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BQ25798: BQ25798 design question

saivenkat b
Prodigy 40 points
Part Number: BQ25798

Can I use BQ25798 to get power from a 5W panel to charge an 2 cell 7.4v battery and buck and get 5v output from battery with 2A output

  • 0
    TI__Guru**** 166996 points

    Hi Saivenkat,

    No. The SYS output follows the battery up to BATREG.  The SYS pin's MINSYS regulation provides a minimum system voltage when the battery is discharged and must be set lower than the battery regulation voltage, which in your case would be 8.xV.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    can you help me understand what is the use fo buck-boost in the IC, weather it buck/boost the the input coming from solar or it can buck boost the output form the battery while charging or it can do both

  • 0 in reply to saivenkat b
    TI__Guru**** 166996 points

    Hi Savienkat,

    The buck/boost converter with output at SYS pin operates in forward/charge/sink mode to charge a 1S-4S battery that is connected to SYS through a internal FET.  The battery is connected to the SYS pin through an internal FET and is in parallel with other loads on the SYS pin.  When the battery voltage is above the minimum system voltage (MINSYS), the SYs pin follows the battery up to regulation. 

    In reverse/OTG/source mode, the buck/boost converter provides a regulation voltage at VBUS from battery power.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Guru**** 166996 points

    In case I wasn't clear, the charger can only work in one mode at a time, either forward or reverse.

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    hey jeff thanks for your reply, I am going back and front in the datasheet I am slowly understanding stuff. Can you confirm does BQ25856 and few other ic's which say they support charging NiMH but inside the data sheet there is no mention about NiMH.

     

  • 0 in reply to saivenkat b
    TI__Guru**** 166996 points

    Hi Saivenkat,

    Most host controlled LiIon chargers can use host processor software to modify their default VREG for NiMH voltages.  However, if charging NiMH with higher than 0.5C charge current, the recommended charge termination is  -dVbat/dt and/or dTbat/dt, which a LiIon charger cannot perform.  If the charger or host has an ADC, the ADC can measure battery voltage and temp, compute the slopes and then terminate charge.  If NiMH charge current is <0.5C, charge can be terminated with a timer and CV clamp.  

    Regards,

    Jeff