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2N7001T: About Vcc isolation

diverger
Expert 1205 points
Part Number: 2N7001T

In application report SCEA062A related to 2N7001T. I see some comparations between NMOS level shifter and 2N7001T. When talk about 'Vcc isolation", it says that for NMOS solution, when the power supply is at 0V, the IO pins are not in Hi-Z. I'm a bit confused by this word. Take Figure 3 for example. In my use case, the input source is powered by Vcca, and the receiver logic is powered by Vccb.

  1. When Vcca = 0, Vin is the input of FET, so the input should be high impedance. The output NMOS's input is pulled to 0V, so the output will output high, as it's pulled to Vccb.
  2. When Vccb = 0, the input is still high impedance. And the output will always output 0V, right? As the Vccb = 0, the receiver isn't powered, so 0V input is safe in this situation.

So, I can't figure out any problem in this usage with NMOS solution. Can you explain the "Non-Hi-z" problem for NMOS solution more clearly?

Thanks. 

  • 0
    TI__Mastermind 44620 points

    Hi, 

    I believe the app note is more focused on back drive protection when the 'I/O' is an output. So for example if VCCB is 0 V and a voltage is applied to the output then this could be an issue with the FET solution. With the 2N7001T if VCCB is powered down, then there is no issue with applying a voltage to the output.

  • 0 in reply to Dylan Hubbard
    Expert 1205 points

    Thanks for the reply, but I still have some quesions.

    1. I can't agree that it focus on 'back drive protection'. Do you mean in the NMOS solution, some components may need back drive protection but 2N7001T not? Then in the NMOS soultion, who need back drive protection? The resistor or the NMOS? Or the receiver?
    2. In practical usage, Vccb may also be the power supply of a high input impedance receiver, so the back drive may not be a problem.  
    3. In situations when the receiver is not powered by Vccb, we may still need a pull-up resistor, as 2N7001T's output may go to high impedance.

    Are there any docs detail the situation where 2N7001T is a more advanced solution related to 'Hi-z' problem?

  • +1 in reply to diverger
    TI__Mastermind 44620 points

    Hi,

    If the VCCB is 0 V and there is any voltage higher than that applied to Vout, you will have backdrive. If this is not a concern for your system, and you feel confident with the NMOS solution then feel free to go that route. There is no specific documentation on this and these are just examples of how 2N7001T can benefit a system.

  • 0 in reply to Dylan Hubbard
    Expert 1205 points

    Thanks for your patient. That is the Hi-z is to prevent any unintended current consumation.