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INA300: Could you use a FET's Rdson as your sense resistor for an INA300 or similar CSA?

Brian Antheunisse
Expert 5215 points
Part Number: INA300

Hi guys,

I have a customer with a battery operated product that wants to add OCP to a rail and turn off a load switch if that happens. Obviously burning power across a shunt is less than ideal, is there any way to take the voltage drop across the load switch and use that voltage to feed into the load switches on/off pin with an INA300 or similar part? Not sure the threshold accuracy needs to be high, so maybe INA300 is overkill, but just wanted to know in general if that was an approach you guys had heard of.

Thanks,

Brian 

  • 0
    TI__Expert 5215 points
    by the way, there is no eFuse or anything that has current limit integrated that can go down to 1V, which is the lowest rail my customer wants to add OCP to. We've already been down that path, so I am exploring a discrete implementation.
  • 0
    Guru 140200 points

    Hi Brian,

    yes this can be done. Even the voltage drop across a fuse or copper trace can be used. But you have to take into consideration that all these "shunt resistances" can have manufacturing tolerances and show a temperature drift. A fuse resistance can even undegoe severe changes after it was hit by a too high current.

    So, I would prefer a precise shunt which I would make as small as possible.

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 21805 points

    Hello Brian,

    Thanks for reaching out on the forum.  So if I understand correctly, your customer wants to measure the load on a 1V bus.  What is the range for this load?  I have some concerns with you you monitoring a voltage across a switch with such a low bus voltage.  The first concern is that switch potentially can have a large variation in the RDSon for that operating region.  Below, is the characteristic curve I found for one load switch.  Here you can see that any small variation in the bus can potentially have significant impact on the Ron value.  Also if your customer's system is not well ventilated that ron value will drift, in which case you might need to adjust your trip point according to temperature.

    As you noted a shunt will resistor will burn some power.  However, you can choose a relatively small shunt with minimal drift to reduce that impact.  Yet with a small shunt, you will need to pay attention to the INA300's input bias current and input offset voltage as those will limit who low it can detect for an overcurrent event.