This uninterruptible power supply, which combines a switching regulator with a linear regulator, consumes little power (see the figure). When V\[subscript\]in is above 7.3 V, the switching regulator (IC1) shuts down, the linear regulator (IC2) generates Vout, and Vin trickle charges the battery through D1 and R1. When Vin drops below 7.3 V, IC2 shuts down and the step-up switching regulator takes over, generating 5 V at 50 mA from the 3.6-V battery (three series-AA NiCd cells).
A 9-V wall adapter supplies Vin. IC2 contains a low-battery detector circuit that senses Vin by means of R\[subscript\]6 and R7. The detector output (pin 7) drives an inverter (Q1), which in turn drives the shutdown inputs IC of IC1 and SHDN of IC2. These inputs have opposite-polarity active levels. The common feedback resistors, R2 and R3, enable both regulators to sense the output voltage, Vout.