One alternative is a dual-comparator/reference IC, which monitors the battery and wall-adapter voltages with respect to its internal reference voltage (Figure 1). The open-drain output of comparator B (with pull-up to 3.3V) provides a low-battery warning in the form of a low-to-high transition when battery voltage drops to 3.6V. The open-drain output of comparator A (with pull-up to 9V) flags low wall-cube voltage in the same way, with a warning threshold of 3.9V.

Figure 1.The MOSFET in this power supply (Q1) saves power and extends battery life by substituting for the diode otherwise required.

Comparator A also controls the PMOS switch Q1, which replaces the OR-connection diode in a conventional circuit. When wall power is removed, Q1 turns on and provides a low-resistance path from battery to regulator.

A related idea appeared in the 5/1/96 issue of Electronic Design.