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Original question:

BQ76942: Role of Additional FET in Discharge FET Drive Circuit

BQ76942: Sizing Charge Pump Capacitor

Ishaan Govindarajan1
Prodigy 40 points
Part Number: BQ76942
Other Parts Discussed in Thread: BQ76200,

Hi! 

It was mentioned in the BQ76942 datasheet that the charge pump capacitor value can range 100nF - 2200nF. Is there a rule of thumb in sizing this capacitor given NFET input capacitances, similar to the 470nF/20nF rule for the BQ76200 (pg. 13 of http://www.ti.com/lit/ds/symlink/bq76200.pdf)?

Thanks in advance!

  • 0
    TI__Guru 54715 points

    Hi Ishaan,

    The typical 470nF charge pump capacitor recommended for the BQ76942 should be suitable for loading up to 20 nF.  The BQ76942 does not have a UVLO like the BQ76200, so if you can accept more droop during turn on of the FETs you can use a smaller charge pump capacitor or more FET loading.  Generally more capacitance is good in that there will be less droop when the driver turns on, but it will cost more for the cap and take longer to charge up to full voltage.  You would want to avoid a situation where the charge pump capacitor is excessively large and the system is operating with the FETs partially on as the charge pump builds the gate voltage.  The best ratio of capacitance will be a design choice for your system.