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Does anyone know how to use a QC 3.0 quick charger to control 12vdc instead of mains

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the one i have my eye on is the FAN6290QF
heres a schematic from its datasheet

from the looks of it, its throttling the output of a flyback transformer, i doubt its regulating through PWM using just that Cout for maintaining a clean output, i think its more likely that the flyback is being controlled based on how much it can output, oddly though theres no interaction between the two controllers.

Anyway, i want to know how to control the FAN6290QF to take 12VDC and control a step down converter, or boost converter, or heck, even just switch between a 5v, 9v and 12v fixed power source, for the purpose of USB quick charging.
My end use is building a battery back quick charger, so, i wont have any mains to work with, not that i would ever consider using mains anyway.

At very least id like to understand how this controller achieves the regulated output? I cant see it stepping down a 12v source because in the event of a failure that would mean that the default output would be 12v, which would blow any non QC compliant device plugged into it.


it just occured to me that in this schematic, the flyback output is directly connected to the to the the USB output. How does that even work?
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oddly though theres no interaction between the two controllers.
Just a quick comment - I've not looked at the datasheets for these parts or fully absorbed what you're trying to do - but there is an optocoupler providing control from the 6290 to the 602. You may have overlooked it as the two sections are drawn separately.
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