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BQ25505: BQ25505 VDC_IN voltage level

Kaveh Mohammad
Intellectual 460 points
Part Number: BQ25505

I have a solar cell with VOC = 3V (@ sunlight) approximately when not connected to BQ25505. 

I have connected my solar cell, battery and other components to BQ25505 chip according to the datasheet. 

When my solar cell is connected to BQ25505, I measure VIN_DC=0.5 (@ sunlight) approximately with a multimeter. Is this normal? 

I expected VIN_DC to be about 2.4V because BQ25505 should set it at MPP point.

Please let me know, 

Thanks

  • 0
    TI__Guru*** 137290 points

    Hi Kaveh,

    If VSTOR<1.9V, the charger operates in cold start and clamps the input voltage.  Once VSTOR>1.9V or a partially charger battery >2.1V is attached at VBAT, the charger works in boost mode and regulates the input voltage to the MPP setpoint.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 460 points

    ok. I have connected BQ25505 to VBAT = 4.1V. 

    But the VDC_IN voltage is somewhere about 0.3V to 0.5V and I see it can increase to 0.7V-0.8V depending on the light I emit on the solar cell. I believe the clamping should be 0.7V? 

    If my VBAT=4.1V I think the chip should quickly charge VSTOR to VBAT level and the chip should not be in cold start. So we dont expect clamping to happen?

    Maybe another issue in my design.

  • 0 in reply to Kaveh Mohammad
    TI__Guru*** 137290 points

    Hi Kaveh,

    When you attached the 4.1V battery to BAT pin did VSTOR jump up to VBAT too?  The undervoltage protection protection FET will not turn on if there is a voltage > 100mV already present at VSTOR or VBAT and/or a voltage at VIN_DC.  You can also just short VSTOR to VBAT to quickly test or permanently if the don't need undervoltage/overdischarge protection for your battery.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 460 points

    Thanks Jeff. Yes when I connect the battery VBAT=4.1V to the BQ25505 chip I can see now that VIN_DC jumps to ~1.5V (@ sunlight) which is higher than what I measured before. 

    Another question: In the datasheet the thereshold condition for cold start is VIN_DC = 600mV. Is this solar cell VOC or VMPP at VIN_DC input when the system is functional? 

  • 0 in reply to Kaveh Mohammad
    TI__Guru*** 137290 points

    HI Kaveh,

    The solar cell must be able to output some current at 0.6V for cold start to function.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 460 points

    I dont understand what is the proper way to get this chip start working. This is the order I am applying inputs:

    I have a solar cell with VOC=3V but since there is no light it outputs about 0V. 

    1- I check VSTOR = 0V.

    2- I connect solar cell with VIN = 0

    3- I connect battery with VBAT = 3.5V

    After all of these, I see VSTOR = 0.6V and not set at VBAT level. 

    What could be wrong here?  

  • +1 in reply to Kaveh Mohammad
    TI__Guru*** 137290 points

    Hi Kaveh,

    What is the ISC for the panel?  Quick test is to short VSTOR=VBAT to see if the boost charging and MPP functions are working.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Guru*** 137290 points

    Kaveh,

    Can I see your schematic?

    Regards,

    Jeff  

  • 0 in reply to Jeff F
    Intellectual 460 points

    ISC is about 50mA. When I short VSTOR to VBAT, it temporarily jumps to VBAT=VSTOR=3.5V but then strangely starts to drift slowly to lower values. When I remove VBAT to VSTOR short, I can see VSTOR drifts much faster toward 0.6V and lower. 

    I am suspecting that my chip has a grounding issue on the PCB central pad that is somehow connecting a pin or so to the ground that cause VSTOR energy leak to the ground. 

  • 0 in reply to Jeff F
    Intellectual 460 points

    My VBAT_OK=2.9V, VBAT_OK_HYST=3.1V and VBAT_OV=4.1V.

  • +1 in reply to Kaveh Mohammad
    TI__Guru*** 137290 points

    Hi Kaveh,

    I agree that your VSTOR appears to have a soft short to ground.

    Regards,

    Jeff