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BQ25756: BQ25756

Shony Sinay
Prodigy 40 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25750

Tool/software:

Hi all,

It is not clear from the BQ25756 datasheet what will be the output voltage of the charger in case that the input voltage is different then the battery voltage (higher or lower).

Is that depends to the battery regulation circuit configuration (FB, FBG)?.

One may conclude from Figure 9-1 that in 12V battery case the battery regulation configuration forces output voltage to be the maximum battery voltage - 16.8V.. What does it means? If this will be the charger output voltage independent to the battery voltage, there could be situation that the battery voltage will be only 8V and when disconnecting the external power the charger output voltage will abruptly jump from 16.8V to 8V - bad for the load.

Thanks.

  • +1
    TI__Mastermind 20570 points

    Hi Shony,

    The output as we refer in a battery charger is the battery. The output voltage of the BQ25756 is the battery regulation voltage. It is not a separate voltage rail. This is set by the resistor divider to the FB pin and also the REG0x00_Charge_Voltage_Limit register. 

    You can refer to section 9.2.1.2.2 detailed setup example. Regardless the input voltage is, as long as it is within the operation range, the device will charge the battery to the configured battery regulation voltage. The charger does not have a independent fixed output voltage from the battery. 

    Figure 9-1 shows a 4S Li-ion battery case which the battery voltage typically ranges from 12V (discharged) to 16.8V (fully charged). The 16.8V is the full charge voltage which is what should be set as the battery regulation voltage in charger design. The battery charger will charge this 4S battery pack up to 16.8V. Hope this clarifies. 

  • 0 in reply to Jing Zou
    Prodigy 40 points

    Thank you Jing Z0u,

    so, if i use 24V LiIon battery pack, I need to configure the resistors to adapt the maximum voltage of 33V, Isn't it?

  • 0 in reply to Shony Sinay
    TI__Mastermind 20570 points

    Hi Shony, 

    You will need to confirm with the battery's spec. The battery spec should specific the full charge voltage which should be used set the regulation voltage on the charger. For example, if you have a 8-cell battery and the full charge voltage specified per cell is 4.2V, then the battery regulation voltage on the charger should be set to 33.6V.

  • 0 in reply to Jing Zou
    Prodigy 40 points

    Yes. This is indeed the case.

    thanks.

  • 0 in reply to Jing Zou
    Prodigy 40 points

    BTW, I have another question (I already asked that in the past but I didn't get a clear answer):

    Seems that the datasheet has wrong statement when it shows in their drawings the point that the system should be connected - The output point should be as in the below picture, otherwise the battery current shunt resistor can not actually represent the battery current. Am I right?

  • 0 in reply to Shony Sinay
    TI__Mastermind 20570 points

    Hi Shony,

    If you need to charge the battery while powering the system, then you will need a charger with power path management. In this case, BQ25750 is probably a better part comparing to BQ25756. The drawing in the BQ25756 datasheet is valid as many applications does not power system and charging at the same time. 

  • 0 in reply to Jing Zou
    Prodigy 40 points

    Hi,

    I indeed need to power the system while charging, but unfortunately the BQ25750 solution is more complex.

    As far as I understand the main difference between the two is in that the BQ25750 supports direct path to power the system from the external source and so it is more efficient in this case, but if less efficiency is OK for me than i can use the BQ25756 in a way i suggested.

    Actually, I think that the BQ25750/BQ25756 EVB is suggesting so.

    do you agree?

  • 0 in reply to Shony Sinay
    TI__Mastermind 20570 points

    Hi Shony,

    Like you said in your previous post, if you have situations where powering system and charging at the same time, non-power path part such as BQ25756 cannot differentiate the current flow to the battery or to the system. Thus, charge control such as termination will not be accurate. We do not support the configuration you had with BQ25756 where connect the system before the charge current sense resistor. 

    Power path has advantage where:

    1. System can be on instant on with depleted voltage upon a valid input source plugging in

    2. Accurate charging current control

    3. Easy termination and better for batter life time

    So if the above is not critical to your application, then you can use BQ25756. 

  • +1 in reply to Jing Zou
    Prodigy 40 points

    Thank you very much Jing Zou