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DAC8750: Chip easily burns

Leonardo Cerri
Prodigy 30 points
Part Number: DAC8750

Hello everybody !

I'm writing here because I have a problem on the DAC8750.

In my PCB, I have two DAC8750 in daisy chain mode and I followed your instructions regarding the protections while designing the circuit.

Firstly, I tried to supply the AVDD with 24 V, letting one of the two chip generate the DVDD (pulling its DVDD-EN pin high) for the rest of the circuit (in particular for the other DAC and for the two optoisolator).

With this configuration, the chip which generated DVDD has often burned out, so I decided to pull its DVDD-EN pin low and generate the DVDD with a voltage regulator (from AVDD).

In this configuration, when I supply AVDD with 12 V, the circuit seems to work fine, but when I supply with 24 V, the DAC burns again.

I attach here my schematics.

Thank for your help !

Leonardo

  • 0
    TI__Intellectual 1570 points

    Hello Leonardo,

    Thanks for providing the schematic along with the issue description. I have a couple of questions that will help me understand the issue better.

    Can you explain the purpose of D15 in the design? Also what do you have connected to port "DA_SUPPLY" for the first DAC8750?

    What specifically happens when the chip "burns out"? Do you observe larger than normal supply current or does the chip stop responding to SPI commands. 

    It will also help if you can provide oscilloscope screenshots showing the AVDD & DVDD power supply pins, together with the IOUT and REFOUT pins as the power supplies ramp up for both DAC8750 chips.

    Thanks,

    Reza

  • 0 in reply to Reza Abdullah
    Prodigy 30 points

    Hello Reza,

    D15 is a design error and it has been removed from the circuit.

    DA_SUPPLY goes to a connector with DA_GND and that's where I supply the 4-20 mA circuitry with 24 V.

    When DVDD was provided by the DAC, the chip which do provide the DVDD literally burns, since I saw a spark in its body case which then resulted damaged. Current consumption grew up and DVDD and GND_DA were short circuited (0 ohm resistance).

    Now, since I supply DVDD with the voltage regulator, I don't see the sparks and at 12 V it works well. Then, when I supply with 24 V, the current consumption grows up suddenly and my DC Power Supplier goes in protection mode limiting the current (0.5 A), signaling the short circuit.

    Now, I'm not able to provide oscilloscope screenshot since I destroyed a pair of circuit and I have to set up another one.

    Thank you very much for helping !

    Leonardo

  • 0 in reply to Leonardo Cerri
    TI__Intellectual 1570 points

    Leonardo,

    Usually this kind of catastrophic damage only happens if and when the absolute maximum ratings of the device are exceeded. For the DAC8750, the absolute AVDD to GND voltage is 40V and for DVDD to GND, the abs max is 6V. The requested oscilloscope captures will let us know if these limits are exceeded when the supplies are ramping up.

    With the DAC8750s off the board, can you repeat the same test, ramp up the power supplies and observe if the power supplies clamp again? The goal here is to confirm the voltage ratings of all the other components on the board such as the bypass capacitors and protection diodes on the supplies. 

    Finally, can you provide the rest of the schematic diagram that shows the connections to the off-page connectors, especially "DA_OUT1" & "DA_OUT2"

    Thanks,

    Reza

  • 0 in reply to Leonardo Cerri
    TI__Mastermind 46290 points

    Hi Leonardo,

    Do you have any updates for us?

  • 0 in reply to Kevin Duke
    Prodigy 30 points

    Hello Kevin,

    unfortunately I'm not able to provide oscilloscope screenshots since I have only one circuit left and I can't risk to damage it.

    DA_OUT1 and DA_OUT2 go to a connector where I can read the output current signal.

    Now, I'm supplying the circuit at 24 V but I use a voltage regulator (12 V) on DA_SUPPLY and everything works fine.

    In any case, I'm pretty sure that I didn't exceed the absolute maximum ratings on DA_SUPPLY, since I've always supplied the circuit at 24 V maximum and I don't think that my voltage supplier set at 24 V could actually provide a voltage of 36 V or higher.

    I think that I will add the voltage regulator on my PCB to be sure not to have problems.

    Leonardo

  • 0 in reply to Leonardo Cerri
    TI__Mastermind 46290 points

    Leonardo,

    At this point it's hard for us to comment any further with the information we have at hand. I will leave it to you at this point to continue your experiments and we will mark this thread as closed. Should you have other questions, please feel free to post again in this thread or in a new thread (after 2 weeks of inactivity this thread with automatically lock).

    The last thing I will offer for you to consider is that at a DC level you may be avoiding any over-voltage condition, but that doesn't mean that some transient isn't possible so that may be something to look out for. I haven't seen any catastrophic issues with respect to power supply sequencing on this part.