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BQ24725A: RBFET connection

Ohad Fundoianu
Intellectual 320 points
Part Number: BQ24725A

Hello, 

Something in that example design appear in the BQ24725A datasheet is not clear:  

The RBFET is connected in a way that Vs>Vd when adaptor is supplying the system. Since it is N-channel (FDS66880A) How will it conduct when adaptor voltage is connected ?   

  • 0
    TI__Mastermind 19830 points

    Hello Ohad,

    Ohad Fundoianu said:
    The RBFET is connected in a way that Vs>Vd when adaptor is supplying the system. Since it is N-channel (FDS66880A) How will it conduct when adaptor voltage is connected ?   

    When the adapter is present, ACDRV is pulled to CMSRC+6V to turn ACFET and RBFET on. When the adapter is not present, ACDRV is pulled to CMSRC to keep ACFET and RBFET off.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Intellectual 320 points

    Hi Christian,

    Please look again on the scheme of the typical application I attached in my previous question - ACFET and RBFET are both N-channel. Look on the scheme and see that RBFET is connected in a way that its Vs bigger than Vd since the S-side of the MOSFET is closer to the adaptor. In that way, RBFET shall not conduct when ACDRV turns it ON since its Vds<0 so FET is not conducting or conduct to the contrary direction. 

    How does this circuit can work when RBFET seems to be connected in opposite direction. 

    Ohad

  • 0 in reply to Ohad Fundoianu
    TI__Mastermind 19830 points

    Hello Ohad,

    Back-to-back mosfets is a common method for reverse current protection. This does work, most of our EVMs have this protection implemented with no issue.

    This question is more related to the operation of a MOSFET, but basically the roles of the drain and source are swapped. If the drain (on an N-channel device) falls below the voltage on the source, current will flow from the source to the drain). 

    Best regards,

    Christian.