in Voltage mode, you have a ramp produced by an oscillator, in Currenct mode, its the same (similar) , except the ramp is gotten from the current sense resistor voltage
Added after 20 minutes:
somethings gotta turn off the switch, and turn it off at the right time, so the output off the error amp sets the current sense trip point.
so the more energy u need passed to the output , the longer the switch is on.