This slide shows a typical LT8709 application schematic that converts a -16V to -30V input to a -12V output at up to 8.5A. The external 2 mΩ current sense resistor sets the maximum peak current though the external N-channel MOSFET switch. The other external 4 Ω sense resistor connected between the ISP and ISN pins sets the maximum output current of the converter.

