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LM4040A: out of tolerance reference

Part Number: LM4040A

I am seeing a high number of LM4040AIM3 10V references failing with higher than tolerance voltage even when tested fresh out of the tube with no soldering.

  • Hi Mike,

    Welcome to the E2E forums. What are the conditions for you test set-up?

    What value are you using for Rs and what are your minimum and maximum loads?

    Regards,
    Diego Lewis
  • We use these as a reference voltage supplied from 24V±10% via a 20k resistor and they are only loaded by a TL082 opamp input used as a voltage buffer. The test is being performed in an ambient of approx. 22 degrees C. We are seeing approximately 8% fail, even when tested without soldering in a SOT23 socket, with no load other than a 6.5 digit DVM. The devices that fail are always high.

  • Mike,

    The LM4040A10 has a minimum cathode current of 75uA typical, 125uA max. With a 20k resistor you get between 0.58mA to 0.82mA of current through Rs. According to the TL082 datasheet, it can pull up to 2.8mA (1.3mA typical) of current with no load connected, so the resistor needs to be decreased.

    I recommend decreasing the Rs value to at least 4kOhm so that you have enough current for operation under worst case scenario. 

    Regards, Diego Lewis

  • Diego,
    The reference is only supplying one of theTL082 inputs which would be typically 30pA (max 200pA at 25C) the 2.8mA would be the supply current to the TL082. If I was to reduce the Rs value, this would increase the reference voltage I was seeing rather than decrease it.

    Regards,

    Mike
  • Hello Mike,

    What voltage are you measuring at the cathode with your DMM?
    Is it possible you could test the device without any loading on the output?
    Could you also look at the cathode voltage on a scope?

    Please also note the deltaVz/deltaIz specification. The cathode voltage can shift with any change in the reference current relative to the standard measurement setup.
    The standard measurement setup is 150uA, so the initial 0.1% tolerance is depending upon the device having 150uA cathode current. If the cathode current is higher, the reverse breakdown voltage can increase with an increase in cathode current.
    The dynamic impedance can also affect the cathode voltage.
    Here is an app note that shows all the specifications that can affect an adjustable voltage reference. This is not an adjustable voltage reference, but many of the concepts apply here.
    www.ti.com/.../slva445.pdf

    Best,
    Michael
  • Hello Michael,

    with the DVM measuring at the cathode I still measure a high voltage, on a small sample these were up to 10.016V, measured with a cathode current of 700uA. This is measured on an isolated test socket with nothing loading the output.
    If I am reading the spec correctly the cathode current can increase to 1mA and only add a maximum of 0.8mV.

    I have some data files of test results I can send you if there is an email I can send them to.

    Best Regards,

    MIke
  • Hello Michael,

    I have now also looked at the cathode of the devices on an oscilloscope and they appear fairly quiet, approximately 2mV of ripple that I can attribute to the DC power supply. The results for the 50 samples we tested fresh out of the packaging without any soldering are below:-

    sample voltage notes
    1 9.99844  
    2 10.00122  
    3 10.00039  
    4 10.006  
    5 10.00768  
    6 10.00528  
    7 9.9981  
    8 10.0046  
    9 10.00472  
    10 10.01515 voltage drifted upwards during test
    11 9.99248  
    12 10.00634  
    13 10.00058  
    14 9.99215  
    15 10.00459  
    16 10.00681  
    17 10.00866  
    18 10.00609  
    19 9.99757  
    20 10.00176  
    21 9.99688  
    22 10.00692  
    23 9.99957  
    24 9.99394  
    25 9.99953  
    26 10.01074 voltage drifted upwards during test
    27 10.00502  
    28 9.99873  
    29 9.99645  
    30 10.00184  
    31 10.00336  
    32 9.99911  
    33 9.9933  
    34 9.9989  
    35 10.00454  
    36 10.00286  
    37 10.00607  
    38 10.01026 voltage drifted upwards during test
    39 9.9958  
    40 10.0054  
    41 10.0134 voltage drifted upwards during test
    42 9.99846  
    43 10.00109 voltage drifted upwards during test
    44 10.00229  
    45 10.00479  
    46 9.99872  
    47 10.00139  
    48 9.99796  
    49 9.99863 voltage drifted upwards during test
    50 10.00185  

    I hope this info is helpful, I have more data from larger samples that show a similar distribution, measured with traceable calibrated meters.

    Best Regards,

    Mike

    1 9.99844
    2 10.00122
    3 10.00039
    4 10.006
    5 10.00768
    6 10.00528
    7 9.9981
    8 10.0046
    9 10.00472
    10 10.01515
    11 9.99248
    12 10.00634
    13 10.00058
    14 9.99215
    15 10.00459
    16 10.00681
    17 10.00866
    18 10.00609
    19 9.99757
    20 10.00176
    21 9.99688
    22 10.00692
    23 9.99957
    24 9.99394
    25 9.99953
    26 10.01074
    27 10.00502
    28 9.99873
    29 9.99645
    30 10.00184
    31 10.00336
    32 9.99911
    33 9.9933
    34 9.9989
    35 10.00454
    36 10.00286
    37 10.00607
    38 10.01026
    39 9.9958
    40 10.0054
    41 10.0134
    42 9.99846
    43 10.00109
    44 10.00229
    45 10.00479
    46 9.99872
    47 10.00139
    48 9.99796
    49 9.99863
    50 10.00185
  • Mike,

    I compiled the data for the 100 samples you provided (in chart at bottom)

    You seem confident in your test equipment, so I won't challenge that.

    ZKA could add 1/2 to 1mV positive shift based on your current source. Not a significant factor.

    Temperature shift from 25C typical (20ppm/C) would be 0.2mV/C with some devices being significantly more. The ones that you have seen drifting up. This is a factor, but not the whole reason.

    Thermal hysteresis is typical 0.08% which is a typical 8mV shift. This is significant. The devices were not soldered, but they may have been in different temperatures between the factory test and today.

    So despite a 10mV factory test window other factors will widen the actual results.

  • Ron,

    Thank you for the reply. I had only provided data for 50 samples, and tried to add a similar chart of distribution which ended up as a repeat of the sample data, however the spread is still the same.

    When I have examined a batch of >300 units soldered into boards using the recommended soldering profile I see a similar distribution centred around 10V but with approx. 8% above the 10mV spec. the highest being 10.037V. The data suggests there may be something different about this 8%.


    Is there a process that will bring the devices back inside the published 0.1% specification?

    Is the drifting voltage on some of the samples typical in some devices? I would have expected them to be consistent.

    It does seem a bit misleading that when we buy parts that to the best of our knowledge have been shipped, stored and assembled within your specified limits then apear to fail your standard specification.

    Regards,

    Mike

  • Hello Mike, 

    Unfortunately the thermal hysteresis, in addition to other forms of variation previously mentioned, can cause this drift as applying a high temperature to the device while soldering can increase the die temperature and ultimately affect its accuracy. 

    It is possible that you could force temperature cycles on the device, say take the device down to -40C or up to 125C and let it soak for a little bit, then bring the device back to room temperature and see how the reference has shifted. You can possibly run multiple cycles of cold or hot to get the reference back into 0.1% specification.

    We test all units to our specification before they are shipped, so that when the devices are shipped, they are guaranteed to meet our datasheet specification. If you feel that something is abnormal about these 8% of devices, then you could also consider submitting a return either to your local distributor or sales person.

    Please let me know if you need anything else or if I can help.

    Best,
    Michael