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BQ25756E: Design constraints and Design requirements

Tio Tit Sin
Prodigy 20 points
Part Number: BQ25756E
Other Parts Discussed in Thread: BQ25756, , BQ25758

Tool/software:

Good day,

I had been trying out on the BQ25756/BQ25756E performance and was thinking is there a way to design it for compatibility up to 6S Cell Charge Voltage. 

As understood, Rfb Top is fixed at 249k Ohm while Rfb Bot is changed based on the voltage range needed. 

In my case, using design calculator provided, it would be approximately: 

a. 2S Batt, 8.4V, Rfb Bot = 56k Ohm (8.37V at 1.536 Vfb)

b. 3S Batt, 12.6V, Rfb Bot = 34.5k Ohm (12.63V at 1.536 Vfb)

c. 4S Batt, 16.8V, Rfb Bot = 25k Ohm (16.85V at 1.536 Vfb)

d. 5S Batt, 21.0V Rfb Bot = 19.6k Ohm (21.08V at 1.536 Vfb)

e. 6S Batt, 25.2V, Rfb Bot = 16.2k Ohm (25.19V at 1.536 Vfb)

 

The concern is not on the Rfb Bot value, but on switching configuration between these required configuration,

I though of using N-gate or slide switch to select Rfb Bot accordingly, but would like to consult TI first, do TI have any recommendation for these controls? 

If it is fixed to one designed output per design, I may need to look for other suitable part.

Thanks in advance and hoping to get your reply soon. 

  • +1
    TI__Mastermind 31155 points

    Hello Tio,

    There's a few different choices to change the voltage.

    1. Yes, you can use an N-gate or slide switch to change the feedback resistance.
    2. You can also use a digital potentiometer to change the feedback resistance.
    3. If your circuit has an MCU and you can change between battery chemistries, you can use the BQ25758 to charge the batteries. The BQ25758 is an I2C controlled DC-DC converter with constant current and constant voltage modes.

    For the first 2 options, make sure that charging is disabled before switching the feedback resistance.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 20 points

    Hi Ethan, 

    Thanks for your kind response I will come out with some prototypes to try it. 

    Just to confirm on my understanding,

    as different Rfb Bot are needed, typically i could not just use one Rfb Bot to cater for all range of charge voltage right? 

    Eg: 32k Rfb Bot can only range from Charge Voltage of 13.22V to 13.76V (Vfb: 1504mV - 1566mV)

  • +1 in reply to Tio Tit Sin
    TI__Mastermind 31155 points

    Hello Tio,

    Tio Tit Sin said:
    as different Rfb Bot are needed, typically i could not just use one Rfb Bot to cater for all range of charge voltage right? 

    Yes, you'll need different values of Rfb Bot to cater to all the different battery charge votlages.

    Tio Tit Sin said:
    32k Rfb Bot can only range from Charge Voltage of 13.22V to 13.76V (Vfb: 1504mV - 1566mV)

    Yes, this correct.

    Best Regards,
    Ethan Galloway