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Q31 allows the charge FET gate to drop below GND_B potential. Without it Q30 would be a voltage follower and the charge FETs would turn on again if the battery voltage was much less than the charger voltage.
Q33 provides a voltage drop to allow variation of the voltage at the Q32 source without triggering the expedite shutdown circuit (Q34, Q37) mentioned in the TIDA-01093 design guide section 2.1.
