Other Parts Discussed in Thread: BQ24295, CSD16301Q2, CSD,
I was previously told that in the event of a reverse polarity condition the BQ29700 will hold the charge FET in the off state. The body diode of the discharge FET is forward biased so the state of the discharge FET gate does not matter. It turns out that the BQ29700 does not do a very good job of holding the gate of the charge FET in the off state... I implemented the BQ29700 with ECH8697R back to back N channel FETs as external protection for a 3.7V 18650 lithium ion cell on the battery output of a BQ24295 switching charger. When you insert the battery backwards the discharge FET body diode is forward biased and the charge FET Vgs is held around 0.9-1.0V. Unfortunately, this allows the charge FET to conduct a very large current (>1A) and dissipate a very large amount of power.
Note that the ECH8697R has a Vgsth of 0.5-1.3V which is slightly lower than the CSD16301Q2 Vgsth of 0.9-1.55V (used on the eval board), but I wouldn't think that changing to the CSD FETs would fix this issue. With all of the reverse current paths removed from our circuit the BQ29700 is able to hold the charge FET in the off state, but then there's no circuit to protect.
Is there anything that can be done to fix this issue? A separate reverse polarity protection FET could be added between the battery and the protection circuit, but the protection circuit really shouldn't need a protection circuit!
