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DAC7750: why are there two TVS diodes?

user4568265
Prodigy 55 points
Part Number: DAC7750

why are there two  TVS  diodes in typical application circuit of page 36? The right TVS is turned on, and the IOUT is shorted to the ground. The IOUT cann't ouput 4-20mA, can it?

  • 0
    Prodigy 55 points
    The design is in page 36 of DAC7750 data sheet. Can anyone help me?
  • 0
    TI__Mastermind 46290 points

    Hello,

    I suppose the answer is that it depends on the diode.

    Some TVS diodes are constructed as, basically, a zener diode with a series standard diode facing opposite directions. In such a structure the zener does not break down until the required reverse breakdown voltage is applied along with the forward voltage associated with the series standard diode. In the other direction the standard diode must achieve reverse breakdown for conduction to occur, which is generally a fairly high voltage.

    It comes down to what is in the particular diode datasheet. If the TVS diode is not constructed as described above and had low forward voltage then your statements are true and the diode would conduct once the voltage at IOUT were sufficiently high to forward bias the diode on the right.

  • 0 in reply to Kevin Duke
    Prodigy 55 points
    Dear Kevin Duke,
    Thanks for your answer.Could you recommend a TVS diode as described above that fit the typical application circuit. I cann't find them.
    L&Y
  • 0 in reply to Kevin Duke
    Prodigy 55 points

    The above artical say that for non-symmetrical supply voltages two unidirectional TVS diodes must be used. I cann't understand why can a bidirectional TVS diode not used for non-symmetrical. Coud you explain that? Look forward to your reply.

  • 0 in reply to user4568265
    TI__Mastermind 46290 points
    Hello,

    To be perfectly clear, in my own testing we did actually use a bidirectional TVS diode and passed the IEC transient testing with unipolar supplies (non-symmetrical). In the datasheet, though, we decided to recommend a "best practice" rather than what was based on our observations.

    The reason we consider the pair of unidirectional diodes to be better for non-symmetrical supplies is because if you used a bidirectional diode in one of the directions the diode would basically be ineffective, or at the least less effective. Either the external clamp-to-rail diode stage or internal ESD cells would begin to conduct at a lower voltage than the TVS diode, so it doesn't provide much benefit to the circuit and stresses the other elements more.

    I seem to recall CDSOD323-15SC not conducting in the forward biased region though the internal structures and V-to-I curve is not published in their datasheet.
  • 0 in reply to Kevin Duke
    Prodigy 55 points

    Hello,

    Thanks for your reply.

         L&Y