The drop also wastes power with an amount equal to load current times forward voltage dissipated in D1. This scheme addresses the disadvantages of Methods 1 and 2. Click here for larger image. Setting U1's output at 5.1 V ensures that it will be higher than that of U2 ensuring that U1 will power the U2 linear regulator when ac power is applied. It provides a regulated 3.3 V with 200-mA output current, and uninterrupted switchover to a battery backup consisting of two AAA alkaline cells.
Linear regulators are easy to use and require fewer components than switching regulators.
Power wasted in the regulator is considerable when the input is much higher than the output. Even if efficiency is not important the heat generated by a linear regulator is generally unacceptable in portable systems. The circuit includes a lithium backup battery for powering the 3.3 V RAMs when neither ac source nor other battery source is present but this backup battery is never allowed to power the 3.3 V MAIN output. A boost circuit that generates -20 V at 20 mA for biasing LCDs can take power from either the main battery or the ac source. Click here for larger image.