BQ24780S: questions about ACOK deglitch time

1.How does IC knows which time  adapter plugs in?

>>  The ACDET pin should be configured with a resistor-divider from the AC input.  When the ACDET pin is above 2.4V, the adapter plug in is detected.  The ACDET must remain above 2.4V for the whole deglitch time of 150 ms/1.3 sec in order for the adapter to be detected and ACFET/RBFET to turn on.  The resistor divider is used to set the minimum voltage threshold for adapter detection by scaling that threshold down to 2.4V at the pin.

2.If the battery is not connected,is adapter always the first time to plugs in?

>>  Yes, first time is the first time after power-on reset.  If VCC is below undervoltage lockout (which it should be if both adapter and battery are removed) then attaching either will be a POR and the next adapter attach will be considered the first attach.

3.Why is the first time's deglitch time is differwnt with the second time,What's the disadvantage if they are both 150ms?

>>  Either deglitch option should work fine.  Generally it is assumed that the extra delay of 1.3 sec deglitch won't be noticed by most people, i.e. if there is an extra 1.3 seconds before the battery starts charging, so a longer deglitch is offered just in case the more conservative option is preferred, but I haven't had any customer who had an issue with using the shorter 150 mS deglitch all the time. 

Regards,

Steve