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BQ25756E: BQ25756

HAMID MORADI
Prodigy 10 points
Part Number: BQ25756E
Other Parts Discussed in Thread: BQ25798

Tool/software:

Hi Dear , 

We are incorporating this BQ25756E component into our new design, but the charging phases do not align with what is described in the datasheet

When the battery voltage is below the VBATLOWN or VBAT_SHORT , the charger starts trickle charging. Once the battery reaches VBATLOWN,
it jumps from trickle to fast charging.  we couldn't see the pre-charge phase

all the register value are default and we did not change any of them  , 

We checked the both CHARGE_STATE register and the input current, both indicating that the charger is not entering the pre-charge state 

we have 8 x 3.65v battery in parallelly

  • 0
    TI__Mastermind 31305 points

    Hello Hamid,

    Thanks for working with this.

    This is expected behavior. The pre-charge state activates when the battery is below VBAT_LOWV (66.7% of VFB_REG) and above VBAT_SHORT (3V). This means that VBATLOWV is at 2.4V and the charger will enter into the fast charge mode once the charger leaves the trickle charge state.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hi Ethan ,

    From charging  graph ,

    Vbat < Vbat_short   >>>>> charge current is  I bat_short

     Vbat_short < Vbat < Vbatlown  >>>>   charge current is IPRECHG  

     Can you clarify the value for Vbat_short  and I bat_short  in charging graph ?

    If the battery voltage is 1.5V and the charger is enabled, what do you expect the charger to do?

    thanks 

  • 0 in reply to HAMID MORADI
    TI__Mastermind 31305 points

    Hello Hamid,

    VBAT_SHORT is 3V. IBAT_SHORT is 150mA. It's the same in the charging graph.

    HAMID MORADI said:
    If the battery voltage is 1.5V and the charger is enabled, what do you expect the charger to do?

    The device will be in trickle charge here.

    Let me know if you have any other questions.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hi Ethan ,

    This means that for a one-cell battery, the charger bypasses the pre-charge state and immediately switches from trickle charge to constant current (CC) charging.

    If this is true, the VBAT_LOWV and IPRECHG  register cannot be used at all . As soon as the Vbat voltage reaches 3V, regardless of the VBAT_LOWV register setting, the charger will start constant current (CC) charging.

     

    Vbat=3V

    VFB pin= (3V x 180K )/ (180K + 249K) = 1.25V 

    VBAT_LOWV reg = 71.4%  >> 1.09V

     

    As we are using the LiFePO  with Vbat low =2V  so we can’t use this charger for our design ,any suggestion ?

    thanks

  • 0 in reply to HAMID MORADI
    TI__Mastermind 31305 points

    Hello Hamid,

    Got it. To help us get a suggestion for you, can you tell me the max possible input voltage and desired charge is for this application?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hi Ethan,

    We are using Solar panel as input voltage which is 12V to 25V , MPPT feature is desired

    Battery is 8 x LiFePO battery with below spec :

    Charge Current  : 300mA

    Charge voltage : 3.65V

    Discharge End Voltage: 2V

     

    Do you think we can’t use BQ25756E ?

     

    Regards

  • 0 in reply to HAMID MORADI
    TI__Intellectual 2010 points

    Hi Hamid,

    I would recommend to use BQ25798.  It has MPPT, supports 3.6V Vbat and 300mA Ichg.

    Regards,

    Charles Harthan