Simple power supply fits into small spaces

Raymond Zheng, Linear Technology

EDN

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 –5 V output and an unregulated 10 V output from a 5 to 6 V 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.

Figure 1.	This circuit combines a regulated inverter and a voltage doubler.

From a 6 V input, the inductorless dc/dc inverter can deliver 100 mA at a regulated –5 V (±5%), and the voltage doubler can provide 50 mA at 10.5 V 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 = 5 V 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)].

Figure 2.	This graphic shows ROUT versus IOUT for the circuit in Figure 1.

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.	This curve shows efficiency versus either output current.

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.

Figure 4.	This curve shows output-voltage regulation versus output current.

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