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BQ25798: Power the load without batteries

Rajat Metha
Prodigy 40 points
Part Number: BQ25798

Tool/software:

Hello Team Teaxas,
   We have System Load and Batteries connected to the same net BAT. We have Diode in series with BAT and Batteries.
Now we want to Power the Load when No batteries are connected.

Sometimes we are able to power the load, sometimes not,

Could you please give us a solution to power the load when no batteries are connected. 

Note: System load and Batteries cannot be on different lines..


Thank You!

  • 0
    TI__Guru**** 165886 points

    Hi Rajat,

    With no battery attached and load on BAT, the charger can only provide up to ICHG current to the load.  If the load dips below ICHG, the battery cap voltages increase to VREG.  If the load exceeds ICHG, the battery cap voltage dips, potentially below BATLOWV in precharge and trickle charge, where charge current is reduced.  Just curious why you cannot place load on SYS?  Is peak load too large for the 6A max discharge current of the internal FET between SYS and BAT?

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    Prodigy 40 points

    why you cannot place load on SYS is because we have already designed a board, now we cannot change this configuration.
    Load requirement 16V and 800mA and transcient current requirement is 7A for 400uSecs
    We have seen that it flickers the output when no battery connnected and only load is connected.

    Please provide a solution considering the requirements.

    Thank You!

  • 0 in reply to Rajat Metha
    TI__Guru**** 165886 points

    Hi Rajat,

    The max charge current is 5A so the converter itself cannot provide the 7A for 400us.  You will need either a battery or a large capacitor at BAT to handle the transient.  You can use i=C*dv/dt to size the capacitance where C=(7A-ICHG)*400us/(VREG-VBATLOWV).  VBATLOWV is the battery voltage at which the charge changes from fast charge to precharge (max of 1A).  The host can set VBATLOWV has a % of VREG in REG0x08(7:6) as long as that voltage >2.5V trickle charge voltage threshold (i.e. cannot use VREG=4.2V*15%=630mV setting). 

    Regards,

    Jeff