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INA300: Differential Input Voltage

Calex36
Intellectual 1695 points
Part Number: INA300
Other Parts Discussed in Thread: INA302,
I am considering using the INA 300 
 
What I want to use it for is to sense across a MOSFET drain / source to detect that it has gone out of saturation or overloaded. The 250mV detection limit matches quite well to my requirements. Obviously where a MOSFET differs from a resistor is that it can be off, which is where my questions are heading.
 
Anyway one point in the data sheet.
 
In the absolute maximum ratings it has:
 
 
So I would expect no damage when the MOSFET is off as VDS will never exceed 40V, we only have 29V max. Good so far. However, note 1 states I can’t expect it to function at these limits. 
 
So, in the electrical characteristics.
 
 
 
Our supply range is about 20 to 29V so Vcm is satisfied. However the differential input voltage is a problem. Now, I expect that the writers intended to indicate that the device can only have the threshold set between 0 and 250mV. Obviously, if the threshold is 250mV, then we must expect the device to function correctly with the Differential input voltage exceeding 250mV or the device would make no sense, else how would the ALERT be signalled with the input was at 275 mV or a mere 10% overload condition? However, by definition TI has stated they do not even imply it will work (again refer to note 1 above).
 
So can you verify please if having a 29V differential input voltage is acceptable and will not cause erroneous behaviour in the device?
Many Thanks 
Calex
  • 0
    TI__Mastermind 26475 points

    Calex,

    I can look at this on one device but the issue will also be timing and recovering from a saturation condition which will happen.  I will get lab access today or tomorrow.  Can you tell me what the expected slew rate will be.  If you do not know I will end up just using the limitation of the generator I can get for 0 to 29V.

  • 0 in reply to Javier Contreras4
    TI__Mastermind 26475 points

    Calex,

    So having the input at 29V will not be an issue but when the mosfet is not active the INA300 will display and alert function.  Below is the condition where the input goes from 0V to 29V.  Keep in mind the low side is not accurate and can trip easily.  I did use and EVM.  Ch 2 is the input and Ch 3 is the Alert pin.  Because of the transition being so large if there are any capacitive coupling elements in the layout you could feedback some noise to the differential inputs and cause the input to see noise causing chatter on the output.  I would not recommend using Latch mode for this due to that.  Also if you are attempting to get an alert for overcurrent you would not be able to do this as you would not distinguish from the FET being off to over current conditions.  We do have a two comparator model in the INA302 if you require that function.

  • 0 in reply to Javier Contreras4
    Prodigy 50 points

    Thank you Javier,

    That answers my question. It will function in our system. 

    The alert function will be feed back into another device to modulate its behaviour in different conditions, having the MOSFET off and the ALERT active is expected. The delay looks like it is only 100us or so, which in our application is negligible. As for chattering an RC time constant will prevent any erratic behaviour.

    Regards,

    Nick