Original question:
I set the fly's rating at 650 mV, 1 mA.
When driving the BQ25570evm board,
VSTOR is effectively charged about 4.2V, leaving CS mode.
The VOUT output also shows 1.8V output normally.
But when I connect the supercapacitor I want to charge to the VOUT,
The supercapacitor is not charging,
PS rating changes to CC mode and voltage drops to half level.
I want to know why this situation is happening now, and I want to know how to solve it.
Sanghyun,
I found an EVM! I attached two 10F in series to VOUT as shown below and then attached to a Keithley source meter current limited to 1mA and 0.650mV with MPP=80%. As the two DMM readings below show (2 seconds apart), the cap is charging but very slowly. Like any dc/dc converter, the input power times the efficiency must be greater than the output power. If the supercap were ideal, with no DC leakage current, it would charge quickly but it has a DC leakage current that the buck converter must provide. As shown in the scope plot, the buck converter only turns on briefly, sending a few current pulses to the capacitor, until the VSTOR capacitor discharges down to the buck converter input UVLO. Then the cycle repeats. If you connect VOUT_EN to VBAT_OK and set VBAT_OK higher, the buck converter will have more time to stay on and send more pulses. I will leave my setup overnight to see it my input power is enough to overcome the DC leakage current.
Regards,
Jeff
Hi Sanghyun,
I confirmed this morning that the supercap charged to 1.8V.
Regards,
Jeff
