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TPS65910xx VRTC Errata

Hi,

We have one more issue with the TPS65910A31 start-up.
Same issue as we mentioned in the below E2E with a different customer.
https://e2e.ti.com/support/power_management/pmu/f/200/t/501127

Few of the custom boards(AM335x+TPS65910A3A1) don't power up...
The VRTC voltage is very low in between 0V and 0.8V.

Same as the above E2E the problem is related to the capacitor value on VRTC pin,
if we change capacitor value on VRTC pin to 0.1uF all the boards were working perfectly fine.

The Boards are working with 0.1uF on VRTC
but we are not sure if this is a proper workaround?
Because as per the TPSxx datasheet, the capacitor value for VTRC should be in the range of 0.8uF to 2.64uF.

Note:
This application don't use Back-up mode or any sort of Low power mode,
the application will be ON always once powered up. AM335x is used in RTC feature disabled mode.

This issued has occurred with many of our customers using TPS65910
and we believe there is some problem in the TPS65910's VRTC during power-up.
Is this some kind of Errata or limitation of this device?

You can also find the details of same issue in the below E2E post also.
https://e2e.ti.com/support/arm/sitara_arm/f/791/t/320847

Confirmation:
Please let us know if it is OK to use 0.1uF on VRTC instead of 2.2uF(TYP)?
It will be helpful for many customers if there is mention on this issue in the manual or wiki page.

Best Regards
Kummi

  • Hi Kummi,


    Behavior like this is often attributed to VRTC's lower current capability during startup, and commonly caused by additional connections on VRTC such as system logic. Reducing the output capacitance may reduce initial currents on the LDO but is not really recommended for normal operation at full load currents.

    If you have a few pull-ups that use VRTC as the source, what are their values? Does increasing their values to 100k alleviate this issue when using 2.2uF?

  • In reply to Richard Solarski:

    Hi Richard,

    Thank you.

    Can we assume this is the limitation of TPS65910's VRTC LDO?

    All our customer's schematics are as per the recommended schematic cheklist and
    the TI's EVM. VRTC is connected to BOOT1 of the TPSxx and VDDS_RTC/RTC_KALDO_ENn pins of AM335x
    and as the AM335x RTC is not used there won't be any such load on the VRTC LDO.
    And no other logic or pull-ups connected to the VRTC.

    Our customer's application is almost into the production and it will be difficult to change the
    circuit but can change the capacitor value(to 0.1uF).

    But as you mentioned "not really recommended for normal operation at full load currents"
    will there be any problem towards the TPS65910's reliability?
    In that case could you please let us know what kind of issues could occur.

    As mentioned as we don't use the RTC there won't be much load on VRTC LDO.
    so will it be safe to continue to use 0.1uF on VRTC?

    Best Regards
    Kummi
  • In reply to Kummi:

    Hi Kummi,


    TI does not have reliability data for this condition since this is outside of the specifications of the datasheet, so I cannot confidently tell you that they won't encounter issues with this configuration in the future.

    I should point out that this would be more of a concern if operating with rated loads and transients, but since they are operating at essentially no load I do not have any other indication that this lower capacitance would cause reliability issues. Many other customers have resorted to this smaller capacitance.


    However, if they do not have pull-ups connecting VRTC to BOOT1 or VDDS_RTC/RTC_KALDO_ENn, I would recommend adding them. Pull-ups would limit the current from the VRTC LDO and should be a more reliable way to address this issue.

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