter note, what is the correct writing?.
    BQ40Z80
    BQ40z80
    bq40z80

    Thank you


    Regards

  • 0 in reply to Lucien Thiriet
    TI__Mastermind 42535 points
    Since all the pins are input pins to the IC, I suggested increase the series R so that you can reduce the current by a small margin. However, to remain in compliance with a 10mA current, you can bump up the series R. If 4.45 is Vmax for a new cell, then 4.45/500 gives you a slightly less than 10mA current that will be within spec. However, the evm parameters are setup for a 100 Ohm resistance.

    The devices are called bq40z80.