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Original question:

Recommended currently active standalone Battery Charger IC for 4S Lithium Ion/LiPo batteries

BQ25792: Reference schematic - for review

Srikanth Bhat
Prodigy 30 points
Part Number: BQ25792

Hello,
Can you please see this schematic and suggest if I have made any mistakes?
This is for 4S battery, with max charging current of 1Amp. 
Input voltage to charger is around 12.7v. 


Also had some questions, clarifications - please can you comment/confirm if they are right?: 
(1) If I understood correctly, this BQ25792 will automatically select between battery or external 12.7 source to supply to the load on SYS pin. If current drawn is within source's set limit, BQ25792 will source the power from the input source and from the battery only when input source fails/disconnects. 
(2) It will automatically source/supplement from battery if current from external source exceeds a preset value set on ILIM_HIZ pin
(3) Is it possible for me to set a totally different voltage for SYS pin to supply the load? I mean a different voltage from battery charging voltage (or the slightly more voltage) and different from input source voltage? 
Example: If input is 12.7v, battery is 4S (so nominal voltage is 4x3.7v=14.8v), but can I set SYS to be at say 12v?

Thank you for your time and attention
Srikanth 
 6165.BQ25792.pdf

  • 0
    TI__Guru**** 166996 points

    Hi Srikanth,

    The schematic looks okay except for the 10kohm resistor to GND to TS pin.  That is meant to simulate a 103AT thermistor at 25C.  If you connect a thermistor to TS, that resistor must be removed.

    Regarding 1 and 2, the BQ25792 use NVDC topology which means the SYS output is from the converter, not directly connected to the input source.  The converter prioritizes the SYS load up until the input power is maxed out.  At that point the battery charge current reduces and could reverse in order to supplement the load.  The converter always provides at least a minimum system voltage (MINSYS or SYSMIN in the datasheet) that you can set but as the battery charges above this voltage, the SYS voltage follows the BAT voltage up to (slightly above) VBATREG (4*4.2 = 16.8V in your case).

    Regarding 3, there is a limited range above VBATREG that you can set MINSYS; however, that results in the battery charging in linear mode.  In linear mode, the internal BATFET between SYS and BAT linearly regulates the current.  The wider the voltage drop across the BATFET, the hotter the charger IC gets.  If it gets too hot, it hits thermal regulation and scales back the charge current to keep the charger's die temp at the thermal regulation point.  Operating in this mode is not recommended because it will extend charge time.

    Regards,

    Jeff

     

  • 0 in reply to Jeff F
    Prodigy 30 points

    Thank you Jeff for the very quick response. 

    Regarding #3: Ok that we have a limited range above VBATREG that we can set MINSYS. How about below VBATREG?
    I guess that is obviously not possible then?

    Example: If input is 12.7v, battery is 4S (so VBATREG is 16.8v), but can I set SYS to be at say 12v?

    Regards,
    Srikanth

  • +1 in reply to Srikanth Bhat
    TI__Guru**** 166996 points

    Hi Srikanth,

    No, you cannot set MINSYS below BATREG.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Thank you Jeff!