While current-shunt-monitor ICs like the INA168 can be connected to current-shunt resistors at supply voltages of up to 60 V, the circuit shown in the figure allows current sensing at even higher voltages. This method can be extended to any voltage for which a suitable MOSFET for Q1 can be found.
Zener diode DZ1 regulates the current-shunt monitor's supply voltage. This voltage "floats" relative to the supply voltage. DZ1 is chosen to provide sufficient operating voltage for the combination of IC1 and Q1 over the expected power-supply range (typically 5.1 to 56 V).
First, select R1 to set DZ1's bias current at some value greater than IC1's maximum quiescent current. The INA168 shown is specified at 45 µA maximum. The bias current in DZ1 is approximately 500 µA at 200 V, much greater than IC1's maximum current. (The bias-current value was selected to limit dissipation in R1 to less than 0.1 W.)
Connect a p-channel MOSFET (Q1) as shown to cascode the output current of IC1 down to, or below, ground level. Q1's voltage rating should exceed the difference between the total supply voltage and DZ1 by several volts, because of the upward voltage swing on Q1's source. Select R2, IC1's load resistor, as if IC1 were used alone. This circuit was specifically designed to operate from 160 to 200 V and sense up to 1 A of current at a 1-V full-scale output.
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