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BQ24074: Out Pin Question

Tyler FB
Prodigy 70 points
Part Number: BQ24074

I'm trying to implement a design that will work with a product that is already in production.

The current product has an AUX port that provides VOut from the BQ24074. If the BQ24074 doesn't have anything connected to "In"; Vout obviously runs off the battery.  Looking at the Functional Block Diagram of the BQ24074, there is a P-Ch FET (Q2) between Out and BAT, with the intrinsic diode blocking Vout from back feeding BAT. There is also a N-Ch FET (Q1) between Out and In, allowing Out to back feed to In. 

My question is what would happen if I apply an external voltage to the AUX port that would connect to VOut, thus powering the system from the AUX port instead? Would Q2 be off ensuring that the external voltage does not get applied to the battery?

Thanks for the help!

  • 0
    Prodigy 70 points

    To supplement my question, this is my thought of what would happen.

    I think that "Out" will get back-fed to "In" through Q1 since the intrinsic diode connects "Out" to "In". This would make the BQ24074 think that an input voltage has been applied. The Vin-Low comparator would then trigger Q2 to turn on connecting "Out" to the battery. This would bypass all the current protection for the battery which most likely cause battery failure. 

    I'm curious if anybody knows what would really happen.

    Thanks again!