SENSEFET App note from ON Semiconductors incorporate a current sense inside a power MOSFET, these devices are straight forward calculating for sense resistance and voltage but have trade-offs for smaller current sensing. Link here (PDF)
Current sensing power MOSFETs provide a highly effective way of measuring load current in power conditioning circuits. Conceptually simple in nature, these devices split load current into power and sense components, and thereby allow signal level resistors to be used for sampling. Since this technique results in higher efficiency and lower costs than competing alternatives, understanding how to use SENSEFET product is an important design issue.
Getting accustomed to these devices is relatively, but not completely, straightforward. They are conceptually simple, but have their own unique set of characteristics and subtle properties. The following discussion examines both, and starts with a description of how SENSEFET devices work.
from Dangerous Prototypes http://ift.tt/2omxmNm