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OMAP L138 Power sequencing RTC_CVDD when using a battery backup?

Judson Wilson
Intellectual 620 points

Hello, I am having a very difficult time figuring out a way to power RTC_CVDD using battery backup.  I see the following basic requirements:

  1. RTC_CVDD must be powered when CVDD is powered.  (Not sure how strict this is?)
  2. RTC_CVDD = CVDD ~= 1.2V

I constructed a regulator circuit to reduce my battery voltage down to the the required levels, so that I have battery backup of the power.  This solution introduces a problem if the user removes the battery, or the battery is dead.

My next solution was to use a 3v3 supply to feed the same regulator as well, (using blocking diodes to prevent the rails from pushing current to eachother).  In the power-on state RTC_CVDD should have the correct power, regardless of whether there is a battery connected.  However, at startup, in our design, the 3v3 can either lag or lead the core-voltage and 1v8 voltage.  Would it be a problem if I used the 3v3 that powers the peripheral I/O? This would cause the RTC_CVDD to lag all the 1v2 and 1v8 supplies by a small amount of time - less than a second.

I cannot find a good way to ensure that the RTC_CVDD comes up CVDD at the same time.

Any help appreciated.

 

  • 0
    Intellectual 620 points

    After additional thought, I think I can sequence the RTC_CVDD up first, before the core/etc are powered, without issue.  Will this cause problems?  Conceivably this could also happen if the system is powered down, the user disconnects, and then reconnects the battery.

  • 0 in reply to Judson Wilson
    TI__Guru 62975 points

    Judson,

    That is fine.  The RTC_CVDD supply is independent form the CVDD core supply so RTC_CVDD can be powered up while CVDD is powered down.

    However, the RTC_CVDD supply is not able to tap power from the CVDD supply so if the battery is the sole source of power for RTC and the user removes it, the RTC will not be able to keep its time.

    -Tommy