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LP3972 Battery Switch Operation

Chee Wooi Cheok
Prodigy 190 points

Under Battery Switch Operation on LP3972 data sheet page 16/62,  It is stated that the battery switch may be disabled when only a main battery is used by setting the “no back up battery bit” in the control register 8h’0B bit 7 NBUB. 

Since switching will not occur with this bit set (i.e. the LDO_RTC will remain connected to the main battery), is that a must to connect the backup battery input (pin 15) to main battery input (pin 6) as stated in the data sheet?

Kindly enlighten what is the side effect if this backup battery input pin is left floating / unconnected (with NBUB bit set to 1)?

Regards

Cheok

  • 0
    TI__Prodigy 70 points

    Hello.

     I have forwarded the question to the engineer assigned to this product.  He shoud be contacting you shortly.  Thank you for your patience.

  • 0
    TI__Expert 3340 points

    Hi Cheok,

    Below is the answer from our designer regarding to your question..

    ************************************

    The brief answer for tying pins 6 and 15 together under your prescribed conditions is a Yes.  

     

    The reasoning is as follows.  First consider the wording and semantic predicated in the data sheet page 16.  

    The word used is "should"- The Backup battery input should also be connected to main battery.  Do not interpret it as "must".  As usual the end user

    can make creative use of the part so long as they understand the limitations and restriction.  The user bears the risk if application conditions do not

    adhere to data sheet and application note suggestions or the electrical limits.

     

    Now the why- The BackupBatt is defined as an input pin, it appears to be an analog input but it is also a digital-like input as well for a couple of reasons.

    There are logic circuits and comparator incorporated to effect the threshold decision making involved for normal operation which may not be turned off

    even if there was no switching between the supply pins.  Furthermore, the protection pad cell also has logic gates and transistor inputs therein.   

    As in any "good engineering practice",  one should never leave an input pin, digital  in particular, floating. Therefore tying pin-15 to a defined potential, 

    in this case the mainBatt (pin-6) the most positive potential is advisable.   This way when noise and leakage are present there will be less opportunities 

    for unnecessary switching incur which may increase power consumption as well as generate unwanted noise in the substrate.