By configuring a charge-pump voltage doubler as a tripler, you can readily derive 15V from 5V (Figure 1). A 15V rail is useful for powering op amps, LCD-bias circuits, and other low-current applications. The connections shown configure the IC1 voltage doubler as a tripler. The no-load output voltage of the circuit is approximately 3VIN -2VD, where VD is the voltage drop across one diode. Use Schottky diodes as shown to minimize VD and its effect on output voltage. Because the circuit's finite output impedance causes the output voltage to drop with load current (Figure 2), a practical limit for load current is approximately 30mA.
Figure 1. This circuit (almost) triples the input voltage for low-current applications.
Figure 2. Finite output impedance causes a decline in voltage with increasing load current.
A similar version of this article appeared in the August 17, 2000 issue of EDN.
