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TLV70018-Q1: TLV70018DDCR Partially starts up when power is applied

Daniel Rhodes
Prodigy 70 points
Part Number: TLV70018-Q1

Tool/software:

Using the TLV70018 on a board with several other LDO.  In the circuit below, C52 is installed with a 10k pulldown.  Regardless, when the 3.3V supply appears, the output of U3 rises to about a volt before turning off again (green).  Finally the enable signal is asserted (yellow as measured on U3.3) and all of the supplies turn on, including U3.  Neither the TLV70033DDCR (blue) or the TLV71211DBVR (red) in the circuit have this behavior.

  • 0
    TI__Genius 15125 points

    Hello,

    Thank you for your inquiry.

    Please allow me 2-3 business days to review your thread and get back to you.

    Best Regards,

    Hannah

  • 0
    TI__Genius 15125 points

    Hello,

    What is your EN pin connected to? 

    Are you sure that this pin isn't accidentally trying to turn the part on during this period?

    Have you tried a different device or is this the only device you have tried?

    Please let me know when you have a moment.

    Thank you,

    Hannah

  • 0 in reply to Hannah Grace
    Prodigy 70 points

    EN is connected to the output of the 1.1V supply which is shown in red in the scope graph above.   It has a 10k pulldown and this signal is low the entire time the 1.8V supply is misbehaving.  This does this on every board with this device installed.

  • 0 in reply to Daniel Rhodes
    TI__Genius 15125 points

    Is there any path that could be back feeding the output once the 3.3V path comes up?

    Could you also please send us another waveform showing the 3.3V rail coming up along with the output of the problem device?

  • 0 in reply to Hannah Grace
    Prodigy 70 points

    The main culprit could be two SN74AXC1T45DCKR which are connected to the 3.3V (VCCA) and 1.8V (VCCB) rails. No other device shares the 3.3V and 1.8V rails.  The AXC power up sequencing document seems to show that this back feeding of the power supply rail should not happen.

    www.ti.com/.../scea058a.pdf

    Below is the waveform showing Vin (yellow), EN (pink), Vout (blue)

  • 0 in reply to Daniel Rhodes
    TI__Genius 15125 points

    Okay, thank you for clarifying this. Did this answer your question or do you have any other concerns?

  • 0 in reply to Hannah Grace
    Prodigy 70 points

    I went through and systematically removed anything that tied a 3.3V power domain to the 1.8V power domain and the glitch remains.  The only thing connected to the 1.8V supply is the IC that is powers, which is only powered by the other rails that are enabled at the same time.  As near as I can see, this is a startup glitch with this LDO.

  • 0 in reply to Daniel Rhodes
    TI__Genius 15125 points

    Hi Daniel,

    I am looking into this now for you. Please allow me 1-2 business days to work on your thread.

    Best,

    Hannah

  • 0 in reply to Daniel Rhodes
    TI__Genius 12480 points

    Hi Daniel,

    Hannah asked me to take some measurements to look into this. After discussing with her, we believe the 1.8V output of the TLV70018-Q1 is being backfed by another supply on the IC that it powers. To see whether the TLV70018-Q1 exhibits a turn on glitch, I set up an evaluation board in our lab and powered it with both fast and slow ramping supplies while the enable pin was held low. I was unable to reproduce the output transient you are seeing. 

    Fast ramp, part disabled:

    Slow ramp, part disabled:

    Slow ramp, enable tied to Vin:

    Best Regards,

    Alex Davis