A USB-powered device is constrained to draw no more than 0.5A from the input supply. Efficiency is paramount in order to maximize the available power from the port, so a synchronous buck regulator with internal switches is the best choice. In addition, this application can be implemented with all-ceramic capacitors for lowest profile and high availability. This MAX1692 circuit delivers +3.18V at 320mA from +5.25V input with 90.7% efficiency.
Figure 1.
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