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XTR111 circuit failure after some testing.

George Photopoulos
Prodigy 120 points
Other Parts Discussed in Thread: XTR111

We have designed and built a test circuit and a PCB for an application we are developing using the XTR111 (see attachments).

After a damaged mosfet BSP171 we replaced it with a different (but similar?) BSP315 which we could find in time.

Our MCU has an Aout 0-3.22v (measured value)  @ 12bit resolution, and the XTR111 is powered with 12vdc.

During my first test for linearity (see attachment for test values) at around 0.80v Vin the output drifted rapidly (0.5mA/sec) to a max of 33.3mA.

That did happen before but with much lower rate and much lower Imax at the end (1mA offset from the expected value), and when I changed the Vin it did get back to normal values.

Now at 0Vin the Iout is around 16~19mA (very unstable) and at around 0.7V goes to the max value of 33.3mA (very unstable 30~33.3mA)

Question: What's wrong!

Schematic: XTR111 4-20mA.pdf

PCB Layout:XTR111 4-20mA PCB.pdf

First linearity test:Linearity Test 31Aug2015.xls

  • 0
    TI__Guru 62745 points

    Hello George,

    Your schematic connections look appropriate so we're not immediately sure what's causing the issues you're experiencing.  Is this happening on several products or only on one device?

    On your PCB layout it doesn't look like you connected the PowerPAD on the bottom of the XTR111, but it's difficult to tell.  For proper performance of the XTR111 the PowerPAD on the bottom of the IC must be properly soldered to the PCB. 

    The data for the first ~0.7V shown in your spreadsheet looks correct based on the input voltage and R1 value of 1600 Ohms.

    To further debug your issue, could you please provide oscilloscope captures of the Vin, IS, VG, and output current (across a load resistor) signals?  Look for oscillations and try to capture images that allow us to observe the frequency and behavior of the output oscillations (if it is oscillating).

  • 0 in reply to Collin Wells
    Prodigy 120 points
    Hello Collins,
    and thank you for your time.

    Yes, the PowerPAD is soldered on the Ground line that passes bellow the IC.

    This is happening to one device.
    A previous one was designed with errors in circuitry in result of magic smoke from the IC.
    Then an identical with the current one was made but didn't work (MOSFET damaged too).
    An now this one.
    Is it possible to damaged the MOSFET again?

    Unfortunately I do not have have an oscilloscope at the moment, but the output although unstable seems linearly offset.
    Also the Rset measured (PINset to GND with Rset on PCB) reads 169Ω, but removed 1599Ω. PINset seems like shorted internally? PINset to GND reads 860Ω (unpopulated).
    Something does not match.

    Regards,
    G.Ph.
  • 0 in reply to George Photopoulos
    TI__Guru 62745 points
    What do you think is damaging the MOSFET devices? Are they overheating or being exposed to voltages beyond the limits specified in the datasheet? The BSP315 has voltage limits outside of what the XTR111 can tolerate so if the MOSFET device is being damaged it's likely that the XTR111 is being damaged as well.
  • 0 in reply to Collin Wells
    Prodigy 120 points

    Hi,

    I don't expect any voltages beyond limits, nor  any overheating happens (at 33.3mA it does get a little hot though). Just  that the workbench I am running this project on, does not have any ESD protection and as (I've been told) that MOSFETs are very sensitive devises and MOSFETs are a new thing to me. Anything I do not have a comprehensive knowledge of, worries me.


    The resistance of the PINset is it correct? I don't see any reason for it to be so low (from the block diagram).

    G.Ph.

  • 0 in reply to George Photopoulos
    Prodigy 120 points
    I just replaced the MOSFET. Same thing.
    I also run it now manually with a 3.3v reference and a 50kΩ trim for 0~3.3Vin.

    G.Ph.
  • 0 in reply to George Photopoulos
    TI__Guru 62745 points

    Hi George,

    When handling integrated circuits, proper ESD handling procedures must be followed or devices may be damaged.  It sounds like this may be happening in your circuit.  

    For easy evaluation of the XTR111 device, consider obtaining an XTR111EVM which will allow you to design and configure the XTR111 on a circuit that has been verified for proper operation.  Proper ESD handling requirements still apply when evaluating the XTR111EVM so try to find an ESD approved work station to test it on.

  • 0 in reply to Collin Wells
    Prodigy 120 points
    Hi again.
    And that's the reason that when I replaced the XTR111 with a new one it came back to life!

    Thank you for your time and advises.
    Case closed.

    Best regards,
    George Photopoulos