PCA/TCA9555 - center pad

Newton,

 Page 1 of the datasheet, see the statement directly under the device package outline:

"The exposed center pad, if used, must be connected as a secondary ground or left electrically open."

Also see page 34, as this is the Thermal Pad and important for heat dissipation.

What is meant by "secondry" ground? So, you can not connect to the ground plane on PCB? We have only one ground plane.

Secondary Ground means it is not intended for current-carrying; the GND pin3 is for that purpose, so connecting the Thermal Pad to the pcb GND plane is acceptable as a means to dissipate heat, but not to be used as a power supply GND path.