The Schottky diode may be critical too. Where I work we somehow got the wrong diode on a board with a similar circuit and the voltage regulator chip got very hot and the voltage regulation was unreliable.I suggest you carefully study the LM2575 datasheet.
The LM2575 is one of the least complicated (external component wise) of the switching regulators The thing you thought was a resistor is an inductor, careful picking the right one as a 3a inductor 300uh is not the same as a 300uh 0.3 watts lolHave you found the resistor value to enable high current charging?
I was thinking about using a voltage divider for that.
Also, i tried connecting pin 2 to pin 4 with 10k potentiometer.
Things are much more complicated than i expected
I recommend you use the charger that came with the phone !!
Another important topic that has been missed in this discussion is that the LM2575 is a voltage regulator - which is only part of the circuitry required to properly charge an iPhone battery. You also need a current regulator that changes it's output as the battery charges.You're headed down a path the will most likely end, best case, with the destruction of the battery. Worst case is that you destroy the phone as well in the process.
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