When a circuit requires reverse-voltage protection on the power input, the usual approach is a series diode (see the figure, a). But if the input voltage is low—say, two or three AA batteries—the 0.5-V loss of a Schottky diode may represent a significant loss in useful battery life. An FET can serve as a series diode with significantly less voltage drop (see the figure, b).
A dual IRF7342 FET was used (p-channel, with both sides wired in parallel) using a 3-V source, resulting in a voltage drop of only 100 mV into a 100-mA load. With a 4.5-V input, the loss is only 50 mV with a 100-mA load. If a low-side switch topology will work in the application, an n-channel FET can be employed (see the figure, c). A dual IRF7341 FET was used (n-channel, with both sides wired in parallel) with a 3-V source, resulting in a voltage drop of only 40 mV into a 100-mA load. A 4.5-V input results in a loss of only 25 mV with a 100-mA load.
If a "soft-start" feature is needed, it can be added with only one capacitor and one resistor (see the figure, d). The values indicated add about 100 ms of turn-on delay, as the FET goes from off through the linear mode to fully on.