Hi,
I'm working on a device with integrated battery charger based on BQ24170. The battery pack is 3S3P Li-ion 18650.
Everything works well, except battery detection after it is reconnected, i.e.:
1. I connect PSU to the charger. Battery is not connected. Power consumption shown on PSU is circa 0A. Voltages measured in this state are shown in green colour on the schematic attached below.
2. Then I connect the battery pack. After a second or two it is detected and normal charging process is started with correct current according to what is set by resistors.
3. Then I disconnect the battery. BQ should detect its absence, but is looks like it is still charging and simultaneously battery detection algorithm is running (voltage on SRN goes up and down in a 1,5s cycle). Current consumption in this state is about 17mA. It seems that battery detection (discharge via SRN pin) does not work correctly because capacitors on VBAT line are not discharged/charged as expected because of current flow from SW pins (there is no other possible source). Voltages measured in this state that differ from those taken in point 1 are shown on the schematic in red.
I even disconnected the system to be sure it does not influence this behaviour and cut connection between Q2 and SRN pin to check if this isn't a source of high and constant voltage on SRN pin.
I also resoldered BQ chip to check if it is not damaged, but nothing had changed. PCB is 4 layer. L2 is Vishay ihlp3232dzer2r2m11 2,2uH/10,2A.
What could be the reason that battery absence is detected only once at power up and then this is not working? Could this be a PCB layout issue?
My schematic:
