Recently, DC/DC switching regulators operated in the range of 20kHz to several MHz. While it has become desirable to have faster switching frequencies, engineers fear the dramatic trade-offs in efficiency. Recently, the industry’s first 8MHz buck regulator with LDO light load mode was introduced. The solution allows a reduction in inductor volume by greater than 90 percent when compared to a 1MHz operation, while at the same time maintaining excellent overall conversion efficiency.
DC-to-DC switching regulators are an efficient means of translating between two DC voltages. For example, they can translate the variable discharge voltage of a Li-Ion battery, which ranges from 4.2V down to 2.7V, to supply a micro processor with a steady voltage of 1.8V, and still achieve conversion efficiencies greater 90 percent. This is of great value, particularly in battery-operated circuits.
Less efficient sister parts to switching regulators that can reduce and regulate an input voltage are linear regulators. The efficiency of linear regulator is defined as the ratio of output voltage over input voltage. For example, generating an output voltage of 1.8V from a 4.2V input results in a mere 43% conversion efficiency.
In spite of their lower efficiency, cell phones today still use anywhere from 10 to 25 of these less efficient linear regulators to perform the DC-to-DC translation. This is because unlike switching regulators, they don’t need an often bulky inductor to enable the voltage conversion (refer to Figure1).
Figure 1. Typical Circuit of an Adjustable Output DC-to-DC Switching and Linear Regulator
Until recently, switching regulators were operating in the frequency range from 20 kHz to several MHz. The inductor size of a switching regulator is inversely proportional to its operating frequency. Buck regulators operating at 500kHz commonly use inductors in the range of 10μH, 1Mhz buck regulators commonly use 4.7μH inductors while 2Mhz buck regulators commonly use 2.2μH inductors. A low profile 4.7μH inductor that is capable of continuously delivering 500mA typically comes with a square footprint of 3-4 mm in each direction and a profile height of 1.8mm or more. Adding to the footprint of typical switching regulator IC, which, for example, comes in 3 x 3 x 0.85mm package, one can see that this becomes a much larger solution when compared to a 500mA linear regulator such as Micrel’s tiny 2 x 2 x 0.85mm MIC5319 (which offers real estate less than a fifth the typical solution size).
Responding to the demand for smaller inductors in space-sensitive applications, leading analog IC suppliers have come out with buck regulators featuring faster frequencies. TI announced a 3MHz device in 2004, followed by Linear Technology and Maxim who both announced 4MHz capable devices last year. At 3-4MHz, the inductor value can shrink down to 1μH thereby reducing the inductor size to around 3 x 3 x 1.2mm. Still, finding small surface inductors with inductance values of 1μH or greater lower than 1mm height was impossible at the time this article was written.
In 2006, Micrel broke the 1μH barrier by offering the industry’s first 8MHz buck regulator. This solution features a tiny 0.47μH chip inductor that measures a mere 1.25x2x0.55 mm; a 95 percent reduction in inductor volume compared a 1MHz solution (refer to Figure 2). As can be seen from the illustration, this is also the first time the inductor becomes smaller than the switching regulator IC package itself.
Figure 2. Solution Size and Height Comparison with Increasing Switching Frequency
Until this solution, DC-to-DC switching regulators with speeds of 8MHz were unthinkable because power supply designers were concerned about the associated reduction in efficiency. Efficiency losses in a switching regulator are comprised of conduction losses and switching losses. Conduction losses are a function of supplied current and the power FET resistance, which remain independent of frequency. Switching losses include gate drive and body diode conduction which dissipate a fixed amount of energy each time the power supply completes a switching cycle. Hence, the more times per second the power supply switches, the more power is dissipated by these frequency-dependent loss components.
To avoid having the performance of an 8MHz buck regulator dominated by frequency dependent switching losses, Micrel addressed efficiency problem by using a high speed proprietary BiCMOS technology that allows for reduced switching rise and fall times at higher frequencies. Figure 3 shows the efficiency comparison of a 2MHz buck regulator (MIC2205) with an 8MHz buck regulator (MIC2285), which have the identical core design with the main difference being the operating frequency. As can be seen, the higher operating frequency doesn’t result in a significant droop in efficiency.
Figure 3. Efficiency Difference Between a 2MHz and an 8MHz Buck Regulator Over Load