HV pin is connected to Line and Neutral through two
rectifier diodes as shown in Figure 2-4; This pin’s voltage is used to detect if AC is present.
Under ideal conditions when AC is
connected, the HV pin should detect the zero crossing where the voltage between
Line and Neutral reaches 0 V twice per cycle. However, due to the parasitic
capacitance of the diode bridge, the voltage at HV will not reach zero. To
overcome this, current is sinked from HV to GND.
Figure 2-6 X-cap discharge internal block
diagram
The switch S2 highlighted in
Figure 2-6 controls the Ixcap test current to determine if AC is still connected. The
logic is looking for the HV pin to be pulled below 9V. Note this test current
goes to gnd.