Feb 4, 2013

USB Charger for Mobile Phones

All of us worry about our mobile phone draining out of charge while travelling. In such a situation, this SMPS-based USB charger powered by just two AA-size batteries will come handy. The advantage of SMPS is high efficiency and low cost. Of course, most mobile phones can also be charged through the standard USB port available in desktop and laptop computers.

A unit load is 100 mA in USB 2.0, and 150 mA in USB 3.0. An individual device may draw a maximum of 5 units load (500 mA) from USB 2.0 and 6 units (900 mA) from USB 3.0.

 This USB charger for mobile phones operates off a 2.4V or 3V battery. It is designed around an average input-current-controlled buck-boost DC-DC converter LTC3127 (IC1) and some discrete components. The circuit converts 3V or 2.4V input into 5V output.

Pins SW1 (pin 2) and SW2 (pin 11) are used to connect the inductor. SHDN (pin 4) shuts down the output of LTC3127 when low. Pin 5 is used to change the mode of operation between PWM and burst modes. If Mode pin is low the mode will be PWM; when it is high the  mode will change to burst mode. PROG (pin 6) is used to set the average input current limit threshold. Vc (pin 9) is used to connect the compensation components, which impart stability to this regulator.

The values of R1 and R2 decide the output voltage level as follows:
VOUT = 1.195×1 + R2R1

You can select appropriate values of R1 and R2 to get 5V output (VOUT).

Most mobile phones have a USB male connector for charging, so a female connector is required for the charger side. Assemble the circuit on a small PCB and enclose in a suitable cabinet. Use a USB female connector and solder the connections as shown in the figure. The table gives the pin details of a USB connector. For powering the circuit, use an AA-size battery pack having either two 1.5V alkaline batteries or two 1.2V (Ni-Cd or Li-ion) batteries.

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