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AM3351: RTC Current Consumption

Jason Lamb
Prodigy 80 points

Part Number: AM3351
Other Parts Discussed in Thread: TPS65218

According to the AM3351 datasheet (Rev. L), the maximum current rating for the RTC domain is 5 mA. I would like to know the typical current draw.

Thanks,

Jason

  • 0
    TI__Guru 63635 points

    I sent an email to a couple of guys that may be able to help answer this question. I will let you know what I find in a couple of days.

    Regards,
    Paul

  • 0 in reply to peaves
    TI__Guru 63635 points

    How are supplying power to the RTC core, are you sourcing CAP_VDD_RTC from an external supply or from the internal LDO via VDDS_RTC?

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 80 points

    I am supplying power to the RTC core from the LDO via VDDS_RTC.

    Jason

  • 0 in reply to Jason Lamb
    TI__Guru 63635 points

    Are you asking with hopes to power it from a coin cell?  If so, I know that will not work.  The quiescent current of the LDO alone would consume a coin cell in a very short time.

    Regards,
    Paul

  • 0 in reply to peaves
    TI__Guru 63635 points

    It looks like I may have been wrong. I thought he RTC power consumption was too large for operating from a coin cell. I found someone that provided me the following two coin cell connection topologies.  Maybe this will answer your question.

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 80 points

    Hi Paul,

    Thanks for your response. I want to clarify between the TPS65218 and TPS65222. We are using the  TPS65218  but then the picture included, which shows the typical RTC power consumption, is the TPS65222. Are the current consumption numbers the same for both the TPS65222 and the TPS65218?

    Thanks,

    Jason

  • 0 in reply to Jason Lamb
    TI__Guru 63635 points

    The RTC only consumes about 11uW when sourcing both supplies from external sources with the internal LDO disabled as shown in the second connection topology. The RTC power consumption increases to 34uW when using a single 1.8V source with the internal LDO enabled as shown in the first connection topology. The difference between 34uW and 11uW, or 23uW is the potion consumed by the internal LDO. 

    This is why I initially said you would not be able to use a coin cell to power the RTC. An external regulator is still needed to regulate coin cell voltage to 1.8V when using the internal LDO. Using an LDO for this purpose almost doubles the total power consumption. The second option uses a switch mode power supply to source both RTC voltage domains which is a much more efficient option.

    Power dissipation external to the device depends on your specific implementation. You would need to discuss these details with the respective PMIC teams.

    Regards,
    Paul