Although you say otherwise, it sure SOUNDS like you've got a damaged component. According to the data sheet:
DONE (Pin 2): Open Collector Indication Pin. When target output voltage is reached, an NPN transistor turns on. Requires a pull-up resistor or current source. Any fault conditions such as thermal shutdown or undervoltagelockout will also turn on the NPN.
So, maybe you've got some other issue.
I would take an oscilloscope and look at the the MOSFET gate and drain, the voltage on the RBG pin, etc.