USB phone charger

A portable phone charger for a Siemens mobile phone. Since I don`t use that phone anymore and recent phones can usually be charged with a USB connector I built a new version of the charger. This time with a proper PCB, a USB connector and a higher current capability. Just as the previous charger it is basically just a step-up conve
USB phone charger - schematic

rter largely built according to the datasheet. In this case I used the TPS61032 because of its high efficiency, low minimum input voltage (1. 8V) and high internal switch current (4A). I wanted the converter to be able to deliver about 1A at 5V, which requires an average input current of 2. 5A at 2V input but the peak current can be much higher. The converter is synchronous, which eliminates the need for an external Schottky diode (and its relatively large power loss). This version also includes a low battery led which turns on when the input voltage is around 1. 95V. At this voltage the batteries will soon drop out. The step-up converter can run down to 1. 8V but usually when the two batteries are at this voltage there is not much energy left and they will soon drop even lower than 1. 8V. Batteries with high internal resistance may come very close to 1. 95V even when not empty, so make sure you use good quality rechargeable batteries with low internal resistance. Because the low battery output of the converter can only sink up to 100 µA (active low) I added a small P-channel mosfet to be able to drive a LED from it. I also added a little switch that determines what`s connected to the D+ and D- data pins on the USB connector. It switches between two common configurations for USB chargers. One is around 2. 0V and 2. 75V on D+ and D- respectively, which is what Apple`s iPhone wall charger does (as well as the Belkin ones). An...

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