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TPS65921 RTC battery backup capability

I have connected a battery to  ball G9 BKBAT VBACKUP  and have maintained the RTC after removing power main power VBAT.

The documentation on the TPS65921 states that ball G9 BKBAT VBACKUP must be shorted to GND,   yet the documenation also states the VRTC.OUT output voltage range for both "Normal mode" and "Backup mode".

The "Differences in Features for TPS65920 and TPS65921" states that the "Backup battery charger feature removed".

Could it be that only the backup battery charger feature was removed and not backup battery feature so the RTC can be maintained via ball G9 BKBAT VBACKUP ?




3 Replies

  • Jeff,

    All feature related to ball G9, BKBAT have been removed on TPS65921.

    It means that you cannot use BKBAT pin to keep RTC alive when main battery has been removed.

    The reference to backup state are still in documentation because if your main battery is present but VBAT is lower than VMBLO then device is in backup state




  • In reply to Alexis Piton:


    That is interesting since I am able to keep the RTC alive using a 3V coin battery connected to the TPS65921 BKBAT when the main battery has been removed from VBAT. The main battery has been removed for a over a week and time has been maintained.   Perhaps VRTC is being supplied via the BKBAT ESD input protection diode or some other sneak path.

    Is it true that once the main battery voltage goes below VMBLO all of the other LDOs are powered down to minimize current draw except for the VRTC LDO ?  If this is the case, can a 3V coin battery be connected in series with a diode to provide a voltage less than VMBLO when the main battery is not present? The diode will also isolate the 3V coin battery from the main battery when present. 

    What is the preferred method to maintain power to the RTC when the main battery is removed with BKBAT grounded? Or was the main battery never intended to be removed and just left to slowly discharge so once it went below VMBLO the backup state was maintained with minimal current draw?

    Thanks for your support,






  • In reply to Jeff Mogensen:


    Backup battery has been defeatured , part of the block is still present explaining that you can keep data but functionality is not guarantee and can create another issue.

    VRTC supply is kept enabled until VBAT goes bellow VBNPR threshold..

    the diode connection will not allow you to recharge the coin cell.

    TPS65921 has been designed to work on platform when main supply cannot be removed.