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BQ24171: Behavior when input current is smaller than charge current

Hisao Uchikoshi
Genius 14860 points
Part Number: BQ24171
Other Parts Discussed in Thread: BQ25896, BQ25601

Hi Experts,

We are testing BQ24171RGY as below conditions.

- Charge Voltage : 4.2V

- Charge current : 700mA

- Battery : Li-ion

- input current : 600mA

In this case, the supply voltage is going down then the charger stops to charge the current. After that, the supply voltage is going up and charging is resumed. This is repeated.

Basically, we are assuming to use the higher input current than charge current.

Does above behavior matter or not?

Regards,

Uchikoshi

  • 0
    TI__Guru**** 166246 points

    Hi Uchikoshi-san,

    The input power times efficiency must be greater than output power in order for you to reach full charge current.  So, VIN*(INLIM-ISYS)*eff < VBATREG*ICHG.  If VIN is close to VBATREG then INLIM needs to be close to ICHG. If VIN is higher than VBATREG, then INLIM can be lower than ICHG.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Genius 14860 points

    Hi Jeff,

    Thank you for your answer.

    Please let me ask my question again. For the usual use-case,  VIN*(INLIM-ISYS)*eff is greater than VBATREG*ICHG so it is not problem. Our application is targeted to use USB power of end-customer and we are considering VIN*(INLIM-ISYS)*eff becomes less than VBATREG*ICHG due to poor USB power source.

    For this case test, we used limited power source like ~600mA. In this test, the power supply voltage is dropped and charging is stopped then the power supply voltage is going up and charging is restarted. Then the power supply voltage is dropped and charging is stopped. These actions are repeated every 1ms.

    I want to know if we should care this action or not.

    Regards,

    Uchikoshi

  • 0 in reply to Hisao Uchikoshi
    TI__Guru**** 166246 points

    Hi Uchikoshi,

    The charger IC will not be damaged but this is not ideal. 

    Can you set the input current limit even lower so that it does not crash the input source?  If not, can you move to a 1S charger that has VINDPM, which lowers charge current to prevent crash, like BQ25896 or BQ25601?

    Regards,

    Jeff