Application note from Maxim intergrated on utilizing a boost converter and a current-sense amplifier to form a regulator that derives +5V from -48V without isolation. Link here (PDF)
Instrumentation amplifiers (IAs) are used where gain accuracy and dc precision are important, such as in measurement and test equipment. The downside of IAs is the cost. However, inexpensive current-sense amplifiers handle high common-mode voltages and share some traits with IAs. As a result, in some applications, such as a ground-referenced -48V to +5V power converter, current-sense amplifiers can replace IAs, thereby reducing cost.
from Dangerous Prototypes http://ift.tt/2BrwVbX