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DAC8775: Can DAC8775 detect the condition as open circuit even Iout is 0mA?

Masaharu Takahashi
Expert 4170 points
Part Number: DAC8775

Hi, Team.

This is MASA

I've been asked by Customer as below.

could you give me the answer ?

Can DAC8775 detect the condition as open circuit even Iout is 0mA?

  • 0
    TI__Guru 53773 points

    Hi Masa-san,

    My colleague should have an answer for you on Monday,

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    TI__Mastermind 46290 points

    Masa,

    No - the open-circuit detection does not work at 0mA output.

    the open load protection circuit is based on monitoring VGS of the output transistor. Basically, it is checking if the output transistor is in the triode region of operation. When VDS of the output transistor is very small, the transistor operates in triode and VGS will go very high to guarantee the output current. However, the maximum gate potential is limited by the gate voltage protection circuit to not exceed VGS=5.5V, and the open load detection flags when the gate voltage protection starts to protect the device.

    The same idea is implemented at sink side, both flags are OR’d for what you can observe in the register map.

    From a past discussion with the Design Engineer, the current can be as low as the off-state leakage current of the output transistor, which becomes the worst at 125C. He believes the off-state leakage current should be on the order of ~10uA. This is all assuming an "ideal" device. There is a second-order effect which basically changes this value to the offset error value for the current output.

  • 0 in reply to Kevin Duke
    TI__Expert 4170 points

    Hi, Kevin.

    it has been a while.

    thanks your comment.

    let me ask question below.

    Q. Won't IC detect open load detection in case of Iout is 0mA(when load is opened), ?

    what I should've assumed that "open load condition" is 0mA.

    -> sorry I can't get the point from your answer since it needs to have some "current" to enable IC to use open circuit detection.

    if load is 0mA (in case of open condition between load and DAC), how IC can sense it because amount of current is 0mA but condition is opened.

     

  • 0 in reply to Masaharu Takahashi
    TI__Mastermind 46290 points

    Masa,

    The high-side current source structure that is used in DAC8775 and in most of these PLC designs would basically see the open-circuit condition as a high-impedance load and still "attempt" to drive the current into the load, except that the current source will not have sufficient head-room to drive such a high impedance load. But, the important thing is that the circuit is actually "trying" to drive current into the load.

    If set to 0mA outputs, there is no activity on the output PMOS and therefore no means to detect saturation. We need some minimal output current setting in order to actually drive the PMOS and detect the open-load condition. So, with the DAC set to 0 code for the 0-20mA output range, for example, we will not be able to detect anything.

    There are some effects of offset error on this mechanism as well, so basically anything within the offset error specification (0.05% at room or 0.1% over temperature) cannot be detected. Continuing the example for the 0-20mA output case, this would mean that any output current less than 20uA (or code 65) could not detect an open-circuit condition, but all other greater output current / code values would.