Help understanding Current Sense MOSFET

Discussion in 'General Electronics Chat' started by Tobias, Aug 4, 2012.

  1. Tobias

    Thread Starter Active Member

    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?
  2. ramancini8

    Active Member

    Jul 18, 2012
    It appears that OPA4B has no feedback so it will saturate.
  3. praondevou

    AAC Fanatic!

    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.