I am currently trying to design a current sensor for an Solid State Power Controller i am developing. Up until now i have been using a LEM HAS hall effect sensor to provide the required current measurements. However to reduce the size of this part of the device i'm switching the a shunt resistor with a current sensing amplifier. I have come across one of Analog Devices(AD) Application notes for an circuit which fits my needs perfectly using the LTC6102 the only issue is i'm having trouble grasping parts of the circuit's operation. I understand the sensors operation clearly when it is working on a lower voltage bus, it is the additional circuitry (shown in the figure) that is added to the 200V sensor which i'm having trouble understanding, even after running the model in LTspice. Can anyone shed some light on the operation of the circuit ?

Thanks

Martin

 

The added circuitry is to keep the voltage across the LT6102 below its maximum rating.

The 51V Zener D1 does this by conducting at near that voltage (only 46V in the simulation because it's operating at a low current) to turn on Q2 and maintain the voltage across U1 at about 43V.

D2 is apparently for reverse bias protection to discharge filter capacitor C2 during turn off of the circuit.

Q1 operates as a common-gate amp biased at U1's negative rail to translate the current from U1 MOSFET's output at a high voltage (207V) to R1 at ground potential.

Make sense now?