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TPS659113 Vbackup

Paul Koenig
Prodigy 190 points

Hi,

   We have a customer who is using a DM814x and would like to use the TPS659113 device.  They have the following questions.

-----

I need help on the Vbackup pin on the TPS659113 from TI. The complete datasheet for the TPS659113 isn't on their website.

But my question: If power is removed. Can a reasonable sized supercap on the VBackup Pin hold up RTC for one year?

If so, what size cap would be required?

----

Does anyone have a feel for the power requirement on the RTC mode and what this would take?

Thanks,

Paul

  • 0
    Prodigy 5 points
    1. What voltage VBACKUP will be charged to

    Please refer to p13 of the datasheet depending on BBSEL bit (also describe p85)

     

    2.    What the minimum VBACKUP voltage is for the part to stay alive

    Please refer to the state machine p43. If VCC7 and BB (Backup Battery)<VBNPR, then the device goes in NO SUPPLY state. For VBNPR value please refer p11

    3.    What needs to be kept alive, just RTC or other parts?

    In back up state, back up registers, osc-clock are supplied, everything under VRTC domain is supplied.

    Regarding your calculation, you need to take into account VBNPR in dV: dV=vbackup-VBNPR.

    For a 10uA current consumption on Back up battery, if you need to last at least 365 days=8760h, then consider a backup battery at least of 10*8760=87600uAh=87.6mAh. Please check datasheets and consider battery self discharge.

  • 0 in reply to Ed Walker
    TI__Intellectual 1245 points

    Using the 10uA backup state current for just the RTC, and 0-2.8V backup voltage, and 2Farads, I get 1.62 days. 

    For 2000mF (2 Farads) and 10uA backup state with Vbat=0, 2.8V charge voltage,

    dv = vbackup - VBNPR (2.1V) = 0.7

    I=Cdv/dt,  so dt=CdV/I = 2F x 0.7V/10uA=140,000 seconds.

    = 2333 minutes = 38.89 hours = 1.62 days

     

    So, using this calculation, you would need around a 400F capacitor in order to power the chip for a year.  You may want to consider Ed's backup battery as a solution instead.