Switched-capacitor converter suits portable applications

Clinton Jensen, National Semiconductor, Santa Clara, CA -- 8/17/2000

Basic switched converters generally provide one simple voltage conversion. The most common application for these circuits are to double, invert, and sometimes halve the input voltage. Because they are unregulated and have no stability problems, you can configure them to perform multiple conversions as well. However, because they have no inherent regulation, it is a good idea to use a regulated voltage as the input source. Multiple converters exhibit some voltage change at the output as a function of loading (because of the output resistance), but this drop is often acceptable. The circuit in Figure 1 is a triple-output switched-capacitor circuit. You can configure the circuit using any switched-capacitor IC that's capable of inversion. This example uses the LM2664, but you can use larger ICs, such as the LM2661 or LM2663, to obtain higher current. The circuit in Figure 1 can simultaneously invert, double, and halve the input voltage over an input range of 1.8 to 5.5V. The combined current capability of the three outputs equals the maximum load current of the IC in either of the standard topologies: inverting or doubling (40 mA for the LM2664 and 200 mA for the LM2663).

The output resistance for any output is equal to or less than the typical output resistance of the basic doubler or inverter. This last statement holds true only if you realize that the doubling output is actually 2V_IN–2V_FD, where V_FD is the forward drop of the diodes used. The doubling output simply uses two diodes to make a discrete charge pump in conjunction with the switch that connects CAP+ to V_IN during one cycle and to ground during the next cycle. The extra diode drops may be a problem in some applications, but they would be insignificant if you connected a linear, low-dropout regulator to the doubled output. The halving output uses the concept of an unregulated inductive step-down switcher with a constant 50% duty cycle. At no load, the output from a 50%-duty-cycle pulse filtered through the inductor and output capacitor is half the peak voltage of the square wave. The same switch used for the doubling function produces the square wave. In the topology of Figure 1, you always obtain the inverting function that is inherent to the IC, and you can use or omit the other two outputs as needed. (DI #2578)

© 2009, Reed Business Information, a division of Reed Elsevier Inc. All Rights Reserved.