Hello everybody, thanks to your suggestions i've solved the issue of the buck converter which didn't work, i used a system of 2 Nmos as a trial, waiting for buying a Pmos as a pwm controlled switch. But now i have a more big issue than before. I'm trying to charge a Li Ion battery, which has 3.5 Volts on ìts terminals. The issue is that, when i apply 5 volts at V1, the battery has also 5 V on its terminals and no current flows across the sensing resistor. It looks as if battery negative terminal is wired internally to ground. I tried my circuit without battery and it works, the issue is present only with battery attached. What could it be? May be the battery defective? I post here a schematic of my circuit. The battery i'm trying to charge is this: http://www.digikey.it/product-detail/it/TL-5276/W/439-1027-ND/512529

 

The manufacter of the battery does not give details but it is a Lithium type. Most Lithium cells have an absolute maximum charging voltage of only 4.20V and your 5V might cause it to catch on fire or trigger a protection circuit in it if it has one.
You might have burned out some wiring inside the battery with your very high current charger circuit.

Its datasheet shows that its capacity and maximum allowed discharge current are very low. Then its maximum allowed charging current will also be very low but this spec is missing in the datasheet.

Here is its English datasheet: http://www.tadiranbat.com/pdf.php?id=TL-5276/W

 

Yes i know, now i'll try with another battery to see if the previous was damaged, beeing careful to not power it with a voltage higher than 4.2. I'll let you know if it goes well. Thank you

 

Yes i know, now i'll try with another battery to see if the previous was damaged, beeing careful to not power it with a voltage higher than 4.2.

You also must limit the charging current to (who knows how much or how little?) or the replacement battery also might burn out or catch on fire.