The exposed pad on the MAX256 is intended as a thermal connection only. It cannot be used as the only ground connection and excessive shutdown current may be seen if it is used as the return current path.
MAX256
Production
The MAX256 is an integrated primary-side controller
and H-bridge driver for isolated power-supply circuits.
The device contains an on-board oscillator...
Datasheet
MAX256 on Analog.com
The exposed pad on the MAX256 is intended as a thermal connection only. It cannot be used as the only ground connection and excessive shutdown current may be seen if it is used as the return current path.