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ADS7028: Power sequnce

Oba
Mastermind 18125 points

Part Number: ADS7028

Hello,

My customer is considering to use ADS7028 with AVDD=5V and DVDD5V.
Then they have sevral questions.

1)
Is any specific power-up/down sequence between the 2 rails required?

2)
Is there any limitation for power ramp up speed?
What if the ramp up speed is very fast or very slow?
Is any ramp up speed acceptable?

3)
Regarding power cycling, is power up from AVDD or DVDD is NOT 0V condition acceptable?

Regards,
Oba

  • 0
    TI__Mastermind 25070 points

    Hello,

    There are no specific power supply sequence needed for this device. Note though that both power rails needs to be settled before any communication to the device.

    Generally any ramp up rate is acceptable, but as noted before, each rail needs to be fully settled before operating the device. On a broader note, ramping up very slowly, ie seconds, may result in a brown out condition. This has not been seen in this device though, but would suggest to avoid this anyways. 

    I am not sure if I understand your third question. I believe you are asking if the device is left idle while a power supply is lower than the operating voltage supply? This would go back to the previous point, where a brown out effect could happen, potentially leaving the device in an unknown state. 

    Regards

    Cynthia

  • 0 in reply to Cynthia
    TI__Mastermind 18125 points

    Hello Cynthia,

    Thanks for your reply.

    Let me expalin more detail for my quiestion 3.

    Usually power ramp-up starts from 0V for both AVDD and DVDD.
    But if the period after the shutdown to next power-up is very short, the next power up might start from non 0 voltage.
    Because the voltage on the AVDD/DVDD might not be discharged enough to 0V before starting next power up.

    Some devices absolutely require poiwer-up from 0V and deosn' accept power-up from non-0V.

    This is the background of queistion 3.
    Is this condition accepted?

    In you previous reply, you used "a brown out" in the below sentense. 

    >On a broader note, ramping up very slowly, ie seconds, may result in a brown out condition

    >This would go back to the previous point, where a brown out effect could happen


    What does this "brown out" mean in this case?

    Regards,
    Oba

  • 0 in reply to Oba
    TI__Expert 3805 points

    Hi Oba,

    Let me add some more details about the power supply behaviour:

    1. AVDD: This is a critical supply rail and powers all the ADC circuit blocks. The minimum operating voltage is 2.35V. The ADS7028 has an internal brown-out reset (BOR) circuit on the AVDD supply that has a reset threshold at 2V - 2.1V. In powered-up state, as soon as the AVDD supply drops below the BOR threshold, the ADC will be held in reset. The reset will be released when AVDD > reset threshold i.e. 2V - 2.1V.
    2. DVDD: This supply rail is used to power the digital input/output circuits and does not affect the core functionality of the ADC. A glitch on this power-supply is not expected to cause disruption to the ADC. The DVDD supply does not have a BOR circuit.

    With this background, this is the expected behviour of the ADS7028:

    1. 0V < AVDD < BOR threshold -> ADC is in reset state.
    2. BOR threshold < AVDD < 2.35V -> ADC is powered up and functional
    3. 2.35V <= AVDD -> ADC is in normal operating region and meets specified performance

    Regards,

    Rahul

  • 0 in reply to RahulVK
    TI__Mastermind 18125 points

    Hello Rahul,

    Thanks for your detail explanation.

    So what do you think abour power-up from non-zero voltage during short period power cycling? (My question 3) 
    From your answer, ideally, it seems to be no problem. But I'm not sure if I can say it should be no problem.

    Regards,
    Satoshi Obata

  • 0 in reply to Oba
    TI__Expert 3805 points

    Hi Obata-san,

    If the supply starts to ramp from a non-zero voltage it is not expected to create any problem because either device will get reset or it would already be in the normal operating region. This is because of the BOR circuit I mentioned in my previous post:

    RahulVK said:
    • 0V < AVDD < BOR threshold -> ADC is in reset state.
    • BOR threshold < AVDD < 2.35V -> ADC is powered up and functional
    • 2.35V <= AVDD -> ADC is in normal operating region and meets specified performance

    Regards,

    Rahul

  • 0 in reply to RahulVK
    TI__Mastermind 18125 points

    Thanks!

    Regards,
    Oba