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TPS62740: Chip enable when the supercap is fully charged

Andrea Azzimondi
Prodigy 20 points
Part Number: TPS62740

Tool/software:

Hello,

I would like to know if it is possible to activate the enable signal when the supercap connected to the TPS62740 is already charged. What behavior should I expect?

I am currently experiencing a malfunction of the TPS62740, and I wanted to know if it could be due to this activation. Perhaps it is not allowed?

Thank you in advance for your support.

  • 0
    TI__Expert 6510 points

    Hi Andrea,

    Thanks for using E2E.

    Please take a look to the data sheet topic 9.4.1 which should help you to get the system running correctly.

    Best regards,
    Sepp

  • 0 in reply to Josef Marsmann
    Prodigy 20 points

    It really seems that residual charge voltages on the supercapacitor could affect the proper startup of the main system and the subsystem.

    Where can I find additional details to better understand this issue?
    Why could a voltage on the supercapacitor impact proper startup?
    What could happen if I do not comply with this condition?

  • 0 in reply to Andrea Azzimondi
    TI__Expert 6510 points

    Hi Andrea,

    I will try to get more information about this behavior and will come back to you asap.

    Best regards,
    Sepp

  • 0 in reply to Josef Marsmann
    Prodigy 20 points

    Thank you, Josef.
    In my application, sometimes I see a continuous voltage of 3.45V on Vout.
    However, the maximum my firmware set with the VSEL pins is 3.2V.
    This should not be possible with the TPS62740.

    One thing I do is set the EN pin to 0 and then back to 1, perhaps too quickly for the supercap connected to Vout to discharge.
    I was wondering if this startup condition could lead to the incorrect regulation of 3.45V.

  • 0 in reply to Andrea Azzimondi
    TI__Expert 6510 points

    Hi Andrea,

    Please see the application note which explains how to use the TPS62740 with a supercap.

    Efficient Super-Capacitor Charging with TPS62740

    Do you following this setup? What is the behavior of the malfunction?

    Best regards,
    Sepp

  • 0 in reply to Josef Marsmann
    Prodigy 20 points

    Yes, I followed this guide and created this design.
    The supercap is SCMT22F505PRBA0 with a 5F capacity.
    In my test, V_PRIMARY is 3.6V, supplied by a bench power supply.
    The VSEL pins are set to regulate 3.2V.

    At some point, I measured a DC voltage of 3.45V on V_SUPER_CAP.

    In my application, the EN pin is frequently toggled.
    My suspicion, though I don't rule out other possible reasons, is the following:
    Starting from a correctly regulated 3.2V on VOUT, the EN pin goes to 0 and then back to 1 without allowing VOUT enough time to reach 0.
    I fear that the buck converter enters a malfunctioning state if it tries to set a voltage lower than what is already present on VOUT, given that a charged supercap is connected to VOUT.

    I would like to understand whether my suspicion is correct (and if my suspicion is correct, to be able to justify it with a theoretical basis) or if there is an error in my design that could lead to more than 3.2V on VOUT.

  • 0 in reply to Andrea Azzimondi
    TI__Expert 6510 points

    Hi Andrea,

    We are not able to detect an issue on the schematic which could lead to the malfunction.

    Could you please provide 2 scope plots (good and bad behavior) to show the VOUT, V_SUPER_CAP and EN voltage when you are switching off and on the EN pin.

    Best regards,
    Sepp