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BQ25504: BQ25504 EVM

Part Number: BQ25504

Hi,
I would like to use the BQ25504 with a 0.7V input voltage and increase it to 3v without using a battery on the Bat pin.
Initially I’m using BQ25504 EVM to do some tests with a 0.9V power supply and 0.2A maximum current (so Pin_max = 0.2W).
I just connected the power supply with a series resistor at the input and the load at the output (VSTOR pin).
My problem is: when I put a 3kohms load on the VSTOR pin, the output voltage doesn't stay steady at 3.1V, it drops to 2.4V (with 10kohms it works). I don't know why because the power supply can provide more power.

Here is a screenshot from a oscilloscope. The red one is the Vbat and the blue one is the VSTOR with 3kohms.

Do you know where this problem is coming from?

Thank you!

  • HI Lucas,

    If the charger works with 3kohm but not 10kohm, then your load is too much for your input power.  With a series resistor between the input power supply, limited to 0.2A, and VIN_DC, the input voltage at VIN_DC will droop as current is pulled.  If it droops below the MPP point (80% of 0.9V?), the boost converter turns off briefly until the input capacitor recharges above that point.  It is difficult to compute the input power for a discontinuous mode boost converter being limited by MPP.   

    If VBAT is following VSTOR then the undervoltage protection PFET between VSTOR and VBAT is turning off.  What is the VBAT_UV setting?  Can you repeat above plot but with VIN_DC and LBOOST as well?

    Regards,

    Jeff

  • Hi Jeff,
    Thank you for the answer.
    Here is the screenshort with the input voltage VIN_DC (blue, 0.9V in this case without a series resistor), the output voltage VSTOR (yellow) when I have 3kOhm, Vbat (red) and LBOOST (green).


    With 10kohms it works because the current is so small I think, so the component keep the voltage.

    When I put 3kohms (so I hope to have 3.1v/3k=1.16mA) after some seconds the output tension drops and it has the behavior of the figure.
    I didn't understand why this happens. The  VBAT_UV is setting in 2.2V. All the other parameters are in accordance with the evaluation board.

    For this test I hoped understand how the component work because in my real application I'll use a power supply with capacitive behavior and I will need up to 50mA and constant voltage at the output.
    Do you think this is possible?