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LM3658: Force voltage output with no battery

Eden Rafealov
Prodigy 40 points
Part Number: LM3658
Other Parts Discussed in Thread: BQ24040, BQ25170

I have a prototype here with LM3658. The application looks the same as the typical application in the datasheets. Right now there is a protection circuit on the battery that shuts off the voltage output below 2.8V. So the system sees "no battery". If I connect 5V to CHG_IN on LM3658 I only have about 1.5V on BATT from LM3658. I guess it's a problem for the circuit if it does not see a battery. However, can one in any way force the circuit to emit a charging voltage after the battery shuts off internally?

  • 0
    TI__Mastermind 19195 points

    Hello Eden,

    You can put the device into LDO mode by floating the TS, this will provide a useable output with no battery connected.

    Best regards,

    Nick

  • 0 in reply to Nick Brylski
    Prodigy 40 points

    Yes I noted that in the datasheet, though I wonder: if a battery is present and I want the charger to function as expected, TS is required, right? Though when the battery disconnects and LDO mode is required to set a charge voltage to the battery to reconnect the battery, TS should be disconnected?

    I suppose if that is the case, I can add a transistor to control the connection of TS through the voltage of VBAT as detailed below. I added a cap and resistor around the FET to increase miller capacitance and therefore turn on time so there is some cross-over when the LDO mode switches on and off...but I would rather not add this transistor if possible

  • 0 in reply to Eden Rafealov
    TI__Mastermind 19195 points

    Hi Eden,

    Let me look into this a more. Are you able to charge the battery at all when the protector activates?

    Thanks,

    Nick

  • 0 in reply to Nick Brylski
    Prodigy 40 points

    Hi Nick,

    It is not possible at all to charger the battery as the internal battery protection "disconnects" the battery from the LM3658.

    I am able to remove the battery and apply an external voltage above 2.8V which reactivates the battery.

  • 0 in reply to Eden Rafealov
    TI__Mastermind 19195 points

    Hello Eden,

    Can you attach waveforms of the BAT pin when applying 5V on CHG-IN? I am seeing much 3-4V applied when the BAT pin is floating.

    The LM3658 is older device and may not work well with all battery protection circuits. I would recommend looking into our newer charger devices like bq24040 or BQ25170 which have improved battery detection routine.

    Best regards,

    Nick