Power Electronics

Multi-Megahertz Switching Gains Momentum

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Cellular phones and other portable applications continue to demand that voltage regulator designs fit into ever-smaller footprints. To satisfy this requirement with their switching regulators, chip developers have been pushing switching frequencies into the multi-megahertz range — up to and above 4 MHz. Such high-frequency switching enables the use of very small external passive components to complete their regulator designs.

Though raising the switching frequency is an obvious way to shrink voltage regulator designs, the technical challenge is not raising the frequency per se, but doing so while achieving reasonable efficiency. In the past, some chip developers have argued against switching at frequencies above 1 MHz, claiming that the higher switching losses make multi-megahertz switching inefficient and, therefore, impractical. However, recently developed ICs such as Micrel Semiconductor's (www.micrel.com) MIC2285 demonstrate that reasonable efficiency is possible even at 8 MHz.

In January, Micrel unveiled the MIC2285 500-mA synchronous buck regulator that switches at a fixed frequency of 8 MHz. A fully monolithic design with gate drivers and MOSFETs on chip, the MIC2285 also integrates an LDO. The switcher-LDO combination allows for two modes of operation. Over most of the load range, the chip can operate as a switcher to maximize efficiency. But at light loads — 60 mA and below — voltage regulation can be provided by the LDO, which offers lower noise operation (about 75 µVrms) than a switcher using pulse frequency modulation or burst mode.

Operating from a 2.7-V to 5.5-V input, the MIC2285 produces an adjustable output as low as 1 V. The regulator is packaged in a 10-pin, 3-mm × 3-mm MLF and works with a 0.47-µH inductor and a 10-µF output capacitor as shown in a typical application example (Fig. 1). Reducing the required inductance to 0.47 µH allows the use of a chip inductor in an 0805 case with a package height below 1 mm. As a result, the height of the complete design can be less than 1 mm.

When stepping down a single-cell Li-ion battery voltage to 1.8 V, at currents ranging from about 50 mA to 500 mA, the regulator's efficiency falls between 80% and 90% (Fig. 2). At lower load levels, the MIC2285 can be switched to LDO mode, which reduces IQ to 20 µA. According to Micrel, process technology enables the MIC2285's high-speed performance. The chip is produced in the company's 0.5-µm CMOS process, which lends itself to the use of a 5-V supply and makes it possible to adjust the regulator's feedback loop very quickly and minimize dead time to optimize efficiency.

The efficiency of the MIC2285 can be compared with Micrel's MIC2245 and MIC2205, which are synchronous buck regulators that switch at 4 MHz and 2 MHz, respectively. The MIC2245 and MIC2205 share the same core design as the MIC2245, are shown with similar application circuits and have been tested under similar conditions. Under the conditions shown in Fig. 2 (3.6-V nominal input and 1.8-V output), the efficiency of the 8-MHz regulator peaks at around 85%. Meanwhile, the 4-MHz regulator peaks around 90%, and the 2-MHz regulator at about 92%.

A comparison of the MIC2285, MIC2245 and MIC2205 application schematics reveals an interesting design characteristic. As the switching frequency goes from 2 MHz to 4 MHz to 8 MHz, the value of the recommended output capacitor does not decrease. Instead it rises from 2.2 µF to 4.7 µF to 10 µF. That's because the compensation scheme requires a fixed inductance (L) × capacitance (COUT) product. So, decreasing the inductor to 0.47 µH requires increasing the capacitance value.

However, this tradeoff allows for a smaller design. “The main aim of the MIC2285 was to reduce size,” says Ralf Muenster, director of marketing and applications of power products at Micrel. “The inductor is the single largest component preventing our customers from shrinking the design. Also, a 10-µF output capacitor in an 0603 case size is relatively inexpensive, since it is such a common ceramic capacitor.”

