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ADS8556: Voltage value on REFC pin and power sequencing of ADS8556

Hung Ching Hsu
Expert 3086 points
Part Number: ADS8556

Hi Team, 

My customer is considering ADS8556 and he has following 2 questions. Could you kindly help?

1. From following block diagram, REFC_A, B, C seems connecting with output of a buffer, customer wants to know what voltage level on REFC_A, B, C. 

Customer wants to choose voltage rating of capacitor based on it. Since 16V capacitor is smaller than 25V capacitor, they want to use 16V capacitor if it is okay. 

2. Does ADS8556 have power sequencing limitation for power supply? Customer is asking this since some ADC from other manufacturer had power sequencing limitation. 

Thanks.

Regards,

Jo

  • 0
    TI__Mastermind 24940 points

    Hello

    1. the reference at these pins will be the reference voltage being used. If the internal reference, then it will be 2.5V, if an external reference is used, then the value of the external reference. 

    2. Generally, there are no specific requirements with regard to the power sequencing of the device. However, when HVDD is supplied before AVDD, the internal ESD structure conducts, increasing IHVDD beyond the specified value. There is more detail in the datasheet, section 9, pg 39

    Regards

    Cynthia

  • 0 in reply to Cynthia
    TI__Expert 3086 points

    Hi Cynthia, 

    Thank you for the reply. 

    Regarding question 2, what will happen when "IHVDD increasing beyond the specified value"?

    Does it mean customer better supply AVDD first and then supply HVDD? 

    And what about power-off sequence? Should customer power off HVDD first and then power off AVDD or both is okay?

    Thanks.

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Expert 3086 points

    Hi Cynthia, 

    Look forward to the comment.

    Thanks.

    Regards,

    Jo

  • +1 in reply to Hung Ching Hsu
    TI__Mastermind 39255 points

    Hello Jo,

    Cynthia is out for PTO. I apologized for missing your post.

    1. When the current is higher than the specified value, the chip may overheat. The situation beyond the specifications can not be guaranteed.
    2. Yes, it's better to supply AVDD first and then HVDD/HVSS. 
    3. Yes, power off HVDD/HVSS first and AVDD later or both at the same time as a good practice.

    Thanks&regards,

    Dale

  • 0 in reply to Dale Li
    TI__Expert 3086 points

    Hi Dale,

    Thank you for the comment. Customer wants to confirm following, could you kindly help?

    1. You mentioned HVSS, does it also need to be supplied after AVDD? 

    2. So it will be no problem if customer use following power sequence and no external protection needed? 

    　a. Supply AVDD first and then supply HVDD

    　b. Remove HVDD first and then remove AVDD

    3. Related to question 2, customer used ADI's AD7328 before. P.33 of datasheet  mentioned Schottky diodes are recommended to place in series with VDD and VSS. Does it also helpful for ADS8556 to prevent IHVDD increasing beyond the specified value if HVDD is accidentally supplied earlier than AVDD?

    Thanks.

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 39255 points

    Hello Jo,

    Please see my answers below:

    1. Yes.

    2. Correct.

    3. The series diodes in that circuit is recommended for nonsymmetrical power supplies, it's a different situation here. I do not think it's needed for ADS8556.

    Regards,

    Dale