Designers and consumers want to maximize battery life in portable and handheld applications—to avoid discarding batteries that contain useful life, and to minimize battery replacements. Small battery-powered devices with wall-adapter plug-ins must switch automatically between the battery and the wall source, and do this efficiently. In many designs, such switching is accomplished with a diode-OR connection.
For a small one- to three-cell battery pack, the voltage drop for a standard diode (0.6 to 0.7 V) is a large percentage of the battery's terminal voltage. Using a Schottky diode (0.3- to 0.5-V drop) improves matters somewhat. But a FET switch reduces the drop to less than 0.1 V.
The circuit of Figure 1 switches between an external supply (a wall plug) and a battery pack consisting of two or three AA cells. The design extends useful battery life by minimizing loss in the FET switchover element (Q1). It also de-bounces the external supply. The FET shown was selected for its low RDS(ON) and low VGS, which is specified down to 1.8 V. Thus, the FET can respond to a nearly discharged battery pack of two AA cells (0.9 V each).
Microprocessor-supervisory circuit U1 acts as a wall-source detector and debouncer. It monitors the wall supply, and its built-in delay ensures that it will switch from battery power to the wall supply only when the wall supply is stable and at or above U1's trip voltage.
The battery will be back-driven (charged) during this delay period of typically 185 ms. Note the effect on load voltage when switching from the battery to the wall supply (Fig. 2), and vice versa (Fig. 3).
The push-pull, active-low output of U1 drives Q1's gate directly, without external components. If U1's time-out delay is too long, consider the pin-compatible MAX6801 (SOT23 package) or MAX6381 (SC70 package), which offer delay options of 1 ms, 20 ms, and higher. Another pin-compatible option is the MAX6375 voltage detector (SC-70 package). It provides no time-out delay, but incurs minimal back-drive effects on the battery.
Note that Q1 is reverse-connected with its drain to the battery and its source to the load. This allows its internal body diode to provide the initial current path to the load. At the same time, it blocks the wall supply from uncontrolled charging (back-driving) of the AA cells when Q1 is turned off.