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BQ25306: Battery charging voltage lower than 3.4V

JA G
Prodigy 40 points
Part Number: BQ25306

Hi TI experts,

I implemented BQ25306 in our design, this ic according to the datasheet has a programmable charge voltage from 3.4V to 9.0V. In our design, the battery we use can be charge up to the max. continuous charging voltage of 3.4V.  Also, in the datasheet the recommended resistance value of R2 is 200kΩ or lower. The battery charge voltage is programmed as VBATREG = 1.1 (1 + R1/R2).

It was a bit challenge for me to find available exact value of R1 to have a set charging voltage of 3.4V. At first I choose R1=422k and R2=200k, using the equation the set charging voltage is 3.421V however this exceeds the battery max charging voltage. I came up with an idea of using a value of R1=412k since this resistance value is the only option for me now, then the charging voltage is 3.366V. My question is what would be the long-term effect to the BQ25306 if I set the charging voltage lower than 3.4V? Would BQ25306 function properly without any issue?

  • 0
    TI__Mastermind 24075 points

    Hi,

       For charge regulation voltage accuracy and battery leakage current purposes, we recommend 200k for the bottom resistor. You can always change this value to a lower value, and accordingly calculate the other resistor required. For example Rbot = 100k, Rtop = 210k gives 3.41 charge regulation voltage. Playing around with these values will give a more accurate charge regulation voltage. I would recommend using it for 3.4V and above as we do not test operation outside of this range.

  • 0 in reply to Kedar Manishankar
    Prodigy 40 points

    Hi Kedar,

    Thanks for your answer. I am testing my design currently with BQ25306 in it. Could you tell me how I will be able to terminate the charging when the battery is charged completely, even when the USB is still plugged-in?

    I see that when the battery voltage level reach above the charge regulation voltage the STAT pin is changing its state all the time until when I remove the USB cable as input source.

  • 0 in reply to JA G
    TI__Mastermind 24075 points

    Hi,

       Termination is automatically done by the charger IC even with adapter connected. What you are observing is a common issue in setups with too much impedance between the real battery and where the BAT pin is connected. The impedance causes a voltage drop (Rimp*Iterm) and when charge is terminated and Iterm removed, causes the voltage to drop more than the recharge threshold, thereby triggering a new charge cycle. This causes you to observe cyclical STAT flashing between charge and termination. The BAT pin is a sense pin and should be tied as close to the real battery as possible.