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Step-down converters rev up efficiency

Posted: 03 Dec 2007 ?? ?Print Version ?Bookmark and Share

Keywords:power efficiency? step-down converter? MCU? DSPs? power MOSFET?

More often than not, power efficiency drives the market success or failure of today's consumer electronics products. By dictating the length of a device's operating life between charges, power efficiency plays a pivotal role in users' perceptions of a product's quality.

Few ICs play a larger role in the power budget of a portable consumer product than the selection of a high-efficiency step-down converter. Used to power the core MCU, I/O and other subsystems, step-down (or buck) converters reduce the average 3.6V Li-ion source to the 2.5V or 1.1V levels most controllers or peripherals require.

Given the importance of battery life to the end products' market success, high efficiency across the entire load range has become an increasingly crucial characteristic in today's step-down converters. In most MP3 players, digital cameras, GPS systems and other portable devices, the processors assign priorities to system tasks and optimize management of the power resources. Typically, the core processor operates in full operational, standby and sleep modes.

Although a system MCU running streaming video may require full load, in sleep mode it may need only enough voltage to refresh the memory. Since the processor in the average handheld device typically sits in sleep or standby mode for large stretches of time, high efficiency across the entire load range can play a major factor in maximizing battery life.

Fresh solutions
To help achieve that goal, step-down converter manufacturers are using various schemes to maximize the efficiency of their devices in light load. For example, Micrel Inc. introduced the MIC2238, a 2.5MHz dual 800mA synchronous step-down regulator using a proprietary Trickle Mode switching architecture to maximize light-load efficiency. Housed in a 3mm x 3mm micro lead frame package, the device delivers 94 percent efficiency at 5mA and a peak efficiency reaching up to 96 percent.

Designers can either run the device in the variable-frequency Trickle Mode (for maximum light-load efficiency with an automatic switchover into full load) or use a constant-frequency option.

Given the need for the core system processor in many of these applications to shift its modes of operation constantly, designers must also take a close look at a step-down converter's transient response. It is important to know how the converter will be used and how well the device stays in regulation as it moves from one load condition to another.

Fairchild Semiconductor Corp., for example, has introduced the FAN5350, a 600mA DC/DC synchronous buck converter featuring a fast, 20mV transient response. The device marries that performance with a very low, 16?A quiescent current and low-voltage ripple to power DSPs, application processors and I/O processors in a range of applications. The converter comes in either a 1mm x 1.37mm wafer-level chip-scale package or a 3mm x 3mm micro lead frame package.

Power-semiconductor manufacturers also compete to deliver the highest-performance converter in the smallest footprint.

Maxim Integrated Products Inc. recently added the MAX8640Y and MAX8640Z, which the company claims are the smallest 500mA step-down converters in an SC70 package. The devices use a proprietary switching scheme to minimize external components' size. They also draw only 24?A of quiescent current.

In July, Texas Instruments Inc. announced a family of step-down converters that come in a tiny, six-pin, 2mm x 2mm, 0.8mm-high small-outline nonleaded package. The TSP62240, TSP62260 and TSP62290 supply 300mA, 600mA and 1A of output current, respectively, while maintaining efficiency levels of up to 95 percent. The TI devices operate at 15?A quiescent current.

Figure 1: Micrel's MIC2238 step-down converter uses a unique Trickle Mode switching architecture to maximize light-load efficiency.

Switching frequency plays an important role in the footprint equation as well. Typically, the buck converter uses pulse-width modulation (PWM) of a switching frequency to control the output of an internal power MOSFET. The device requires an external Schottky rectifier diode with an external inductor and output capacitors to produce a regulated DC output. The converter's switching frequency determines the physical size of the external inductor and capacitors. Converters using a higher frequency can use external components that are smaller in both size and component value.

By operating at a relatively high 2.25MHz, for example, the LTC3560 from Linear Technology Corp. can be used with ceramic capacitors and inductors measuring less than 1mm in height. The step-down converter supplies 800mA from a six-pin ThinSOT package.

Similarly, Advanced Analogic Technologies Inc. introduced a family of step-down converters over the last year, beginning with the AAT1149, that come in compact SC70JW packages and operate with small, low-profile 0603 inductors. The converters operate at switching frequencies up to 3MHz.

Enpirion Inc. takes an entirely different approach to integration. It combines the step-down converter with an inductor to reduce parts count and system footprint. The EP53x2Q combines a PWM controller, MOSFET switches, compensation network and a power inductor in a single 5mm x 4mm x 1.1mm QFN package. The new family of devices supports loads of 500mA, 600mA and 800mA. A 5MHz switching frequency minimizes the size of filter components and enables a commensurate improvement in transient performance, according to the manufacturer.

Microdrive support
Designers in the portable-system arena are increasingly using microdrives in today's products. Although drive manufacturers have made enormous strides in power efficiency, more often than not the microdrive consumes a mammoth percentage of a portable system's power budget.

Unlike the core processor, however, a hard drive typically operates in only two operating conditions: on and off. Generally, that mode of operation demands a power source that can meet the specific voltage and current requirements of the drive as efficiently as possible. Fast transient response is also important. Because starting up a drive requires a momentary surge in load as the motor begins to turn, designers need a converter capable of reacting quickly to meet that requirement without affecting the rest of the system.

Standard buck vs. buck-boost
To meet those requirements, a designer can opt to use either a standard buck converter, with a low dropout voltage, or a buck-boost converter. Each approach offers distinct advantages.

A standard buck converter can go into dropout or a 100 percent duty cycle condition and offer very high efficiency by tracking the battery voltage. It will also operate with smaller external components. A buck-boost converter offers an output voltage that can be either greater than or less than the input voltage and that is adjustable based on the duty cycle of the switching transistor.

Figure 2: Available in a wafer-level chip-scale package, Fairchild Semiconductor's FAN5350 buck converter minimizes system footprint while delivering up to 94 percent efficiency.

Although a buck-boost typically requires slightly larger external components and delivers slightly lower efficiency than a conventional buck converter, by boosting the supply voltage it allows the system battery to operate at a lower operating voltage, extending the operational life of the system.

An excellent example is the LM3668 from National Semiconductor Corp. Optimized for operation from a Li-ion source between 2.5V and 5.5V, the device offers seamless transitioning between buck and boost modes and supports output voltages of 2.8-3.3V at 1A, with better than 90 percent efficiency.

Given the propensity for today's portable systems to offer wireless connectivity, power-semiconductor manufacturers are increasingly combining a high-efficiency step-down converter for the core processor with one or more low-dropout regulators to power noise-sensitive RF functions.

For UMTS, Edge, CDMA2000 and wideband CDMA handsets, Micrel's MIC2807 combines a 2MHz buck converter with two LDOs in a 2.5mm x 2.8mm micro lead frame package. The output voltage of the buck converter can be adjusted with an external DAC to maximize talk time on the cellphone. In PWM mode, the converter supplies a 600mA output. In bypass mode, it delivers up to 1A. The two LDOs supply 200mA and 30mA, respectively.

To power application processors and associated RF circuitry in portable media players, MP3 players and DMB or DVB-H mobile TV-enabled cellphones, the NCP1526 from ON Semiconductor Corp. integrates a synchronous buck converter capable of delivering 400mA at 1.2V with an LDO providing 150mA of current at 2.8V. The device is housed in an ultrathin, 0.55mm microDFN package, minimizing its footprint and profile.

- John Mayer
EE Times

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