Circuit variations produce negative voltages

Clinton Jensen, National Semiconductor Corp, Santa Clara, CA -- EDN, 9/1/2000

Basic switched-capacitor converters generally provide one simple conversion. They commonly double, usually invert, and sometimes halve a positive input voltage. Because they are not regulated converters and do not have stability problems, you can easily configure them to also do some negative conversions. However, given that they are unregulated, it is a good idea to use a regulated input voltage to obtain a predictable output. All these converters suffer from some voltage change on the output as a result of loading, but this voltage drop is often acceptable. The potential space and cost savings are often worth the trade-off if the circuit fits the application needs. Figure 1 and Figure 2 present two application circuits. The first is useful in systems that require multiple negative voltages. You can set up a basic switched-capacitor converter capable of inversion to halve a negative voltage. Figure 1 shows the schematic of the circuit, using an LM2664 as an example. The circuit is capable of the full rated load (40 mA, in this case). The operating-voltage range is –3.6 to –11V. The circuit can be useful for creating multiple bias voltages for op amps, power amplifiers, or displays.

The second application is useful when a negative voltage is present and you need an equal but inverted (positive) voltage. In Figure 2, an LM2665 produces a 5V output from a –5V input. The topology shown works with any switched-capacitor circuit designed for the doubling function. This circuit can also supply the full rated load current and works with an input-voltage range of –2.5 to –5.5V. The output resistance of the circuit equals that of the basic positive doubler. The circuit can be useful for op-amp biasing when only a negative voltage is present. Another benefit is that, if you have a loosely regulated negative supply for an op amp, the positive supply tracks the negative supply and keeps the output dc-biased near to or at ground. Another possible application is in systems using two-cell lithium-ion batteries or four alkaline batteries and needing ±5V supplies. (DI #2580)