The demand for negative-voltage power supplies is increasing with the popularity of applications for portable devices. It can be expensive and relatively complicated to generate a negative supply from a positive input source, especially when the design requires both positive and negative outputs. Figure 1 shows a simple and cost-effective solution that combines a voltage inverter and a voltage doubler in a single charge-pump circuit. It produces a regulated –5V output and an unregulated 10V output from a 5 to 6V input. The circuit requires only five small, ceramic, surface-mount capacitors and two diodes in addition to the charge-pump IC in an SOT-23 package.

From a 6V input, the inductorless dc/dc inverter can deliver 100 mA at a regulated –5V (±5%), and the voltage doubler can provide 50 mA at 10.5V with ±7% variation. The inverter's output-voltage regulation follows the relationship (V_IN–5)>(I_OUT×R_OUT); you can determine the values of R_OUT and I_OUT for V_IN=5V from the graph in Figure 2. (The R_OUT and I_OUT values for other V_IN values are available in the LTC1983's data sheet.) If the variables don't meet this inequality condition, the part runs in open-loop mode and acts as a low-output-impedance inverter in which the output voltage is V_OUT1= –[V_IN–(I_OUT×R_OUT)]. You can define the output voltage of the voltage doubler as V_OUT2=2V_IN–2V_D, where V_D is the forward voltage drop across the diodes. Figure 3 shows the efficiency of the circuit, which exceeds 81% and peaks at approximately 85%. Figure 4 shows the inverter's output-voltage regulation versus inverter output current. The IC includes short-circuit and thermal protection.