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BQ25895EVM-664: Why actual current trigger IINDPM lower than Input current limit setting

Part Number: BQ25895EVM-664
Other Parts Discussed in Thread: BQ25895


I have question about IINDPM detection

Which current be detect for IIDPM? is it current at VBUS or current at SW?
According to datasheet IIN_DPM_acc, it seems detect current from SW

I also tested the BQ25895 EVM and set condition as below
To observe IINPDM be trigger when ISYS increase.
VINDPM:4.4V (absolutely)
IINDPM:900mA by input current limit setting
Uncheck ILIM pin
Floating BAT
Just add load in SYS (0~900mA)

I found the IINDPM be trigger at VSYS is 4.19@730mA, VBUS is 5.0V@688mA as below screen capture
My point is why IINDPM be trigger is not at VBUS@900mA

  • Hi Leon, 

    On this device input current detection for IINDPM is done across the internal RBFET (Q1). The section in the datasheet on IIN_DPM_acc shows that the current detected at the SW node will not be higher than the set input current limit. 

    I recreated your test on a BQ25895evm and found IINDPM triggers at VSYS = 4.20V @840mA and VBUS = 5V @ 760mA. The IIN_DPM_acc states the sw node is typically 825 mA with a max of 900mA when IINLIM is set to 900mA. Therefore you can expect your device to trigger IINDPM at a current below VBUS @900mA.



  • Hi Garrett:

    Thanks for verification and answer.

    One more question, if you continue to increase VSYS loading, you can found the VBUS current will be locked at 825mA around.
    It seems IIN_DPM_acc typical value, What is behavior when IINDPM be trigger?
    Does it trigger IINDPM by detect current at SW node and lock current at VBUS? but both IINDPM_acc value?

  • Hi Leon, 

    As mentioned above input current detection is done at the RBFET (Q1). When IINDPM is triggered this FET is used to limit the current at VBUS into the device. Current is pulled from SW for the IINDPM_ACC value in the electric characteristics table, but the SW node is not where current is detected to trigger IINDPM.



  • Hu Garrett:

    A little confusion, as you mention "IINDPM triggers at VSYS = 4.20V @840mA and VBUS = 5V @ 760mA. The IIN_DPM_acc states the sw node is typically 825 mA with a max of 900mA when IINLIM is set to 900mA"
    Do you think IINDPM be trigger at 840mA is reasonable, since the IINDPM_acc typical is 825mA at SW node when ILIM is set 900mA? If yes, it mean IINDPM be trigger according to IIDPM_acc, is it?

    So let check it again, if when ILIM set 900mA
    1.What typical current is pulled from SW node that IINDPM will be trigger?
    2.What typical current will be limited at VBUS?
    3.Above current refer to which specification in datasheet?

  • Hi Leon, 

    Do you think IINDPM be trigger at 840mA is reasonable

    IINDPM is a protection to ensure the input power source is not being overloaded. Yes IINDPM triggering when VSYS at 840 mA is reasonable, as that was the observation of my test, but understand that IINDPM is controlled by current sensing internal to the device not based on current observed at the output to SYS. Internal control ensures that the current at the SW node is within the range specified by IINDPM_ACC on the datasheet.

    1) The typical current pulled from SW node when IINDPM is triggered is IIN_DPM_ACC in the datasheet. Min 750 mA, max 900 mA and typical 825 mA. 

    2) The typical current at VBUS will be no greater than the current observed at the SW node during IINDPM. 

    3) The datasheet does not list a specification for the typical current at VBUS when in IN_DPM. As you pointed out the specification on IIN_DPM specifies that the current observed at SW node is below the ILIM setting of 900 mA. 

    Best Regards,