This is a simple circuit for automatic switchover between battery and USB port. This circuit uses more general step-up converter architecture. Compared with an analog switch or diode-OR circuitry , this circuit is simpler. Here is the circuit :
This circuit uses the MAX1795/MAX1796/MAX1797 step-up converter. To connect the output of a step-up converter and the USB power line, a low-dropout linear regulator (the MAX1792 LDO) is used. The voltage is applied into LDO output and the load due to the the battery voltage.
The LDO is protected by selecting an LDO with an adequate output-voltage rating or including a schottky diode (D1). The maximum LDO output that is allowed by diode is Vin + 0.3V. The output of LDO is set by R1-R2 resistor-divider. To reduce load current the R1-R2′s value should be as large as possible. The divider current is negligible in comparison with the load current because the output voltage of the converter is 3.3V. [Source: maxim-ic.com]
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