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.
When IC2 shuts down, its output turns off. However, when IC1 shuts down, the whole chip assumes a low-power state and draws under 1 µA. L1, D2, C1, C2, R2, and R3 are part of the 250-mW switching regulator. Diodes D3 and D4 wire-OR the power connection to IC2, and C3 improves the linear regulator's load regulation.
When active, IC1 provides an overall efficiency of 76% for load currents between 5 and 50 mA. Vin may range from 8 to 17 V, but the value of R1 should be set for a trickle charge of no more than 10% of the battery capacity.
The 8-V minimum voltage figure for Vin prevents unnecessary cycling between the main and backup power sources.