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DAC80501: Sequence for VREF

Ryuuichi machida
Guru 12655 points
Part Number: DAC80501

Hello,

Let me confirm about below.

* According to datasheet, user can use both internal REF and external REF.
However, if user want to use external reference, user need to change register setting by REF_PWDWN.
Then, I have following question.

Q. If user input external voltage to VREFIO without setting REF_PWDWN, is there possiblity that this will be broken ?
(Ex, if user input 4.096V as REF, since internal REF is 2.5V, reflux current will flow to output of internal REF. I understand that value of this current depends on impedance of internal REF and external reference, but I'm not sure there is no spec for sink current of internal REF...)

BR, 

  • 0
    TI__Mastermind 19086 points

    Hi,

    Internal reference can sink 5mA current and Reference input impedance is 100K.

    You can connect external reference and immediately do register write to change the device configuration to external reference. Device will work fine.

    Regards,

    AK

  • 0 in reply to Akhilesh K
    Guru 12655 points

    Hello,

    Thank you for your reply.
    How about answer for following question ?
    Q. If user input external voltage to VREFIO without setting REF_PWDWN, is there possiblity that this will be broken ?

    >current and Reference input impedance is 100K.
    According to datasheet, internal REF is enabled as default. output impedance for IO is defined as 0.1ohm.
    Why do you describe 100K ohm (I think that this is case which user set REF_PWDWN and change to use external REF.)

    BR,

  • 0 in reply to Ryuuichi machida
    TI__Mastermind 19086 points

    Hi,

    We are not supposed to operate the device with both external and internal reference mode. when you provide external reference, you need to set the device to use external reference by disabling internal reference.

    Possibility of device breakdown will be there if the external reference is applied and not changing device mode of operation for long time. We didn't characterize this time so, I cannot comment on the same.

    what I described about input impedance is REFIO input impedance.

    Regards,

    AK

  • 0 in reply to Akhilesh K
    Guru 12655 points

    Hello

    Let me confirm about below.
    If impedance when user do not set REF_PWDWN is 100kohm, I think there is little chance to break device.
    Because, if voltage difference b/w internal VREF is 3V (external VREF = 5.5V(max value) - 2.5V), impedance is such high impendace, only "uA" unit current will flow to internal REF.
    I think if user do not set REF_PWDWN impedance should be applied 0.1ohm. In that case, high current from supply source will flow to internal REF.
    What do you think my opinion ?

    BR,

  • 0 in reply to Ryuuichi machida
    TI__Guru 53773 points

    Hi Machida-san,

    Your opinion is correct.  The temporary current will not damage the device.

    Thanks

    Paul

  • 0 in reply to Paul_Frost
    Guru 12655 points

    Hello Paul-san,

    Thank you for your reply.

    >Your opinion is correct.  The temporary current will not damage the device.
    Which one do you point out ?

    1.If impedance when user do not set REF_PWDWN is 100kohm, I think there is little chance to break device.
    2. if user do not set REF_PWDWN impedance should be applied 0.1ohm.

    BR,

  • 0 in reply to Ryuuichi machida
    TI__Guru 53773 points

    Hi Machida-san,

    1. When the REF is powered-down, then there is no risk of damage.

    2. If the references and output buffer are still powered-on, then the output buffer will be sinking ~5mA into the DAC device from your external reference.  This is limited such that damage will not occur, but this can reduce performance as the device will be heating from this additional power dissipation and your reference will have some loading.  In addition, any impedance on the REFIO pin, PCB, and package will now have a higher current greater voltage differential. 

    I do not think there is risk of damage as the abs max is not being violated.

    Thanks,

    paul

  • 0 in reply to Paul_Frost
    Guru 12655 points

    Hello Paul-san,

    Let me confirm about below.

    > This is limited such that damage will not occur, 

    I can understand internal LDO have up to 5mA sink from datasheet, however, if internal LDO have current limit, I wonder why TI define input current for all pins on Abs spec(According to datasheet, there is "current into any pin" description for input current of abs spec.).

    I think that device may breakdown if following condtions are met.
    1. external reference have supply current capability over 10mA.
    2. there is no load except REFIO pin with not setting REF_PWDWN.
     
    Could you please re-confirm whether internal reference have current limit for sink ?

    BR,

  • +1 in reply to Ryuuichi machida
    TI__Guru 53773 points

    Hi Machida-san,

    The internal reference's output amplifier has limited current drive capability, so when you connect it to a source that has higher current drive capability, it will be forced into compliance as the output will be at it's short-circuit limit.  This looks like an increase in output impedance.  So in this case, your external reference will be sourcing 4.096V and ~5mA, forcing our internal reference to 4.096V.  This will not damage the device.  If you external references violates the abs. maximum voltage (4.096V > VDD), then the ESD cell will activate and there is a risk of damage, so you will have to control the supply sequencing.

    thanks,

    Paul

  • 0 in reply to Paul_Frost
    Guru 12655 points

    Hello Paul-san,

    Thank you for your reply and explanation.
    I understand what you explained. If I have another question, I will open another thread, so I will close this.

    BR,