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TPS65910AA1 VRTC & AM3359 VDSS_RTC

Paul Thompson
Prodigy 20 points
Other Parts Discussed in Thread: AM3359, TPS65910A

Hi,

I've designed a board using the AM3359 MCU that is powered with a TPS65910AA1 PMU, (where due to availability issues we used an XAM3359 part).

In designing the board I refered to the TI TPS65910A User's Guide For AM335x Processors (SWCU093, August 2011) which recommended that the PMU's VRTC output supply be used to power the MCU's VDDS_RTC input supply.

We had three boards made up, one of which was working fine & has subsequently failed (& we haven't quite got to the bottom of why). In investigating this failure, I noticed that there is an up-issue to the User's Guide (SWCU093, version B, September 2012) which recommends that the VRTC supply from the PMU is not connected to the VDDS_RTC input supply to the MCU, in contradiction to the August 2011 version of the User's Guide.

Looking on this forum & in the User's Guide, I've not been able to understand why this change has been made.

More importantly, we are about to make up some more boards (this time using "real" AM3559 devices) whose VDDS_RTC inputs will also be wired into the PMU's VRTC output supply.

Can you please advise, why has this change of VRTC supply between the PMU <-> MCU been made & can devices fail as a result?

Will the MCU/PMU still operate correctly if their VRTCs are wired up as per the August 2011 version of the user guide?

Regards & thanks,

Paul.

Dreampact Limited.

  • 0
    Prodigy 20 points

    Hi,

    Could someone, preferably from TI, please help me with this?

    Why has this change been made (namely the VDDS_RTC of the AM3559 is no longer powered from the VRTC of the PMU)?

    I am about to build another batch of boards - do I need to modify them or will they be OK with VDDS_RTC powered from VRTC as per the original PMU application note?

    I would be most grateful for any assistance with this.

    Paul Thompson.

    Dreampact Limited.

  • 0 in reply to Paul Thompson
    TI__Intellectual 1820 points

    Hello,

    TPS65910A : in OFF state, maximum rated output current  of VRTC is 100uA, for internal + external use.

    On some platforms, VRTC is not able to deliver enough current in OFF to ACTIVE transition to supply VDDS_RTC of the processor.

    We advise to use:

    -        an external LDO for supplying VDDS_RTC of the processor

    Best regards,

    Flore

  • 0 in reply to Flore Auzanneau
    Intellectual 600 points

    Dear Flore,

     

    We too experienced similar issue but we could isolate SoC VRTC and PMIC VRTC...when checked ( in isolated condition), PMIC is providing 2.8V instead of 1.8V. And it is also observed that VBAT dropped to 2.8 V from 3.6V ( with constant current of 100mA initially just for safer side). What would have gone bad. Is it that PMIC is bad which is giving 2.8V VRTC to SoC.

    YKFS

     

  • 0 in reply to Y2K
    Prodigy 10 points

    I met the same probelm,VRTC of PMIC output is 2.4v.