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XTR117 Current Issues

Du Lo
Intellectual 400 points
Other Parts Discussed in Thread: XTR117, LM2840, TPS54062

I built this circuit on my board and have noticed some current discrepancies.

With no MCU/ADC load on the 5V Vreg line other than the resistor into IIN, the XTR117 is behaving normally. I can measure 5V DC at Vreg and the IOUT is what I am expecting.

Powering the MCU/ADC with a bench top power supply and a current meter, but completely isolated from the XTR117 circuit, i.e. cut the traces so the two portions are completely isolated from each other, I get a reading of 5mA at 5V.

Introduce the XTR117 to the MCU/ADC circuit and power up with 24VDC bench supply and I get a reading of 12mA at 24VDC. 5V Vreg reads 4.71V

Remove 24VDC bench power supply and power the 5V directly with a bench power supply. This time without isolating the XTR117 from the MCU/ADC and I'm reading close to 60mA at 5VDC.

What is going on when I join the MCU/ADC to the XTR117? Why is the current consumption all of a sudden jumping up to 60mA at 5V? Too much capacitance with the MCU/ADC capacitors? I tried removing the larger caps and still no changes.

  • 0
    TI__Guru 62745 points

    Hello,

    While you've described the different situations well, we would still like to see a full schematic with the MCU/ADC circuitry shown as well so we can verify the connections and understand your issues better.

    Your later test cases make us believe that the MCU/ADC is possibly drawing too much current from the VREG output.  Doing so will cause the VREG output to decrease as well as cause the output current to increase to a level greater than you've set it to. 

    The MCU capacitors are not likely the source of the issue.  What is the total current consumption of the MCU/ADC when active? 

  • 0 in reply to Collin Wells
    Intellectual 400 points

    Hello Collin,

    I've tried removing the MCU and the Serial transceiver from the circuit, so only the ADC chip is present, but it's still pulling above normal current.

     

    The MCU/ADC/Transceiver circuit alone on a 5V bench power supply only pull 5mA at 5V.

     

    Thank you for your help.

  • 0 in reply to Du Lo
    TI__Guru 62745 points

    Hello,

    Unfortunately I don't believe this application will work the way it's currently designed.  2-wire 4-20mA transmitter circuits must be designed to consume < 4mA of current in order to create a 4-20mA output.  With 5mA from the MCU/ADC/TX + the quiescent current of the XTR, this application will likely not be able to produce output currents lower than 5.5mA. 

    Furthermore, it would appear that the combined quiescent current of the PSOC chip, transceiver, and ADC alone well exceed the 5mA level that you're measuring.  Perhaps in an idle state it's only drawing 5mA, but I believe once the circuitry is active it will exceed the maximum allowable level of 4mA based on the datasheet specifications I've copied below.

    The PSOC chip draws between 8 - 14mA from a +5V supply, the ADC will draw 1.5 - 7.5mA, and the TX device draws at least 1mA.  This doesn't include the current required to power the "Scale" as you've called it in the schematic which will also require current.

    Could you please provide more information about the goals of this application including the type (and resistance) of the "Scale" sensor as well as accuracy and other requirements. 

    Also, what is the transciever used for?  We do not recommend placing non-isolated transcievers into a 2-wire 4-20mA circuits because as the IRET pin changes voltage relative to the system RTN (GND) the logic threshold levels of the transciever will change relative to GND as well making it very difficult to interface with.  

    PSOC Current Consumption:

    ADC Current Consumption:

    TX Current Consumption:

  • 0 in reply to Collin Wells
    Intellectual 400 points

    Hello Collin,

    Good points. Thank you.

    The project is for an industrial scale to be placed in an intrisically safe environment. The transceiver is used to connect to a user board to calibrate the scale, once the calibration is done, the user board is disconnected from the board and the transceiver won't be used anymore.

    The resolution we'd like to see is +/- 0.1mA, which I believe was calculated to be achievable with a 12-bit ADC.

    The next thing I'm going to try is to not use the Vreg on the XTR117 and use a switcher to generate the 5V. I've ordered some LM2840 to try.

    Thank you,

    Du

  • 0 in reply to Du Lo
    TI__Guru 62745 points

    Hi Du,

    We've had success using the TPS54062 for this type of application in the past and I would recommend it. Take a look at the following forum posts:

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/138328/498986.aspx

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/158332/576355.aspx#576355

     

  • 0 in reply to Collin Wells
    Intellectual 400 points

    Hello Collin,

    I've ordered the TPS54062. Thank you.

    In the meantime I built the LM2840 circuit and the current went down, but at 24V it is still pulling 4.25mA with just the ADC connected. Without any load on the LM2840, so just XTR117 and LM2840 I am drawing 2mA @ 24VDC.

    I will try the TPS when it arrives, but I might have to re-evaluate each component and replace them accordingly if it still doesn't work out.

    Thank you,

    Du