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 src="/data/attachment/portal/201007/ET12539201007191111341.gif">
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 src="/data/attachment/portal/201007/ET12539201007191111342.gif">
Figure 2. The output voltage error in Figure 1 increases with load current.


Figure 3. Efficiency also increases with load current in Figure 1.

This switched-capacitor circuit provides better regulation than that of a simple voltage divider, and better efficiency than that of a simple combination of divider and op-amp buffer. Its main drawback is the increase in output noise with load (see Table 1). VIN is restricted (by the IC specifications) to a maximum of 5.5V, which is the maximum voltage allowed between pins 2 and 4 or between pins 1 and 4.

RLOAD
(Ω)INPUT CURRENT
(µA)VOUT
ERROR
(mV)OUTPUT
CURRENT
(µA)RIPPLE
(mVP-P)EFFICIENCY
(%)∞36.5————10M36.5—0.25—0.3410M37.7—2.5—3.32100k48.90.125—25.5610k1561.4250~180.041k124013.52490~599.72470263028.55260~898.8310011,410126.923,700~3098.71
A similar idea appeared in the August 1, 1997 issue of EDN.