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BQ77216: Back to Back Protection FETs not completely turned on

Ron Alers
Prodigy 30 points
Part Number: BQ77216

Tool/software:

The power protection FETs I used were N-FETs.  They are hooked up with common Drain. These have a built-in protection zener from the Source to the Gate, so I did not add one.

The problem is the first FET Q1 does not turn on completely and has a 2.5 V drop across it.  The second FET, Q3, only has its expected diode drop.  Why does this not work, and can you recommend a fix or different parts?  The schematic is included.

  • 0
    TI__Mastermind 37360 points

    Hello Ron,

    This will not work. The output of the BQ7721600 device is open-drain/active-low. So you would need to have a pull-up on the COUT/DOUT, so that it is able to turn-on.

    You can try adding a pull-up and test this again.

    Now, keep in mind that this device does not have current detection, so it will not have any body-diode protections, in case current is flowing through the body-diode of a MOSFET.

    Best Regards,

    Luis Hernandez Salomon