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ADVREF pin current draw?

Discussion in 'Microcontrollers, Programming and IoT' started by eem2am, Jun 16, 2013.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    We are using an external reference for our micro (for the ADC and DAC)

    Table 35-33 (page 719) of the SAM3N00B datasheet appears to say that the current drawn from the ADVREF pin by the micro's ADC module is 250uA.
    Is this correct?
    If it really is that much then that means the micro's external voltage reference circuitry (based around a TLV431) will not be able to keep the reference constant at 3.064V (due to the volt drop across the 220R resistor in the external voltage reference circuit)
    The following is the external reference circuitry connected to the ADVREF pin of the SAM3N00B microcontroller.......

    External Voltage reference circuit feeding the ADVREF pin (also attached in pdf)
    http://i41.tinypic.com/2qxrmug.jpg



    Table 35-35 on page 720 says that a further 300uA is drawn into the ADVREF pin by the micro's DAC module.
    This makes things even worse.

    Does this much current really get drawn into the ADVREF pin by the microcontroller?

    SAM3N00B microcontroller datasheet:
    http://www.atmel.com/Images/doc11011.pdf
     

    Attached Files:

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    Last edited: Jun 16, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The first thing you need to check is the minimum current through the 431. If you go below that, you will lose regulation rapidly.

    Ensure the current through R93 is sufficient to provide the minimum current to the 431 as well as the maximum to your load. This calculation need to be done at the minimum supply voltage too.

    My reading is that the 8 bit DAC may require 250uA, and the 10bit DAC 300uA, but you may be right that it requires an extra 300uA -- I can't help you there.

    A colleague of mine did some experiments with a TL431 and found that the values given in the datasheet for "typical" minimum current required for regulation were very close to actual values for the devices he tested. However, the datasheet says that up to 1mA may be required, so it would be best to have perhaps 2mA rather than the 1.1mA you have chosen, and ensure that you stay above 1.5mA at the minimum supply voltage.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Excellent advice Steve.
     
  4. eem2am

    eem2am

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    Aug 3, 2009
    TLV431 only needs 100uA max i believe.

    Its just when the ADC and DAC are working together that it could be 550uA? (250uA + 300uA)
    It seems bizarre that a voltage reference pin of a micro could draw half a mA?
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    You're right eem2am, the TLV431 is specified for a minimum "cathode" current of 100 uA. It also has a 1.24V reference voltage, instead of 2.5V. Thanks for the heads up.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    True. I just looked at the TL431.

    The TLV431 is specified for "Low Operational Cathode Current 80μA Typ, but as you note, 100uA is the maximum minimum (err, you know what I mean).
     
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