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XTR117: XTR117 Vref connection

Pranav Dosajh
Prodigy 160 points

Part Number: XTR117
Other Parts Discussed in Thread: XTR115, XTR116

HI 

  I am trying to interface xtr117 with MCU's PWM  the problem I am facing is that the I in  pin of the XTR117 need a constant current offset which is supplied by the Vref . 

many of the application design uses a 4.096 reference voltage to provide the necessary offset current.

My MCU operates at 5V where should I provide this refernce voltage to get the constant current offset. 

Regards

Pranav

  • 0
    TI__Mastermind 25875 points
    Hello,

    Please understand that most of our engineers are out for the holidays. It is most likely that we won't address your question until early January 2017.
  • 0 in reply to Pete%20Semig
    TI__Guru 64895 points

    Hello,

    The XTR117 does not include a precision reference output.  The XTR115 and XTR116 devices include a 2.5V and 4.096V reference output respectively.  Consider using one of those two devices if you need an integrated precision reference.

  • 0 in reply to Collin Wells
    TI__Guru 64895 points

    Also, take a look at this related forum post which includes a schematic and some discussion regarding how to use the XTR117 devices with a PWM input.

  • 0 in reply to Collin Wells
    Prodigy 160 points
    thanks for the reference design it made the task easier. I have already received samples for ISO7220CD will it make any difference if i use ISO7220CD than ISO7310CDR.

    Regads
    Pranav Dosajh
  • 0 in reply to Collin Wells
    Prodigy 160 points
    For the reference design I have some doubts

    e2e.ti.com/.../2048181

    1) The VREG pin of xtr117 is connected to VDD, Is there a separate VDD which needs to be supplied to xtr117 or the VREG o/p is sufficient to provide the offset current to IIN pin of xtr117 and give supply to the digital isolator ISO7310CDR.

    2) What is LP3p6? I am assuming it to be the supply voltage provided for the MCU interface.

    Kindly solve this issue.

    Regards
    Pranav
  • 0 in reply to Pranav Dosajh
    TI__Guru 64895 points

    Hello,

    Thanks for reviewing the design.  The ISO722x devices will not work in this application because they require too much quiescent current to be powered from a 2-wire transmitter.  Devices powered from the VREG output must consume less than 4mA or the transmitter can't go down to the 4mA zero-scale levels as desired.  Take a look at this blog for more information:  Blog III:  IRET < 4mA

    1.)  The VREG pin on the XTR117 is an output which can supply the required current for the OPA and digital isolator.  As mentioned above, the current drawn from the VREG output must be less than the minimum desired zero-scale output.

    2.)  You are correct.  this was a 3.6V supply that was used to power the MCU that was generating the PWM output.

  • 0 in reply to Collin Wells
    Prodigy 160 points
    Thanks for your reply the reason for selecting ISO722x was purely on the basis of availability because the ISO7310 is not available here.
    anyway I will try and manage to get ISO7310.

    My doubt still remains as you said the VREG is not capable of providing the required offset current to the IIN pin of the XTR117. So a seperate supply is required,
    according to the reference design the op-amp, supply to the isolator and the offset current is provided by the VDD since the op-amp's ground and the isolator's ground is connected to the IRET pin of the xtr117 the VDD has to be separate from the MCU power supply for good performance.
    there is another thing what is the purpose of joining VDD and VREG pin if VREG is not used to supply current to any of the components attached.

    Kindly give more details of this VDD used in reference design, how to separate VDD from the mcu supply and what voltage is used.

    Regards
    Pranav Dosajh
  • 0 in reply to Pranav Dosajh
    TI__Guru 64895 points
    Hi Pranav,

    We did not state that the VREG output is not capable of providing the required offset current to the IIN pin of the XTR117. To offset the output of the transmitter to 4mA, only 40uA of input current is required which is easily achieved with the VREG output. I did mention that the VREG output is meant for coarse regulation of supply voltages to op amps, isolators, etc. in the system and wouldn't provide the same level of accuracy as a dedicated reference such as the ones included with the XTR115 and XTR116 devices.

    Regarding your other questions and comments, I think you're misinterpreting the schematic drawing. The VREG connection on the XTR117 is an output. Connecting an external voltage to the VREG output will likely damage the XTR117. We have simply named the output of the VREG pin of the XTR117 with a net-name of "VDD." We are not implying that an external voltage named "VDD" needs to be connected to the VREG pin of the XTR117.
  • 0 in reply to Collin Wells
    Prodigy 160 points
    Thank You Collin for clearing my doubts now I understand the working of the design. Firstly I will test with XTR117 VREG pin and get back to you.

    Regards
    Pranav Dosajh
  • 0 in reply to Pranav Dosajh
    TI__Guru 64895 points
    Hi Pranav,

    We're glad you've got it figured out. Let us know how it goes.