August 1, 1997

Inverter forms high-efficiency rail-splitter

Terry Millward, Maxim Integrated Products, Reading, UK

In Figure 1, a switched-capacitor voltage inverter, IC₁, configured as a "rail-splitter," provides a bipolar, dual-rail power supply. The circuit is useful in single-supply systems that include one or more dual-supply ICs. The SOT-23 package and associated components require little board area. After you apply power, the flying capacitor, C₂, connects alternately across the storage capacitors, C₃/C₄ and C₅/C₆. This operation equalizes the voltages on those capacitors and draws current from V_IN or V_OUT as required to maintain V_OUT=1/2V_IN.

If the loads across V_IN­V_OUT and V_OUT­0V are equal, the IC remains in a quiescent state and draws approximately 36 µA. To keep V_OUT at the midrail level, the flying capacitor need only supply the difference current arising from unbalanced loads. The IC's quiescent current dominates the efficiency for load currents less than 100 µA, but for currents greater than 1 mA, the efficiency is greater than 90%--an important feature for low-power or battery-powered systems. The voltage error and efficiency vary with the load current (Figures 2 and 3).

This switched-capacitor circuit provides better regulation than that of a simple voltage divider and better efficiency than that of a simple combination of a divider and an op-amp buffer. The circuit's main drawback is the increase in output noise with load (Table 1). V_IN is limited to the maximum voltage allowed between pins 2 and 4 or between pins 1 and 4; that is, 5.5V. (DI #2054)

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