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Original question:

TPS65217 with non-rechargeable battery backup?

AM3358: RTC battery switchover

dynamphrous
Prodigy 50 points
Part Number: AM3358
Other Parts Discussed in Thread: TPS65217, TPS65218D0

I'm designing a board around a AM3358 chip, and using a TPS65217C for a PMIC. My question revolves around powering the RTC. I am using a coin cell battery, and I also have output VLDO1 of the 65217C as my VDD_RTC. What can I do to make the RTC run off the PMIC while its being powered by an external power supply, and only switch over to the coin cell when the LDO output on the PMIC shuts down.

I'm currently using a TPS71718 LDO for the coin cell regulator, and a scheme of AND gates for doing the delay required for the RTC_PWRONRSTn line. Very similar to what is found in the TMDSSK3358 schematic. 

And are there any example layouts for this type of powering scheme? I'm having trouble finding reference designs of this type of circuit. 

  • 0
    TI__Genius 13567 points

    Does your board have a need for the battery charger in the TPS65217? I would recommend using the TPS65218D0 since it has specialized coin-cell regulators for the RTC in place of the TPS71718. Reference user guide. The TPS65217C and D do not support RTC-only mode due to power sequencing limitations.