Hello,
I'm seeing strange behavior from my BQ with the load voltage powering the rest of my circuit. When the battery is plugged in, everything works as expected and the BQ outputs the voltage of the battery, ~3.9V down to ~3.7V as it discharges and the PG pin goes low (illuminating the LED I have connected). When I plug in USB, which is the source of +5V into VBUS, the STAT pin also goes solid low, lighting up that LED and indicating good charge.
The status register indicates that the device is not operating in DPM. The only registers I have changed are increasing the Input Current Limit in REG00 to prevent the device from entering DPM on boot and disabling the watchdog in REG05. Everything indicates that the BQ is operating normally, but it's not actually charging the battery, as the battery voltage slowly drops over time.
If I try to power the board with just the 5V into VBUS from USB and no battery inserted, there is 0.6V seen at the output of BQ, but PG and STAT are both still pulled low to 0V. There is 0V seen at the positive terminal of the battery connector (with no battery connected). This seems to indicate that this is why it's not actually charging when it's plugged in, as the BQ 'thinks' it's giving the battery charging voltage but it's not pushing any voltage. The full 5V into VBUS can also be measured at the PMID and REGN outputs, so it's definitely connected through the IC.
According to this line of the data sheet:
"The power path management regulates the system slightly above battery voltage but does not drop below 3.5-V minimum system voltage (programmable). With this feature, the system maintains operation even when the battery is completely depleted or removed."
I should see 3.5V at the output with just 5V into VBUS and no battery. I have verified that there is a full 5V going into the BQ at the VBUS pin during this odd operation mode, so it's not the actual USB +5V that's sagging.
I cannot check the registers of the BQ when it's running in this weird disabled state as the 0.6V output is not nearly high enough to pass through the downstream 3v3 regulator that powers my microcontroller (see update in reply).
I have double checked that all passives surrounding the BQ have solid solder connections, and I have tried touching up the BQ solder joints by hand to make sure they are all solidly joined (which is a little difficult due to the VQFN package).
Is there something obvious I'm missing about this behavior, or should this circuit be working? I've attached my schematic, and I'm happy to share my layout if that helps.
