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XTR111: No output current.

Stéphane LEMAIRE
Prodigy 10 points
Part Number: XTR111
Other Parts Discussed in Thread: XTR300, TIDA-01536

Tool/software:

Hello,

I would need some support on the XTR111 component. The Vout control of the MCP4726 is working correctly; I can vary the DAC from 0 to 5V. I control the EDiICENABLE signal perfectly, EDI_ERROR is at 3.3V, and the voltage on pin 5 "REGF" is at 5.2V. The transistor Q1 is a BC857ALT1G and Q2 is a BSP170PH6327XTSA1.

The problem is that there is no current at the output of IC_P, regardless of the voltage on Vout and pin OD.

Do you have any idea what might be wrong?


Thanks in advance.

  • 0
    TI__Guru 55466 points

    Hi Stephane,

    Can you please verify and/or let us know the following:

    1) What is the value of the load resistance? Is the other side of the load resistor referred connected to the same XTR111 local GND?

    2) How are you measuring the output current?  Are you using a current meter? or measuring voltage across the grounded load resistor? If using a current meter, are you referring the negative terminal of the current meter to XTR300 local GND?

    3) Can you kindly re-verify the OD pin 9 is set to 0V (or pulled low) with a voltage meter to enable the XTR111? 

    4) Can you verify you get the expected voltage across RSET?

    5) How many devices or PCBs show this issue? Is this a prototype board assembled by hand OR this board went to a solder reflow process?

    If only one device shows the issue, can you please replace the XTR111 and re- attempt the measurement?

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 10 points

    Hello Luis, and thank you for your support.

    3) I had made a mistake with controlling pin 9 (OD), but now I’ve set this pin to zero. However, output 8 (/EF) is now indicating an error.

    I set the DAC output to 2.52 (measured value at the DAC output).

    1) and 2) I placed a 150-ohm resistor between IC_P and IC_N, and I’m using a voltmeter to measure the voltage across this resistor; I currently read 6.2mA.

    4) I read a voltage of 2.52V across R71 (pin 7).

    5) I currently have 2 prototype boards assembled by reflow. They behave exactly the same, with the same voltage and current values.

    Pin 5, REGF: I read 5.13V and 3.07V for REGS(pin 4)
    Pin 2, IS: I read 22.8V (VSP is at 24V)
    Pin 3, VG: I read 5.35V


  • +1 in reply to Stéphane LEMAIRE
    TI__Guru 55466 points

    Hi Stephane,

    The Error Flag occurs when the XTR111, current output  is out of compliance, where the XTR111 can support a maximum voltage of (+VVSP - 2V) at the IS pin.  In your bench, the IS pin measures 22.8V, while VSP supply is +24V so this confirms that the device is out of headroom.

    The loss of headroom occurs when the series load resistance is to high, or when the resistor is not connected properly to the XTR111 GND, or an inadvertent wire break occurs, and/or when the P-FET transistor is not connected correctly.

    --  What is the L32/L31 part number? What is the series resistance of L31 and L32? For debug purposes, can you please connect the load resistor between R70 (15-ohm) and XTR111 GND directly, removing L31/L32 on the current path?  In general, we don't recommend using a high inductance load for the XTR111. 

    Please refer to the following XTR111 reference design circuits for suggested current output transmitter protection:

    XTR111: TIDA-01536, TIDA-00760

    -- Can you please share or verify the PCB layout showing the PFET drain/source connections? On a few cases of XTR111 unrelated queries in the past, the PFET transistor drain / source connections have been inadvertently reversed on the PCB board layout.

    Thank you and Regards,

    Luis

  • +1 in reply to Luis Chioye
    Prodigy 10 points

    Hello Luis,

    You are correct; the issue was caused by the filter inductors. I removed them and replaced them with zero-ohm resistors, and now the circuit is delivering the expected current. Thank you so much for your support, Luis.

    Best regards.

    One last question, my board will be installed in a noisy environment. What do you recommend for filtering this output?

  • 0 in reply to Stéphane LEMAIRE
    TI__Guru 55466 points

    HI Stephane,

    That's great news, glad the device is working.

    Please check below the TIDA-01536 Design Guide, the reference design features a precision DAC and XTR111, with a digital isolator.

    The design includes TVS diodes, and a couple of resistors/capacitors at the XTR111 output for protection, to provide immunity to EMI/RFI interference and transient voltage disturbances as described in the IEC61000-4 tests.  

    Single-Channel, Isolated, 3-Wire Current Loop Transmitter Reference Design

    Reference design links:

    Thank you and Kind Regards,

    Luis