I want to make a USB cell phone charger. I know USB operates in 5 volts which means i need to drop down voltage if i have battery potential greater than 5 Volts. I have 12 volts sealed lead acid battery (SLA) of capacity 9AH. I need to drop down the voltage for which i am using LM2596 buck regulator ( Switching power supply) and it can deliver 3 amperes of current. But my cell phone charger(came with the product) is rated for 2A. So i was wondering how can i limit the current so that it doesnot blow my Cell phone. Thanks
The cell phone charger spec is a maximum. I.e. the maximum current that can safely be drawn from the charger is 2A. If the buck converter is rated for 3 amps, then you have more than enough current available to charge your phone. Your phone will draw the current it needs.
Some phones (especially Apple devices) are finicky about the data lines. You can't just leave them floating (with nothing connected). Adafruit has a good discussion about reverse engineering the voltages that are required on the USB data lines for charging Apple devices.
Provided you correctly set the voltage by the feedback network, you cannot blow your phone by current rating. The phone will draw precisely as much power as it needs.
Also FYI, you should connect the data wires together (better with a 100 ohm resistor) to inform the phone that this is not a computer, but a charger. I used to have a phone (Sony Ericsson j108i) which refused to charge without proper USB detection or at least this workaround.