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Original question:

TPS65910 BACKUP VOLTAGE RANGE

TPS65910: Backup Voltage

kntxp
Prodigy 10 points
Part Number: TPS65910

Hello,

I want to backup the RTC of the TPS65910 with a Supercap when the device is shut off. What is the minimum voltage required on VBACKUP to maintain the RTC information? The datasheet does not seem to be very clear about this. I assumed the minimum votage to be 1.9V, but the RTC seems to turn off even at a higher voltage.

  • 0
    TI__Mastermind 38821 points

    Hello, kntxp,

      Please see the section copied below from datasheet. The min VBNPR threshold is 1.9V. Please double check the VRTC should on on when VBACKUP=1.9V. 

      6.6 Backup Battery Management
    The device includes a back-up battery switch connecting the VRTC regulator input to a main battery
    (VCC7) or to a back-up battery (VBACKUP), depending on the batteries voltage value.
    The VRTC supply can then be maintained during a BACKUP state as far as the input voltage is high
    enough (>VBNPR threshold). Below the VBNPR voltage threshold the digital core of the device is set
    under reset by internal signal POR (Power-on Reset).

    Thanks!

    Phil

  • 0 in reply to Phil Yi40
    Prodigy 10 points

    Hello Phil,

    thank you for your answer. The RTC seems to already loose backup when the voltage falls below 2.15V. How is this behaviour explainable?

    We kinda accepted the increased threshold and will now try to charge our supercap to 5V instead of 3.3V by setting the BBSEL register to 11b. But doing this we have concerns about a note in the datsheet BBSEL registers:

    (1) Note: • BBSEL = 10, 00, or 01 intended to charge battery or superCap • BBSEL = 11 intended to charge capacitor

    Why is BBSEL=11 only intended to charge capacitors and no Supercaps? Is there a chance to damage the supercap or the PMIC when using BBSEL=11?


    kntxp

  • 0 in reply to kntxp
    TI__Mastermind 38821 points

    Hello, kntxp,

      Can you measure the VRTC output when supercap still has voltage >1.9V? The VRTC should be 1.8V and RTC function keeps working. 

      Please check the datasheet section "6.6 Backup Battery Management" for more details.

      I'll check with other team member for your last question and then answer you later. 

    Thanks!

    Phil

  • 0 in reply to Phil Yi40
    Prodigy 10 points

    With a backup voltage of 2.094V the RTC works fine (VRTC=1.81V)

    With a backup voltage of 2.068V the RTC does not work anymore (VRTC = 0V)

  • 0 in reply to kntxp
    TI__Mastermind 38821 points

    Hello, kntxp,

      It looks like the unit has an issue with the backup circuits.

      How many units have the same issue from how many boards in total (issue ratio)? 

    Thanks!

    Phil

  • 0 in reply to Phil Yi40
    Prodigy 10 points

    Up to now, I have tested 5 out of 5 devices that show an equivalent behaviour

  • 0 in reply to kntxp
    TI__Mastermind 38821 points

    Hello, kntxp,

      I'll do the same measurement tomorrow on the EVM I can find and then give you an update. 

    Thanks!

    Phil 

  • 0 in reply to Phil Yi40
    Prodigy 10 points

    Hello Phil,

    do you already have some measurement results?

    Did you check the question about the BBSEL regsiter with your team memebers?

    Thank you
    kntxp

  • 0 in reply to kntxp
    TI__Mastermind 38821 points

    Hello, kntxp,

      Yes; my measurements have the same results as yours. We may need to change the 1.9V to 2.1V in next datasheet update.  

      About the BBSEL register, when BBSEL=11, the charger output can be up to VBAT which is up to 5.5V. Usually coin cell battery is up to 3.6V and SuperCap is about 4V;  so only capacitor with > 6.3V can be used for the VBAT level charging. 

    Thanks!

    Phil