This question is specific for ACT2802X series 5V/2.5A Backup Battery Pack Manager IC.
On page 15, there is a schematic that provides a solution for auto plug-in detection. However, I cannot understand how this is achieved? What is the thing that comes out from the USB connector's shield in the schematic? I cannot also find a USB connector with "plugged" output.
PB pin is described as:
Push button input, connect a push button from this pin to AGND, internally pulled up by a 3MΩresistor to battery. PB starts up boost converter if HZ pin is grounded and VIN is not present. For ACT2802B/ACT2802C, if this pin is pushed for 3s, flashlight is enabled.
For ACT2802QL-T/ ACT2802-T1026, When HZ=0, if PB is pulled low more than 100ms, boost converter is enabled; For ACT2802B/ACT2802C, when HZ=0, if PB is pulled low more than 30ms, boost converter is enabled.
PB High Input Voltage = 0.9V (typical)
PB Low Input Voltage = 0.75V (typical)