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TPS65950: RTC losing time & date when used with a 3V primary cell

Tim Tanner
Prodigy 40 points
Part Number: TPS65950

We use an Overo Gumstix SOM in a product. The SOM uses the OMAP processor and TPS65950 for power management. The RTC was powered from a rechargeable coin cell but even the largest cell proved to be insufficient to keep the RTC alive if the product is stored for years in a cupboard.

We replaced the rechargeable cell with a CR2477 3.0V 1000mAh  primary cell with a BAS16J diode to prevent the TPS65950's charging circuit from attempting to charge the primary cell. There is also a 150R series resistor.

Sometimes when the system is powered off the RTC fails to maintain the time and date. This is intermittent but can be provoked in to being a hard on fault by increasing the series resistance to 33K.

The RTC normally consumes about 6uA from he primary cell but when not maintaining time and date this drops to 2uA or even fractions of a uA.

Is there a recommended circuit to use with primary cells?

  • 0
    TI__Mastermind 27230 points

    Hello,

    I checked the thread and assigned it to a product specialist. He is off today and will be back tomorrow.

    Regards

  • 0
    TI__Genius 17380 points

    Hi Tim,

    Did you update your software to not charge the backup battery? That would probably be better than adding the diode which will have some additional power loss. 

    One issue we have seen in the past is if the main power supply drops below VBACKUP before the power down sequence has completed then power can be pulled from VBACKUP briefly. With large series resistance (or forward voltage), then it could be dropping too low.

  • 0 in reply to Kevin LaRosa
    Prodigy 40 points

    Hi Kevin,

    We did not change the software to not charge the battery. We could not find out how to do it. The Gumstix runs Linux with the Qt UI framework. If you know how we can make this modification please let us know.

    We are aware of the voltage dip on power off. We tried a 100nF capacitor directly on the Gumstix VBACKUP pin. This did not help the RTC still did not advance when the unit switched off.

  • 0 in reply to Tim Tanner
    TI__Genius 17380 points

    Hi Tim,

    Unfortunately I'm not knowledgeable about the software for implementing it either.

    You may want to try increasing the capacitance to see if it reduces the loss of RTC.