I have a BQ29700 connected to a lithium battery using the datasheet values and FDN028N20 mosfets.
On some boards it works just fine, even when starting with a fully discharged battery at near 0V. On a few other boards, however, I can't get the device to turn on the Charge mosfet. I have tried battery voltages as low as 1V and as high as 3.7V, and in all cases the Cout signal is always slightly negative with respect to Vss.
Even when shorting V- to Vss, the behavior is similar.
I am attaching three images showing the voltages at each node in the circuit.
When I connect a charger, the Cout voltage goes down below Vss. Vgs on the mosfet is 200-300mV so it still lets a few mA of charge current through. I can see how the V- to Vss voltage looks like a fault (even though it's because the FET never turned on to begin with.
If I short V- to Vss, I would expect it to clear the fault and turn on the charge mosfet, but as you can see below, it remains off.
Finally, if I remove the charger, the charge FET finally turns on, but the discharge FET turns off. Not sure why, but this doesn't affect correct operation.
I would expect the charge FET to remain on when I connect the charger and only turn off if the difference between V- and Vbat exceed 1V, but this does not happen even when keeping V- shorted to Vbat.
Any ideas on what is going on?
