Where do I connect the exposed pad of a QFN package?

Unless otherwise specified in the datasheet, you can connect the exposed pad to ground (GND), or leave it floating, but it may not be connected to any other signals or potentials. The exposed pads in QFN (any) packages parts increase the maximum power dissipation of the packages. When an exposed pad is offered as an option, it is specifically to increase the power dissipation capabilities of the IC package. For packages which always have the pad, the pad may or may not be needed for power dissipation. In most applications, the exposed pad is connected to ground. Hence, the center pad can be connected electrically to GND without any harm to the PCB.

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