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BQ25504: BQ25504 VSTOR charging time calculation

Part Number: BQ25504

Hi,

I'm new to the EHM units and BQ25504 based capacitor charging.

I'm trying to figure out how to calculate the charging time for a capacitor connected to the VSTOR of the BQ25504.

The VIN of the BQ25504 is connected to a power supply unit (for testing), 3mW power (1V, 1mA). The load is a MSP430.

The BQ25504 charges the VSTOR capacitor from 2.4V to 2.8V, but I would like to know the formula for calculating the charging time from V1 to V2, where V1 > 2.4V (i.e., after partial charge) and V2 = 2.8V.
Do we have to tweak the conventional RC circuit formulas ?

Vc = Vi (1-e^(-t/RC))

where Vc = capacitor voltage, Vi = capacitor charging voltage.

what is R ? is it the equivalent resistance of the BQ25504 unit ? (how do I find that ?)

What is Vi in our case? (i.e., the BQ25504 output ? does the BQ25504 always produce a 3V output ?)

thanks

  • Hi Rosh,

    It is impossible to compute charge time because there are too many variables when computing instantaneous output power for a discontinuous mode (DCM) pulse frequency mode (PFM) boost converter.  From the power balance equation, efficiency = VOUT*IOUT/(VIN*IIN), VIN is your MPP point but IIN is the average of the triangular shape inductor current pulses that are spaced apart as a function of the input source's output impedance. See datasheet sheet figure 18 for an example. In addition, VOUT=VSTOR is slowly increasing up to VBAT_OV set point (3V in your case) and efficiency is slowly decreasing.  My advice is to take several measurements at different input source levels.

    Regards,

    Jeff