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Confusing Documentation Regarding Backup State for RTC in TPS65920

Howard Robson
Intellectual 415 points
Other Parts Discussed in Thread: TPS65920, TPS65930, TPS65950

There seems to be some confusion in the documentation regarding the backup state and capabilities of the TPS659xx family.

Document SWCA025 states on page 1:

"The TPS65930 and TPS65920 devices have the same backup state functionality as
TPS65950 device. Hence the backup state functionality described for TPS65950 in this
report also applies to TPS65930/20."

This seems clear and unequivocal.  However, document SWCU052A states on page 211

"BACKUP System is powered only with backup battery and maintains only VBRTC supply.
Note: This state is inherited from the TPS65950 device and is not appropriate for
the TPS65930 or the TPS65920, because they do not accept the use of a backup
batter."

Also, document SWCU052A states on page 217:

"No events put the TPS65930 and TPS65920 in BACKUP state, because these devices do not have
access to a backup battery."

Both these statements are in conflict with the other document, so what am I to believe?  It does not seem to make sense that the devices would have an RTC that cannot be backed up.  I require this capability, but do not require all the other additional functionality of the TPS65950.  Also, both the TPS65930 and TPS65920 have a BKBAT pin, which I assume is the pin by which the devices is connected to the backup battery (documentation not very clear) so what use is it without this functionality?

  • 0
    TI__Genius 15100 points

    Hi,

    Sorry for the confusion in the documents.

    Information in SWCA025 is correct. The information in SWCU052A will be corrected. In TPS65930/20 the battery charging capability was removed  and in the documentation everything related to charging was taken away including the back-up battery.

    However, the backup battery is not part of the battery module, it is part of the power domain and the functionality is same as in TPS65950.

     

  • 0 in reply to Gandhar Dighe
    Intellectual 415 points

    Hi Gandhar,

    Thank you for your reply.  This is still somewhat confusing as SWCA025 covers backup battery charging so I would assume that then the TPS65930/20 do also have backup battery charging capability, yet you say "In the TPS65930/20 the battery charging capability was removed", and this seems to contradict my assumption and what is stated in SWCA025?

    Basically, I want to know if I can connect a battery to retain the real time clock when the main power is not available, and furthermore, if this battery can be a rechargeable type.  Finally to which pins do I connect the battery - i.e. which of the pins VBAT, VBATS and BKBAT are used?

  • 0 in reply to Howard Robson
    TI__Genius 15100 points

    Sorry if my comments were confusing. I tried to give some more information by saying that "backup" is part of the power domain and not the battery charging domain. So, though battery charging module is removed in TPS65930/20 "backup" domain is NOT affected because of it being part of the power module.

     

    Direct answers to your questions:

    Yes, you can have back up battery connected to BKBAT pin to retain RTC information. Backup battery can be recharged. Please refer to the state diagram to see transitions into and out of backup state.

  • 0 in reply to Gandhar Dighe
    Intellectual 415 points

    Thank you, now I get it.

    I am trying to find the current drain of th RTC to determine battery life.  The only reference I can find is in the TPS65950 Data Manual and it is on page 55 and it appears to be 2.25uA (Table 4-20).

    Some more confusion here, the equivalent Table in the TPS65930/20 Data Manual (Table 4-12 on page 45) does not have a "Backup" mode. Presumably VBRTC is the voltage that supplies power to the RTC in the backup state and on page 27 it states "VBRTC is not used in this configuration"?