VDD pin is the power supply for the device. There should be a 1-µF capacitor directly from VDD to PGND. VREF pin provides the supply rail for internal logic, it should be by-passed to GND as close as possible to the device using a 0.1-µF capacitor. PVDD pin is the supply pin for the power devices, it should be by-passed to PGND using a 10-µF capacitor.