Help understanding Current Sense MOSFET

Thread Starter


Joined May 19, 2008
I am trying to understand how to make this MOSFET work. Ultimately I want to use the current sense output to go to a comparator. If the voltage output that correlates with a max current I want the MOSFET to see, the comparator will pull the enable pin on the MOSFET driver low and shut down the MOSFET.

I am attaching three PDF files. The first is my try at a schematic without the comparator side of the schematic included. It is just how I am interpreting the AN10322 PDF, which is an app note for the MOSFET. The last PDF is the spec sheet on the MOSFET

My first question is do I have the schematic right?

Second question is about the RSense resistor value. What is n? I want to scale it so 50 amps is 5v.

Third question, I understand the first op-amp is converting amps to voltage (OA3b), whats the purpose of the other op-amp (OA4b)?

Finally is a OPA227 Op-amp a good part to use for this application?



Joined Jul 9, 2011
The Drain-to-sense current ratio is 500. That means for 50A it will provide 100mA. If you wanted to transform this into voltage you will either need a OPAMP with very high output current or better you transform the current into voltage and amplify the voltage as shown in figure 4 of the application note.

Example: 50A --> 100mA, If Rsense was 10 Ohm this creates 1V, you then only need an amplifier of 5 to get to your desired output voltage of 5V.