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REF5025: Start up issue (in conjonction with ADS1246)

Ibrahim Kamal
Intellectual 460 points
Part Number: REF5025
Other Parts Discussed in Thread: ADS1246

Hi,

I'm using a REF5025 connected to a ADS1246

I have a very strange behavior: at start-up, the ref output does not reach the target 2.5V. It stays below 1V. Sometimes, after a random long period of time (seconds), it eventually reaches the target. During this period, a huge current is drawn, around 100 mA, whether by the REF or the ADC, then everything seems to be ok.


Side note : DVDD of ADS1246 is not powered.

In which directions should I look?

  • 0
    TI__Mastermind 19550 points

    Hi Victor,

    Have you tried swapping the REF50 with another REF50?

    Can you measure the current across R23 during startup?

    Do you have scope shots you can share?

  • 0 in reply to Marcoo Z
    Intellectual 460 points

    Hello Marcoo,

    It was not the REF50, it was the ADS1246. After replacing it, it works like a charm, I get very nice readings.

    The high current must have been drawn through the REFP pin of the ADC.

    Given the schematic above, do you think the big capacitors on REFP can be the problem? when AVDD turns off, REFP can be higher than AVDD, leading to a current through the diodes.

    Thanks,

    Victor

  • 0 in reply to Ibrahim Kamal
    TI__Mastermind 19550 points

    Hi Victor,

    This is good that the issue is found.

    Let me add the ADC team to help out with this issue as they will know more about the ADS1246

  • 0 in reply to Marcoo Z
    TI__Guru 51051 points

    Hi Victor,

    Apologies for the delay in response. I believe you are correct. If you take a look at the absolute maximum ratings table, 7.1 in the datasheet: https://www.ti.com/lit/ds/symlink/ads1247.pdf?ts=1593118853718&ref_url=https%253A%252F%252Fwww.google.com%252F

    You will see that the references pins should not exceed AVDD by more than 300mV, otherwise the device can be damaged. More than likely the previous ADS1246 had burned out, causing an internal short which led to the high current a limited voltage output by the REF5025. 

  • 0 in reply to Alexander Smith
    Intellectual 460 points

    Hi Alexander, 

    I have several PCBs, with the same potential problem, but all of them seems to work fine.

    I'll update the design for the next prototypes. What would you recommend to tackle this issue, without degrading performance ? adding a Schottky in parallel? Or a series resistor?

    Thanks,

    Victor

  • 0 in reply to Ibrahim Kamal
    TI__Guru 51051 points

    Hi Victor,

    Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

    I would ensure that the voltage present on VREF does not exceed AVDD by more than 300mV. Can you delay enabling the REF5025 until AVDD has been established? 

  • 0 in reply to Alexander Smith
    TI__Mastermind 19550 points

    Hi Victor,

    To add, I would not use a series resistor due to it affecting accuracy of the REF50.

    Is the issue that the REF50 turns on too fast before the ADC?

    Or that the REF50 is still on after the ADC turns off?

  • 0 in reply to Alexander Smith
    Intellectual 460 points

    Hi Alexander and Marcoo,

    Of course I don't want to reach the Absolute Maximum Ratings.

    I don't think the issue is the startup, but rather the power down. Both ADC and REF are powered by the same LDO. The risk is that the charge in the REF output capacitor can discharge through the REFP pin.

    I think this is prevented in the ADS1246EVM with 2 BAT54, unless they are here for another purpose...

    @Marcoo can it damage the REF50 too?

  • 0 in reply to Ibrahim Kamal
    TI__Mastermind 19550 points

    Hi Victor,

    The ADS1246 EVM also has the 47 ohm resistor from the RC filter that can help.

    I usually don't like series resistors but if you need to limit the current then you can add one. R (min) = VREF / Iout(max) = 250 Ohms. With a typical current draw of 30nA, there should be a ~8uA voltage drop.