Actually I'm considering two options for my buck current source :
- hysteretic buck : Sense inductor current with small high-side shunt resistor, use a comparator with hysteresis, drive MOSFET.
The comparator inputs need to accept common mode voltage up to Vcc, because I want it to keep working with as little dropout as possible between input and output (for instance, 10V out, 10.5V-28V in)
The comparator inputs need to accept common mode voltage down to GND, this makes the output fully protected against shorts to ground of BOTH load wires (which is the reason to use high side current sense)
- constant off time : Sense current in the VCC rail of the top MOSFET when it conducts, when it goes over threshold, turn off MOSFET, wait, repeat
The comparator inputs only need to accept common mode voltage up to Vcc, not ground, in this case. So it might be easier to find a suitable one.
Since the voltage I want to put at the input of the comparator comes from a current shunt in the Vcc, I cannot shift it : it is pretty small. My current design uses a MAX4376 current sense amp, which is all right, but I'd like to simplify the design even more. I don't need high precision in the current.