Цитата(XVR @ Jul 6 2015, 15:11)

Неа, это Q5
да согласен.
тогда поищите "bq40z50 Advanced Gas Gauge Circuit Design" (slua660a.pdf)
там на стр.11 есть схожая схема подключения (и даже Q4 совпадает) и цитата:
Код
Q4 is provided to protect the discharge FET (Q3) in the
event of a reverse-connected charger. Without Q4, Q3 can be driven into its linear region and suffer
severe damage if the PACK+ input becomes slightly negative.
Q4 turns on in that case to protect Q3 by shorting its gate to source. To use the simple ground gate
circuit, the FET must have a low gate turn-on threshold. If it is desired to use a more standard device,
such as the 2N7002 as the reference schematic, the gate should be biased up to 3.3 V with a high-value
resistor
в вашем случае защищается Q5