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DAC70501: Settling time of of VREIO

Shiga Yuichi
Prodigy 80 points
Part Number: DAC70501

How long does it take for the VREIO pin to output 2.5V after the device is powered on (VDD = 2.7V)?

Also, how long does it take for the 2.5V output of the VREFIO pin to turn off after rewriting the CONFIG REGISTER to disable the device internal reference?

After that, how many  seconds can I start applying the reference voltage from the outside?

  • 0
    TI__Mastermind 30568 points

    Hi Yuichi-san,

    The reference will be powered up during the POR which is given as a max of 250us. I don't have an exact spec for the reference disable time, but the reference will power down very quickly (100us or less). One thing to keep in mind is that your reference capacitor will be charged to the internal reference (2.5V) when it is powered down. You didn't mention the value of your external reference, but if it is less than 2.5V then the charged reference cap will try to discharge through your reference. The DAC70501 reference pin has an input impedance of 100k, so you'd want to wait a few time constants (100k * refcap) depending on the value of your external reference for the capacitor to discharge before applying the external reference. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Prodigy 80 points

    Hi Katlynne

    Thanks for your replying.

    I will apply 5V as a reference voltage.

    When starting the DAC 70501, I will set it to 0V. After I rewrite the register to disable the internal reference, I will start the external 5V.

    In this case, Is it OK to wait 100usec from the register rewrite confirmation timing (rising edge of the SYNC signal) to start boosting the external 5V?

  • 0 in reply to Shiga Yuichi
    TI__Mastermind 30568 points

    Hi Yuichi-san,

    Yes, if you are using a 5V external reference then you do not need to wait for the reference capacitor to discharge. 100us should be a sufficient time to wait for the internal reference to power down after the rising edge of sync.

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Prodigy 80 points

    Hi Katlynne,

    OK, I wiil wait 100us at least after rewrite register.

    Please tell us a little more to understand how it works.

    Q1. Does the reference capacitor you mentioned means an external bypass capacitor stabbed with the arrow in the figure below?

    Q2.If the answer to Q1 is Yes, is the timing correct in the figure below?



  • 0 in reply to Shiga Yuichi
    TI__Mastermind 30568 points

    Hi Yuichi-san,

    Yes, the external capacitance you have shown and there is a small internal reference capacitance.

    Your timing diagram is correct, but because your external reference is greater than the 2.5V external resistance, you shouldn't have to wait the extra time shown in blue for the reference capacitance to discharge. 

    Best,

    Katlynne Jones