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DAC7760 Damage

Eric Visser
Prodigy 20 points
Other Parts Discussed in Thread: DAC7760

Hello,

I am using the DAC7760 in a device where the user can select whether he wants the analog output to be 4-20 mA or 0-10V. This was implemented by having the DAC7760's current and voltage outputs active at the same time, and then using a jumper to decide what is connected to the connector pins.

When the jumper is in voltage position, I can change the load with the board powered, including short-circuiting it completely, and no damage occurs to the IC. However, if I remove the jumper from the board (from either the V or mA poistion) and connect to the V position when there is power on the board, the IC short-circuits my VIN and burns the IC. It's happened to me 3 times already. It happened when the load on VOUT was a short circuit and when it was connected to an opamp input (very high impedance), so it seems to be unrelated to the load.

I don't understand why inserting the jumper "live" is any different than changing the load on the connector. The only difference I see is that it adds a 470 pf capacitor instantly and that the insertion of the jumper can cause momentary rapid connects/disconnects.  Any ideas what can be causing this, or want can be done to fix it.  

Attached is my schematic. Vin = 24V

EDIT: I tried adding a permanent load and VOUT (10k) that is there even when the jumper is removed. I then had another 10K load connected to my AOUT1 signal, making the load 5k. When I removed the jumper, effectively changing the load from 5k to 10k, the IC blew.... I up to 5 dead ICs! What is wrong with my design?

  • 0
    TI__Genius 10615 points

    Hi Eric,

    I can't notice anything wrong with your schematic. The behavior is very strange and unexpected. The device has a 30 mA short circuit current protection that should keep the device from breaking down. I will be testing it in the lab sometime this afternoon to see if I can replicate the behavior.

    What is the DAC voltage output setting when this occurs? Full-scale I assume?

    If you could obtain an oscilloscope capture of the voltage breakdown it would be very useful.

  • 0 in reply to Eugenio Mejia
    TI__Mastermind 46290 points
    Eric,

    Just to add a detail on to Eugenio's post...

    The oscilloscope captures that he requested would be very helpful. To be specific, it would be helpful to see if there is any transient on the VOUT or IOUT line as well as the AVDD line that could be triggering any unexpected behavior of the internal ESD structures of the device.
  • 0 in reply to Kevin Duke
    Prodigy 20 points
    Hi,

    I don,t have any oscilloscope meausurements, but I can tell you a few more details.

    Yes, when I am doing this, I am at full scale voltage (10V). In some cases, the IC "blows up / burns" near pins 34 to 37. Seems to be caused by too much current entering pin 36 (AVDD). In other cases, the IC is not completely dead. It draws about 50mA more than it should on the 24V supply when the voltage output is open circuit. The voltage output is noisy and not quite high enough 9V instead of 10V for à 10k ohm load. The current output seems to be working fine.
  • 0 in reply to Eric Visser
    TI__Mastermind 46290 points
    Eric,

    The oscilloscope capture is, to some extent, a very critical piece of diagnostics for us in this case. Your observations are helpful but we will need more clues.

    As Eugenio mentioned, we will try to replicate something similar in our lab here and get the captures ourselves. If we are unable to replicate the failure we may need you to attempt to capture the waveforms and/or submit some of the failed units for failure analysis for us to have a chance to trace the root cause on a deeper level.

    We will keep you posted as we continue investigating on our end.