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DAC8771: Data converters - INTERNAL forum

Christina Lim
Expert 5655 points
Part Number: DAC8771

Hi, 

My customer raised a complaint about a sudden surge of current consumption on DAC8771 when they trigger open circuit condition. 

There are a total of 4x DAC8771. All 4 DAC are using same settings.

 They connect the ammeter to the whole system, when in normal operation, DAC Data Register (0x05) with none zero value, no open circuit happen, the current consumption of our whole system is 0.22 to 0.3A.

 However, when we purposely trigger open circuit condition, the current consumption for whole system goes up to 0.66A (heat up happen at this moment).

 We did a trial to narrow down to DAC8771 current consumption spike as below:

  • Whenever the open circuit fault happens, detected by Status Register (0x0B) -> F, we set DAC Data Register (0x05) to 0, the current consumption of the whole system immediately drops back to 0.22 to 0.3A

 

Please let us know if you notice any inappropriate in our settings below.

 The bits I type out are the bits we set, the bits that not type out follow defaults.

 1. Reset Config Register (0x02) -> CLRENA = 1

                                                   -> REF_EN = 1

2. Select DAC Register (0x03) -> CHA = 1

                                                -> DSDO =

3. Configuration DAC Register (0x04) -> OTEN = 1

                                                             -> RANGE = 0110

4. Select Buck-Boost Converter Register (0x06) -> DC = 1

5. Configuration Buck-Boost Register (0x07) -> PNSEL = 01

6. DAC Data Register (0x05) -> depend on user input

 The above setting will be used throughout the firmware flow.

 May I know if you see anything incorrect in the settings or does a schematic is required to review the case better?

Thanks,

Christina 

  • 0
    TI__Expert 4200 points

    Hi Christina,

    How long of a duration is the current spike the customer is seeing?

    When the load is open in current output mode, the VPOS voltage will increase to around 32 V in full tracking mode, in an attempt to drive the current. This could be the reason for the large increase of current being observed. The IOUT alarm bit will also be set during this condition.



    If the customer is looking to immediately stop the Buck Boost when this happens, they can write 0x0020 to the Alarm Action register (0x0D). This Alarm Action setting will disable the Buck Boost converter and Current Output amplifier when an open circuit is detected, and also reset the Alarm condition.

    I would recommend trying this and seeing if it resolves or lessens the current spike.

    Thanks,
    Lucas

  • 0 in reply to Lucas Burnette
    TI__Expert 4200 points

    Hi Christina,

    I would also like to see the customer's schematic if they are able to provide it.

    Thanks,
    Lucas

  • 0 in reply to Lucas Burnette
    TI__Expert 5655 points

    Hi Lucas, 

    The current spike is continuous until we clear open circuit condition.

     

    We are not looking for stopping buck boost converter.

     

    Instead, what we are looking for is we want to continuous monitor open circuit condition.

    Can I clarify one thing with you?

     

    1. In the datasheet, we saw this paragraph.

    I believe this is same as your suggestion below. Is this paragraph mean that, when we use alarm action register to turn off buck boost, we cannot monitor open circuit anymore? Our application requires us to continuous monitor open circuit condition.

     If you have some idea, how we can continuous monitor open circuit condition while keep the current consumption low (low temperature), please help to advise also.

    Thanks,

    Christina 

  • 0 in reply to Christina Lim
    TI__Expert 4200 points

    Hi Christina,

    I misunderstood the customer's problem earlier. The current spike is a constant condition and not just a transient pulse of high supply current consumption.
    The figure you shared is the "solution" I mentioned earlier, but would stop the monitoring of the open circuit until you set the Buck-Boost and Output again, so it doesn't fit your customer's problem.

    I would need the customer's schematic and information about the power rails they are using to verify further, as I do not see this high current when trying to recreate the scenario on the DAC8771EVM.

    Thanks,
    Lucas