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BQ25751: how the input current limitation operates

sun qiyun
Intellectual 280 points
Part Number: BQ25751

Tool/software:

Dear

Could you please help check my schematic about input limitation function?

In my design, I set the input current limitation is 2A. My understanding is when the current from SMPS_30V is over 2A, the input current would be fix at 2A, right?

I test the circuit by connecting a load to 30V_SYS and connect a 24V/4Ah lead acid battery to BAT_CHG. 

When test, I set the load as 2A, I find the battery charging is close, the load is operating normally. When I set the load as 2.5A, the load also operates normally.

Could you please help explain how the input current limitation work?

Thanks!

  • 0
    TI__Mastermind 32246 points

    Hello Sun,

    Please refer to section 7.3.5 Power Path Mangement in the Datasheet:

    To clarify the BQ25751 INDPM feature:

    1. The BQ25751 features Dynamic Power Management (DPM), which continuously monitors the input current and input voltage.
    2. When input source is over-loaded, either the current exceeds the input current limit, or the voltage falls below the input voltage limit. The BQ25751 will reduces the charge current until the input current falls below the input current limit and the input voltage rises above the input voltage limit.
    3. When the charge current is reduced to zero, but the input source is still overloaded ,the input voltage continues to drop.
    4. Once the input voltage drops below the ACUV limit (VACUV < VREF_ACUV), the charger stops switching and the device automatically transitions to Battery Only Mode so that the system is supported by the battery

    If the charge current is already reduced to zero, and the input supply is still overloaded. The system can continue to draw current until the VAC drops below ACUV.

     

    If the adapter is actually able to support a higher input current, hten what is set using the ILIM resistor. The system will continue to draw power until the adapter reaches the max limit. After it reaches the max limit,  VAC will start to drop, once it drops below ACUV the ACFETS will shutoff and the device will go into battery only mode. We have no way to limited the input current in this case.

    Best Regards,

    Christian