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BQ25504: Input voltage drops to 40mV when VSTOR = 1.77V.

Lucas Souza1
Prodigy 60 points
Part Number: BQ25504
Other Parts Discussed in Thread: BQ25505

Hi everyone,

I'm using BQ25504 in the evaluation board (ref EVM-674) and I have a problem when my output tension is 1.77V.

I'm using a power supply limited at 0.7V and 1.5mA so as you can see in the figure below the output (in green) and bat (in blue) voltages are increasing.

 

When the output voltage is 1.77 (as mencioned in the datesheet) the input drops to almost 40mV and the current is the maximum. In this case I'm not using MPPT and there is not load in the output. At Bat pin I have a 1mF capacitor. In next figure I put the behavior of the input when the output is 1.77V like I said.

Do you know why is this happen ?

I have another board with the BQ25505 and I don't have this problem. One difference I saw between these components is in the PFET between BAT and VSTOR pins.

Thank you,

Lucas.

  • 0
    TI__Guru*** 143880 points

    Hi Lucas,

    Both BQ25504 and BQ25505 have the same cold start circuit that turns on when VSTOR drops below ~1.9V.  Both have an internal PFET between BAT and VSTOR which opens, to prevent over discharging the battery, when VSTOR is low.  The BQ25505 threshold for the turn off is 2.1V while the BQ25504 threshold is set by external resistors connected to VBAT_UV.  In both cases, even when the PFET is off, to prevent battery discharge to VSTOR, the PFET's body diode still allows current flow from the VSTOR to BAT.

    I am not aware of significant difference in the cold start circuits between the two chargers. Is the loading different between the two boards?  What is the BQ25504 threshold?  

    Regards,

    Jeff  

  • 0 in reply to Jeff F
    Prodigy 60 points

    Hi Jeff, thank you for the quick answer. 

    The BQ25504 threshold is 2.2V.

    So, I agree with you. The loading is different between the two boards.

    When I'm using the BQ25505 with a power supply limited at 0.7V and 1.5mA, the input voltage drops to 0.35V during the charge and keep until the end of the charge at 3V, for example. But, with the BQ25504 when VSTOR is ~1.77 the input voltage drops to some milivolts (~40mV), as you can see in red, and it keep this value until the end of the charge.

    Best regards,

    Lucas.

  • 0 in reply to Lucas Souza1
    TI__Guru*** 143880 points

    Hi Lucas,

    I would have expected the BQ25504 to also clamp the input voltage to ~350mV during cold start.  If you increase the power supply current limit slightly but add a series resistor that gives 1.5mA at the same MPP voltage, does the BQ25504 behave the same?   Cold start operation is not tested during production so the clamp voltage can vary but I wouldn't expect it to vary down to 40mV.

    Regards,

    Jeff  

  • 0 in reply to Jeff F
    Prodigy 60 points

    Hi Jeff, 

    Yes, with a series resistor and a higher current limit I have the same behavior at the input of the DC-DC.

    As you said I wouldn't expect a very low level too at the input when cold start ends.

    With MPPT activated (in this case at ~80%V_in) I have the same as showed below:

    Regards,

    Lucas

  • +1 in reply to Lucas Souza1
    TI__Guru*** 143880 points

    HI Lucas,

    MPPT only works once VSTOR>1.9V.  The cold start circuit has its own input clamp.  I recommend changing to BQ25505.  It is the next generation of BQ25504.

    Regards,

    Jeff