5-V UPS Has Low Power Consumption

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 V_out, and V_in trickle charges the battery through D₁ and R₁. When V_in 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 V_in. IC₂ contains a low-battery detector circuit that senses V_in by means of R\[subscript\]6 and R₇. The detector output (pin 7) drives an inverter (Q₁), which in turn drives the shutdown inputs I_C 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, V_out.

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. V_in 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 V_in prevents unnecessary cycling between the main and backup power sources.