The first-released version of the MIC2285 is an adjustable part and uses external feedback resistors to set the output voltage. To keep the operating current low, high values are selected for these resistors. But with large-value feedback resistors, the parasitic capacitance to ground creates an additional pole. The 82-pF shown in Fig. 1 creates a feed-forward zero to offset the parasitic capacitance. But, Muenster notes that his company will also offer fixed-output models that integrate the resistors on chip and eliminate the need for the 82-pF capacitor.

Micrel's MIC2285 may be the first commercially available buck regulator IC to switch at 8 MHz. However, several buck regulators on the market switch at frequencies in the 3-MHz to 5-MHz range.

One example from Maxim Integrated Products (www.maxim-ic.com) is the MAX8560/61/62 family of 500-mA synchronous stepdown regulators, which switch at frequencies up to 4 MHz. Introduced in August 2003, these regulators are offered in 3-mm × 3-mm TFDNs as well as ThinSOT23s, and can operate with a 1-µH inductor and a 2.2-µF output capacitor (Fig. 3). Operating from a 2.7-V to 5.5-V input, the MAX856x produces an output adjustable down to 0.6 V.

The MAX856x data sheet reveals that efficiency at 1.8-V output with the 1-µH inductor peaks at around 85%, although the shape of the efficiency curve differs from that of the MIC2285. Also note that while the MIC2285 uses PWM control, the MAX8560 employs hysteretic control, regulating the output voltage from the inductor node and using voltage positioning rather than regulating to a fixed output voltage as the MIC2285 does.

Another high-frequency regulator is Linear Technology's (www.linear.com) LTC3411, a 1.25-A, synchronous stepdown dc-dc converter that switches at frequencies up to 4 MHz. Input voltage range is 2.63 V to 5.5 V, and output voltage is adjustable down to 0.8 V. This chip is offered in 3-mm × 3-mm DFNs (and MSOPs). The typical application circuit calls for a 2.2-µH inductor and a 22-µF output capacitor.

Linear also offers the LTC3409, a 600-mA buck regulator in a 3-mm × 3-mm DFN. This chip operates from a 1.6-V to 5.5-V input. The regulator operates at a fixed switching frequency of 1.7 MHz or 2.6 MHz, but can be synchronized to 3 MHz. The typical application circuit shows a 2.2-µH inductor and a 10-µF output capacitor.

Another device is Texas Instruments' (www.ti.com) TPS623xx, a 500-mA synchronous buck converter that switches at a fixed 3 MHz. Introduced in 2004, this regulator is offered in a 2-mm × 1-mm chipscale package as well as a 3-mm × 3-mm QFN. Designed to operate from a 2.7-V to 6-V input, the regulator generates a fixed or adjustable output down to 0.6 V. Like, the 4-MHz MAX8560, the TPS623xx can operate with a 1-µH inductor. The typical application circuit shows this inductance value with a 4.7-µF output capacitor.

Though a comparison of these various buck regulator ICs is beyond the scope of this article, it's clear that there are a variety of tradeoffs involved in the application of these parts. First, there is the previously mentioned tradeoff of switching frequency and efficiency. But even at the same switching frequency, efficiency varies among the different regulators discussed.

Since coil losses affect converter efficiency, the inductor choice should be taken into account when comparing the performance of the different regulators. Some vendors go to great lengths to document the different inductor options. And while the motivation for jumping to 8 MHz was to reduce the inductor size, the inductance values (1 µH to 2.2 µH) used with the 3-MHz and 4-MHz regulators are available in parts with 1-mm profiles. In a few cases, these inductors are even available with package heights below 1 mm.

Other factors to consider are the differences in overall design footprint with the recommended external components, transient response, quiescent current in the various modes of regulator operation and noise. And while the input- and output-voltage ranges may be similar, parts may differ according to minimum on-time or duty-cycle requirements, which may or may not be specified on the data sheets.

Output-voltage accuracy and output-voltage regulation are other performance factors that may differ from part to part. Functional differences may include the provision for frequency synchronization, power-saving modes and protection features. For links to the ICs mentioned here, see the online version of this article at www.powerelectronics.com.

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