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AM3358 RTC Power

Burt Ran
Prodigy 90 points

Other Parts Discussed in Thread: AM3358

I have two questions about Am3358 RTC power, there is one schematic about am3558 EVM version 1.5C from TI website.

1)from this schematic I can not understand RTC power solution on this board.

There is RTC LDO in PMIC(TPS65910) and I found this LDO output is not used for AM3358 RTC domain power supply,
but there is another LDO(U5) chip used for AM3358 RTC power supply. why add another LDO? I can not understand.

2) And I want add one battery for AM3358 RTC fucntion and could you give me some suggestions?

If I take AM3358 evm reference, and maybe I add one battery on am3358 vdds_RTC pin, do you agree?

  • 0
    TI__Guru**** 393215 points

    Hi Burt,

    The VDDS_RTC power supply must come from a stable 1.8V source. A battery cannot be connected directly to this supply.

    The external LDO is used for VDDS_RTC in the GP EVM Rev. 1.5C, because the TPS65910A3 RTC LDO switches to low power mode in OFF state, meaning that it can provide less than 100uA in this state, which isn't enough for the AM335X RTC. If you use TPS65910A31 instead, it can provide the full 20mA, eliminating the need for an additional LDO. See http://www.ti.com/lit/ug/swcu093c/swcu093c.pdf for details.

  • 0 in reply to Biser Gatchev-XID
    Prodigy 90 points

    Thanks for your answer.

    (1) I need your confirmation. After power on my system, and make sure TPS65910A3 always in active state, its RTC LDO could meet AM335X requirements, So, I could use TPS65910A3 RCT LDO provide VDDS_RTC and deleting LDO(U5);

    Another question:

    If you agree with me about item1

    after power off my system, how long AM335X RTC work  according to this design(I am not sure if RTC only work, and it also need 20mA current)? And use the same battery for TPS65910A3 in the GP EVM Rev. 1.5C.

  • 0 in reply to Burt Ran
    TI__Guru**** 393215 points

    Burt Ran said:
    (1) I need your confirmation. After power on my system, and make sure TPS65910A3 always in active state, its RTC LDO could meet AM335X requirements, So, I could use TPS65910A3 RCT LDO provide VDDS_RTC and deleting LDO(U5);

    Yes, the RTC LDO can deliver up to 20mA and the AM335X RTC requires less than 0.45mA. See this wiki for AM335X power consumption: http://processors.wiki.ti.com/index.php/AM335x_Power_Consumption_Summary

    Burt Ran said:
    after power off my system, how long AM335X RTC work  according to this design(I am not sure if RTC only work, and it also need 20mA current)? And use the same battery for TPS65910A3 in the GP EVM Rev. 1.5C.

    For this case an external RTC will definitely give much longer battery life (typically 0.5uA against 30uA for the AM335X RTC).

  • 0 in reply to Biser Gatchev-XID
    Prodigy 90 points

    AM335x RTC , it looks like not good, there are three reasons:

    1. We can not add one battery for RTC module,so, RTC could not work after system power off, it is very inconvenient and not accepted.
    2. This RTC backup current is about 30uA, it is too high for RTC function. Generally, it is about 1uA.
    3. If we use this RTC and processor must work on RTC only mode after system power off(if we can add one battery on it), processor must be wake up from
    RTC mode if it return active mode.

    Do you agree? So, we should better not to use it.

  • 0 in reply to Burt Ran
    TI__Guru**** 393215 points

    It's your design, your decision. I pointed out above that an external I2C RTC will give you a much longer battery life. When the processor starts you can configure the internal RTC to run from Peripheral PLL clock and you can update it from the external RTC if necessary.