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XTR115 output is not stable

Cawen Gu
Prodigy 10 points

Other Parts Discussed in Thread: MSP430F4270, XTR115

Dear   Sir:

           We use XTR115 in our product,VCC supply MCU(MSP430F4270),MCU output DACout voltage to drive XTR115 which can output 4~20mA.MCU current is 1.8mA.

          But we find that XTR115 output current is not stable(4mA-->6.16mA),and No periodic.We test XTR115 output Vreg and VCC Volatge,Their volatge is not stable, VCC voltage is up:2.5Vd-->3.3Vdc, Vreg volatge is drop:5.0V-->4. 78V.

          Can you gave me some suggestion which can improve CLO current stable?

Thanks!

 

  • 0
    TI__Guru 65015 points

    Hello Cawen,

    Can you please share a schematic of your system that shows all circuitry powered by the XTR115 VREG and VREF voltages as well as some information about the loop-supply and load that the XTR115 will interface to?  The XTR115 is a 2-wire 4-20mA loop transmitter and the operation is commonly confused with a 3-wire transmitter which causes device damage or other strange behavior so I'd like to verify that your circuit operation is correct.  The fact that the VREG and VREF outputs are not correct leads me to believe that there is something fundamentally wrong with your circuit. 

    You mention that the MCU current is 1.8mA, but what is the current consumption of the sensor and conditioning circuitry that interfaces with the MCU?  Asking another way, what is the total current that you sink back through the IRET pin of the XTR115?  The load current for the VREG and VREF regulators flows back through the IRET pin.  If you look at the block diagram, the final output current is the sum of the current through the emitter of the BJT and the IRET current.  The OPA controls the base of the BJT so that the final current on IOUT is correct and the BJT simple provides the remaining necessary current that is not provided from IRET.  ( I_BJT-Emitter = IOUT - IRET).  Therefore, if the total load which sinks back through IRET is above 4mA then it won't be possible to force the output of IOUT to anything less than the current provided by IRET due to Kirchoff's Current Law (Current In = Current Out).  Realistically there is some required headroom deu to internal current consumption of the XTR115 and the maximum IRET current is closer to 3.7mA before the device will lose its ability to output 4mA.  Is it possible that in your programming loop you have a sequence of events that causes the uC + Sensor and conditioning circuitry to consume closer to ~6mA?  If so, this event would look like a periodic change in the output current and may explain your results.  Overloading (drawing too much current) will also cause the VREG and VREF outputs to change so if you have a continuously changing load this could look like instability of VREG and VREF.

    Those are a couple of quick guesses regarding what may be going on but I need more information including a schematic and some details about the system to provide further guidance.

    Thanks,
    Collin Wells
    Precision Linear Applications