I want to understand the datasheet specification.
Vcontrol pin and IN pin both have voltage rating +40V with respect to Vout and not ground ?
If true then I can connect +55V on Vcontrol pin and IN pin get output of +50V.
The -0.3V Absolute Maximum Rating for the LT3080 that you show in your image indicate that the LT3080 has parasitic diodes from OUT to IN and OUT to VCONTROL. I draw the parasitic diodes on the LT3080 block diagram in the image below. You can make a 55VIN to 50VOUT LT3080 circuit, but you have to clamp IN=VCONTROL to OUT and also IN=VCONTROL to SET so the 40V Abs. Max. ratings are not exceed. I talk more about a circuit for arbitrarily high output voltages in this Engineer Zone post. The title of the EZ post is "Is it possible for the LT3081 to generate 40V from 45Vin if the potential difference from Vin to Vout is within 40V?"
