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DAC8771: Iout Load open issue

user5163865
Genius 3525 points
Part Number: DAC8771

Hi ,

Our customer has a question about DAC8771.
Does this device work properly under the following conditions?

 Iout & Vout shorted
 Iout load resister opened
 POC is set
 Buck / Boost ON( full tracking mode)
The typical usage assumes a 500 ohm resistor.
However, end users may open the load.
So they did a test to open the load.
But this device broken...
I believe this device is not broken, as the load OPEN is usually a condition.
If POC clear (default), the device is not broken. The customer also said that the current value is correct.
In order to solve this problem, I will check the customer with the necessary information.
Can you give me some advice on what to check?
Best regards,
Hiroshi
  • 0
    TI__Genius 14220 points
    Hi,

    Thank you for your query. My colleague Garrett will respond to you in the US timezone.

    Regards,
    Uttam Sahu
    Applications Engineer, Precision DACs
  • 0
    TI__Expert 5820 points
    Hi Hiroshi-san,

    Yes, the device will operate correctly under the conditions you listed.

    I am not sure I understand the issue. Why do they think the device is broken? When the load is open in current output mode the voltage will increase in an attempt to drive the current to around 22V in full tracking mode. The IOUT alarm bit will be set during this condition.

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Genius 3525 points

    Hi Garret,

    Thank you for your reply.

    I will check if this circuit is really broken.
    Hiroshi
  • 0 in reply to user5163865
    Genius 3525 points
    Hi Garrett,

    When the circuit was broken, an ammeter was connected.
    I was questioning how to measure current at OPEN, so I checked the measurement method.
    (I thought it was impossible to measure the current of the open circuit.)

    This method simply removed the resistor and connected the DMM.
    Therefore, I think that the internal resistance of DMM actually acts like a load.
    For example, if the internal resistance of the DMM is too high and exceeds the compliance voltage, I think there is a possibility of failure.
    Is it correct?
    I think there is usually no protection for this behavior.

    *Additional request
    I would like you to review the schematic, can I email you directly?
    As far as I see, it is almost a D/S circuit.

    Since it works normally with a 500Ω load,
    I believe that simply stopping the ammeter's measurement will solve the problem.

    Best regards
    Hiroshi
  • 0 in reply to user5163865
    TI__Expert 5820 points
    Hi Hiroshi-san,

    If a DMM is connected to measure the current, it is not an open circuit condition as the current will flow through the DMM. As you thought, the DMM simply acts like a small load resistance.

    There is no risk of damage from having a large load resistance. Think of open-circuit as having practically infinite resistance between Iout and GND. The device will not be able to drive the correct current if the load resistance is too large.

    Sure, we can PM to exchange contact information. I will go ahead and mark this thread resolved as we can continue discussing over email.

    Thanks,
    Garrett