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TPS65217 with non-rechargeable battery backup?

Randy Maruschock
Intellectual 320 points
Other Parts Discussed in Thread: TPS65217

For the TPS65217 (B in particular) I am interested using a non-rechargeable coin cell battery as the backup, such as a 3V CR2032.  Is the TPS65217 set up for such a scenario?  Or is this a bad idea with the default register for CH_EN being 1, so the battery may see a charge current until the bit can be flipped?

  • 0
    TI__Expert 4525 points

    You can automatically/permanently disable the charger simply by leaving the TS pin floating.  This will indicate that the battery temperature is out of range and thus the charger will not turn on.

     

    However, there are a couple other problems worth noting:

    --A 3V supply will not work with the 3.3V outputs.  Therefore you would need to have those disabled if you were running from the coin cell.

    --The default UVLO of the PMIC is 3.3V.  You may change the UVLO threshold to a lower value, but this setting (and all settings) are reset to default if you go to SLEEP mode.  Hence you would need to stay in ACTIVE mode (to keep lower UVLO setting) but disable the 3.3V rails (to avoid power-good fault).

     

    So, the coin cell would most likely be used to supply the RTC and maybe keep the DDR in refresh mode.  It would not be used to power an entire system.

     

    An alternative solution would be to buy a cheap external 1.8V LDO and connect the coin cell to that.  (I'm assuming the main point of the coin cell was to keep the RTC alive)

  • 0 in reply to Stephen Nortman
    Intellectual 320 points

    Thank you for the response.  Your assessment is correct, the sole purpose of the the battery in my application is to keep the RTC ticking.  It sounds like my options are to use external circuitry to create a battery back up circuit, or use a higher voltage battery (above 3.3V) so to avoid any issues with UVLO.  It sounds like the external backup circuitry may be a bit more reliable as it avoids any interruption to LDO1 if a state is ever reached that shuts it down.

  • 0 in reply to Randy Maruschock
    TI__Expert 4525 points

    Yes.  This PMIC was created with a Li-ion battery in mind.  To use a coin-cell battery as RTC backup, the external LDO is a good solution.

    One more thing to note.....assuming you are using the AM335x processor, you will see a pin called RTC_PWRONRSTn.  This pin must be high in order for the RTC to remain active.  You can simply use the external LDO to put this pin at 1.8V.  However, as noted in the processor datasheet, the RTC_PWRONRSTn pin must stay low for 1ms after the RTC supply turns on.  So, you would need to add a delayed path...perhaps a simple RC delay.