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 input 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.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
