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BQ25306: Why is there a diode on the gate of Q4 for the external path?

Part Number: BQ25306

Following up from this previously posted question- https://e2e.ti.com/support/power-management/f/196/t/977787

Just to clarify, the input current regulation(I INDPM) is 3A min. So in a topology where I am powering the load through PMID and charging the battery simultaneously I am limited to a total of 3A between the load and the battery charging current, correct?

Also is there a specific reason there is a diode on the gate of Q4 for the external power path?

  • Hi Lauren,

    Yes, the total input current is 3A.  If you pull current from PMID pin, it will not be available to the buck converter to provide charge current to the battery.

    The diode from VBUS to Q4 gate, instead of direct connection, prevents a path from SYS through Q4 gate-to-source capacitance back to VBUS and also from BAT back to VBUS through the gate-to-drain capacitance.  A transient spike could be feedback to VBUS otherwise.

    Regards,

    Jeff