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DAC161P997: DAC161P997 unpowered with reference design transformer

John Slaney
Prodigy 70 points
Part Number: DAC161P997

If you have a DAC161P997 with no power on the isolated portion of the design, what sorts of protections would you recommend for the DIN/DBACK/ACKB pins if the microprocessor may still attempt to communicate?  Diode clamps to VD and GND?  Thanks!

  • 0
    TI__Mastermind 46290 points

    John,

    Largely this depends on the current. I would point to the comments in the Absolute Maximum Ratings table:

    Current limiting would probably be pretty easy with resistors.

    The voltage part may be more difficult since most likely COMA is floating in this case, so we would want to protect this with some breakdown diode.

  • 0 in reply to Kevin Duke
    Prodigy 70 points
    I think the challenge here is that the transformer won't be properly driven if I add much in the way of resistance, and it would seem like a common situation where you have an unloaded/unpowered 4-20mA transmitter. The transformer coupling is shown in the data sheet without really addressing this. I'm designing an "active" 4-20mA loop (i.e. I provide the isolated power), so I'll see if a current sink outside the loop resistor will hold it on and prevent the chip from becoming unpowered.
  • 0 in reply to John Slaney
    TI__Mastermind 46290 points
    Hi John,

    Understood.

    In the end I can only reliably point to what is in the datasheet, but experience tells me that this is more of a current problem than a voltage problem so this is really what we need to be focused on. If the current is limited to 5mA or less I am quite certain there wont be any issue.