AN1199| Application Note
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POWER-SUPPLY CIRCUITS
Keywords: Voltage-Inverter IC Forms High-Efficiency Rail Splitter
Aug 01, 1997
APPLICATION NOTE 1199
Voltage-Inverter IC Forms High-Efficiency Rail Splitter
A switched-capacitor voltage inverter configured as a "rail splitter" (IC1 in Figure 1) provides a bipolar (dual-rail) local power supply that is useful in single-rail systems featuring one or more dual-rail ICs. Moreover, the tiny SOT-23 package and associated components require very little board area.
For Larger Image Figure 1. This compact and efficient charge-pump circuit implements a local dual-rail supply for single-rail systems. After power is applied, the flying capacitor (C2) connects alternately across the storage capacitors C3/C4 and C5/C6. This action equalizes the voltages on those capacitors and draws current from VIN or VOUT as required to maintain VOUT VIN. If the loads across VIN-VOUT and VOUT-0V are equal, the IC sits in a quiescent state and draws about 36uA. To keep VOUT at the mid-rail level, the flying capacitor needs only to supply the difference current caused by unbalanced loads. Efficiency is degraded by the IC's quiescent current for load currents below 100uA, but above 1mA the efficiency is greater than 90%--an excellent feature for low-power or battery-powered applications. (Voltage error and efficiency vary with the load current, as shown in Figures 2 and 3.)
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