I am designing a 4 cell LiFePO4 battery monitor using AD7280A and a PIC microprocessor running at 3.3v. The 3.3v supply is connected to Vdrive. As soon as I set PD to 3.3v to activate the AD7280, the Vdrive pin back-feeds about 4.6v into the PIC power supply (the PIC is only using a few mA). Is this expected behaviour? I have replaced the AD7280A in case I damaged it, but this behaviour on the new chip is the same. The rest of the circuitry around the AD7280 is as per the data sheet.