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BQ25570: Inrush current limiting to a VOUT connected supercapacitor.

Gregor Kalan
Prodigy 80 points
Part Number: BQ25570

Hello,

I am building a circuit to power a microprocessor as a data logger of temperature using solar for the day and battery for the night. The problem i have is that the microprocessor has a about 20s time, when it consumes +80mA with some bigger spikes of 200mA that are solved by a +47uF cap. After the time periode the microprocessor goes to deep-sleep for 1h and consumes about 0.9mA in that time. Then the cycle repeats. The microprocessor operates with a voltage range of 3.0 - 3.6V, if the voltage dips below that the processor resets and that i do not wont as it could potentially cause it to loop.

I do not know the max current that the BQ25570 can provide for that time frame of 20s at the required voltage of 3.0 - 3.6V.

So i wanted to use a supercapacitor to provide a the power for that time frame but i am worried that the inrush current of the cold start will be so high that it will fry the BQ25570. I dont know the current regulation capability of the BQ25570. 

Solve this problem I was looking at current limiting diodes but do to no solid documentation and a max current of 24mA, that is not usable.

I could use a Ultralow Power, Low Input Voltage, Current-Limited Load Switch but that adds additional load on the circuit. The battery is capable to power it (2600mAh). I didnt do the power calculations jet if the device is self sustaining. 

The solar panel i plan to use is a 4V /0.64W. I know that the power is above the BQ25570 max power but there will be resistance of a wire from the solar panel do to lenght. 

So my questions are: If there is a better way to drive the microprocessor and what it is? What could I use to limit the current if I use a supercapasitor? Is there a better way to in the self sustaining scenario?

Regards, 

Gregor

  • 0
    TI__Intellectual 1640 points
    Hello Gregor,
    The maximum output current is upto 110mA, and the cycle by cycle current limit of the charger is 230mA , also 0.64W for the input of BQ25570 is exceeding the operating conditions.
    I would recommend you to limit the peak input power to 0.51W
    Also, are you connecting the microprocessor to VSTOR Pin?
  • 0 in reply to Upal Patel
    Prodigy 80 points
    Hello Upal,

    Thank you for the reply.
    The solar cell will be changed to a 0.45W cell (I am limited by the cell manufacturers).

    If I understand the VSTOR pin it is the output of the bust charger 1.8 V (typical) only if the solar cell is giving power? I was not planning to connect it to the microprocessor as the logical high is 2.5V min and there are now more ADC available. There could be a refresh with a different microprocessor and an ADC to monitor the VSTOR.

    Best Regards,

    Gregor
  • 0 in reply to Gregor Kalan
    TI__Intellectual 1640 points
    Hello Gregor,
    The VSTOR pin is the output of the boost charger, when VSTOR>1.8V (VSTOR_CHGEN) the charger operates in boost mode.
    The PFET between VSTOR and VBAT closes when the VSTOR voltage reached the programmed VBAT_UV (1.95V)+VBAT_UV_HYST (15mV) voltage.
  • 0 in reply to Upal Patel
    Prodigy 80 points
    Hello Upal,

    Thank you for the reply.
    Ok, so the VSTOR voltage ranges from ~2V to 4.2V, depending on the:
    - mode of the BQ25570
    - voltage of the battery (rages from 3V - 4.2V)
    So it cant provide a constant voltage is this correct and would require external regulator to keep the voltage at 3.3V . Is this correct?

    Best Regards,

    Gregor
  • 0 in reply to Gregor Kalan
    TI__Intellectual 1640 points
    Hello Gregor,
    The VSTOR and VBAT max voltage depends on VBAT_OV, the VBAT_OV is programmable with the external resistors.
    You could program the VBAT_OV to 3.3V, but the downfall of that is you won't be able to charge your battery beyond 3.3V.
    Yes, you'll require an external regulator to regulate 3.3V.
  • 0 in reply to Upal Patel
    Prodigy 80 points
    Hello Upal,

    Thank you for the reply. You have been very helpful.

    Final question.

    Can you connect two BQ25570 to the same battery but leave every thing else separate. The design has now updated to use a display. That would put way to much load on a single BQ25570. The power lanes will not be connected except the battery would be shared.

    Using two BQ25570 would solve the power and add more charging time with a two separate solar panels (not connected), at different angle. The OV would be set at the same voltage (some small error due to resistor accuracy), other parameters may be different.

    They would use the same battery due to the space restriction.

    Is there going to be some kind of weird frequency or something like that would not let both of the BQ25570 charge at the same time? Is it even a good idea to connect two of them like that?

    Best Regards,

    Gregor
  • 0 in reply to Gregor Kalan
    TI__Intellectual 1640 points
    Hello Gregor,
    Thank you, I'm glad I've been of help to you.

    The issues I see while connected 2 BQ25570 to 1 battery are:
    1. You might have slight different input from the solar panel, do you think you can provide accurate VOC and ISC from the solar panel to both the inputs? Because if it's different, then you'll be charging single battery with two different voltages or currents, which is not at all recommended.

    2. When you connect 2 BQ25570 to a single battery, you might sink current into the other BQ25570 IC's VBAT pin, which could also create complications on the internal loops.