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TPS65721: DCDC won't turn off :(

ChipChucker
Prodigy 20 points
Part Number: TPS65721
Other Parts Discussed in Thread: , TPS65217, TPS65218D0

My goal is to have DCDC1 turn off, once the battery has finished charging from AC  (the AC will remain connected).

But I can't get DCDC1 to turn off if AC is live.   Setup is like so:

PB_IN pin remains pulled up to VSYS at all times.

I set DEFDCDC1.B7=0 (which allows control via the pin),  then drive pin HOLD_DCDC1 low. 

This should turn off the DCDC1.   And it does turn off when AC is dead.  But it does NOT turn off when AC is live.   

Anyone have any ideas?

  • 0
    TI__Mastermind 30280 points

    ChipChucker,

    Unfortunately, I have tested this on the TPS65721EVM-516 before and have verified that DCDC1 and LDO1 of the TPS65721 cannot be disabled when AC power is applied. I understand that this should be made clear in the datasheet but the datasheet has not been updated yet to clarify.

    The question that needs to be answered is: what purpose is there for disabling DCDC1 and LDO1 when AC power is applied?

     The simple logical explanation for keeping DCDC1 and LDO1 always-on when AC is available is that AC (line power) has infinite current capabilities over time, but a battery will discharge if DCDC1 and LDO1 stay on all the time. I agree with you that it does not match the datasheet, but it appears to me that the device is functioning properly: HOLD_LDO1 and HOLD_DCDC1 input pins and digital bits are ignored when AC power is applied.

     

    The logic observed is a 3-input OR-gate and same for DCDC1 and LDO1:

     

    If (AC power = = 1 || HOLD_DCDC1 Bit = = 1 || HOLD_DCDC1 Pin = = 1)

                { DCDC1 = ON }

    Else      DCDC1 = OFF

     

    If (AC power = = 1 || HOLD_LDO1 Bit = = 1 || HOLD_LDO1 Pin = = 1)

                { LDO1 = ON }

    Else      LDO1 = OFF

  • 0 in reply to Brian Berner
    Prodigy 20 points

    Hi Brian... thanks for the quick and definitive answer.

    >>AC (line power) has infinite current capabilities over time.

    Alas, not in my case:  our "AC" is supplied by a bank of 18650 cells (or one of those USB PowerBanks).  :(

  • 0 in reply to ChipChucker
    TI__Mastermind 30280 points
    ChipChucker,

    There are other PMICs with similar features that allow you to disable the DC-DCs and LDOs manually via I2C.

    The TPS65217 is a similar architecture as the TPS65721 and has a confusingly similar part number.

    The TPS65218D0 does not include a battery charger, but is the next generation of this type of device. An external battery charger would be needed for this device.