Sensing current utilizing MOSFET’s Rds(on) and Inductor’s DCR in different location discussed in this app note from Alpha and Omega semiconductor. Link here (PDF)
In current mode converters, current sense resistor can be placed in three locations: input loop, output loop and freewheeling loop. The pros and cons are discussed in details. The principles and features of peak current mode and valley current mode are also presented. The cautions and tips for using Rds(on) of high-side MOSFET, low-side MOSFET and DCR of the inductor as the current sense resistor are also mentioned at the end.