5V power supply teams low-dropout regulator, charge pump

Auxiliary power of 3.3V is replacing the 5V auxiliary power that "silver boxes" supply in most computer systems, but some circuits still require a 5V supply. Such systems impose the messy task of creating a central 5V auxiliary supply from the 3.3V auxiliary supply and then routing 5V power throughout the motherboard. An alternative exists, however, for systems in which only a few ICs need 5V: Employ inexpensive charge pumps as low-power 3.3V-to-5V converters and place them directly at the 5V loads. Regulated charge pumps do this job, but they are uncommon, and they often command a premium price. You can build a regulated charge pump by combining an unregulated charge pump with a low-dropout regulator that reduces the voltage to 5V. Unfortunately, that method requires a low-dropout regulator rated for at least 7V, because an unregulated charge pump can deliver 7V when its 3.3V input goes to the upper limit of tolerance. That fact eliminates the possibility of using the latest low-cost, low-dropout regulators, whose small geometry limits their maximum input to 6.5V.

You can reverse the order by placing the low-dropout regulator in front of the charge pump, thereby reducing the 3.3V to 2.5V before doubling it. That approach allows the use of a low-cost, low-voltage, low-dropout regulator, but the charge-pump output impedance then becomes an issue. A low-cost charge pump, such as the MAX-1683, operating with 1-µF capacitors exhibits a typical output impedance of 35Ω, making it unusable at currents above a few milliamps. The circuit of Figure 1 shows a better way to cascade the charge pump with a voltage regulator. The low-dropout regulator, IC₁, reduces the 3.3V input to a lower value, and the unregulated charge pump, IC₂, doubles that value to 5V. To cancel the voltage drop that charge-pump output impedance causes, the circuit feeds the 5V output back to the low-dropout regulator, which alters its output to maintain output regulation. The available headroom of at least 1V allows output currents to approximately 30 mA or even higher with larger capacitors. Although it requires two ICs instead of a single regulated charge pump, this approach can be cheaper because high-volume applications use unregulated charge pumps and low-current, low-voltage, low-dropout regulators. Furthermore, because the low-dropout regulator and charge pump are available in SOT-23 packages, the overall footprint of the circuit in Figure 1 is comparable to that of a regulated-charge-pump circuit.

Table 1 demonstrates the circuit's ability to maintain output-voltage regulation and deliver currents as high as 30 mA; the input, output, and flying capacitors are all 1 µF. Similarly, Table 2 shows the regulation for output currents to 45 mA; all capacitors are 3 µF. As you can see, load current does not affect efficiency, which is approximately equal to the output voltage divided by twice the input voltage. Capacitor values affect the ripple voltage and available output current but have little effect on efficiency.