Buck regulator forms high-power, inverting –5V supply

Configuring a step-down switching-converter IC as an inverter yields an efficient, high-power, –5V supply that can of deliver currents as high as 4.5A at the 12V input or 3.2A at the 5V input (Figure 1). Conventional inverting power supplies do their switching using a p-channel MOSFET (Figure 2). That configuration works well at lower currents, but has limited use above approximately 2A, depending on the input and output voltage levels and the MOSFET you use. If you compare a standard buck circuit with the circuit in Figure 1, you can see that the converter's "output" in Figure 1 connects to ground, and what used to be ground becomes the –5V output (Figure 3). Because the on-resistance of an n-channel MOSFET is lower than that of a comparably sized p-channel device, a power supply with n-channel MOSFETs usually provides more current at higher efficiency. To turn on, however, an n-channel device requires a gate voltage approximately 4V higher than the source voltage, which is usually the supply voltage.

The circuit in Figure 1 achieves high output current and high efficiency by reconfiguring a high-power buck converter, IC₁, as an inverter, thus exploiting an all-n-channel design. Efficiency is 90% with a 12.35V input, –5.02V output, and 4.7A load. The efficiency is 84% with a 4.56V input, –5.02V output, and 3.3A load. You can easily accommodate –5.2V applications by changing the values of R₁ and R₂. (Operation at –5.2V incurs a small penalty on maximum output current.) Input and output ripple voltages directly relate to the input and output capacitors' ESR (equivalent series resistance), so you should carefully select these capacitors. Circuit layout is also extremely important, as for all dc/dc converters. You may want to consider the MAX1636 evaluation kit from Maxim (www.maxim-ic.com). The kit includes a small pc board with optimized layout and all components necessary for operating the MAX1636. Because the board's layout is similar to the one required in Figure 1, the kit can serve as a rough layout guide for this Design Idea.

Check out our Best of Design Ideas section!