Advanced hard drives clamp mobiles' power drain

The Apple iPod set a trend among portable handheld consumer devices: It was among the first battery-powered MP3 music players to use a microminiature hard drive. That was followed late last year in South Korea by the Samsung SPH-V5400, arguably the first camera phone to make use of a 1.5-Gbyte hard drive. Clearly there is a move toward replacing flash memory with hard-disk drives (HDDs) in portable devices to increase storage capacity in an effort to meet consumer demands for more songs, pictures and video. Hard drives do come with power consumption penalties, however. Now, a cell phone not only triples as a voice, data and pocket-media center, with surround sound, high-resolution video and still imaging-including autofocus and high-intensity flash; it also gets handed the problem of driving miniature motors for disk drives.

Fortunately, advances in power management techniques and components are combining with novel concepts in data sequencing and intelligence partitioning (between hard drives and applications processors) to greatly reduce the potential power drain of components once considered anathema to handsets.

Already complex

As it stands, the power management and delivery chain for camera phones is already sophisticated and complex, involving as many as 17 separate voltage regulators. The use of microminiature HDDs requires yet more voltage regulation. The parts must be capable of delivering a relatively large amount of current on short notice (in burst mode) and a smaller amount of current as the disk spins, and to come offline and go to sleep (certainly, to consume no current of their own) when the drive needs to stop.

The key to extended battery life with feature-laden handhelds lies in the voltage regulator topologies and the efficiencies associated with each. Higher- efficiency switch-mode regulators have largely replaced linear low-dropout regulators (LDOs) in a wide variety of low-voltage circuits, but they can push switching noise into sensitive circuits. Additionally, switchers have had a tendency to lose efficiency under light current loads. Clearly, it takes a kind of genius to get the power management device to deliver just the voltage and current needed but otherwise stay out of the way.

Examination of the microminiature drives available for handhelds shows a relatively small number of choices. One is manufactured by Toshiba Corp.; another is made by Hitachi Ltd. (with technology inherited from IBM). A startup called Cornice Inc. (Longmont, Colo.) claims to have shipped 1 million 1-inch MP3 "storage elements" to companies like Philips and Samsung. Like other HDDs, this one has an IDE interface and stores up to 3 Gbytes.

The Toshiba unit used in the original Apple iPod was a 30-Gbyte drive, with a 1.8-inch platter and 15-millisecond seek time. A teardown report by David Carey, president of Portelligent Inc. (Austin, Texas), suggests that the drive is buffered by 32 Mbytes of SDRAM. That is about 20 minutes' worth of skip-free music, Carey estimates. (A full description of the iPod, including a close analysis of the power management section, is available at the Portelligent Web site, www.teardown.com).

Behind the interesting specs, disk-based music players like the iPod Mini all face one big power management issue: responding efficiently to the current spikes demanded by the disk drive as it starts to spin. In the power consumption profile assembled by Portelligent, there is roughly a 5-second period in which power consumption will rise to 1.5 watts as a song is being accessed for play from the disk. During the actual playback, the power consumption drops to 0.4 or 0.2 W, depending on whether the user keeps the backlight on while the song is playing.

Power will go up and down within a 0.5-W range, depending on the user's application of the iPod's click wheel and display. But the drive accesses and seeks clearly generate the largest power consumption spikes, according to Portelligent's report.

To provide a controlled 3-volt supply rail from a 3.6-V lithium ion battery source, manufacturers once recommended LDOs to ensure low noise; then, others perfected step-down switching regulators using a "buck" topology. However, it remained uncertain, with early-generation cell phones, how much battery capacity was left after the battery voltage dropped below 3 or 3.3 V in sustained use. Continued use would cause the measured battery voltage to drop below 3 V, and some estimates suggested as much as 10 percent of the battery's capacity could still be left at 3 V.

Some manufacturers argued that a step-up topology-a "boost" regulator-might be appropriate to lift the supply rail voltage back up to 3.3 V once the battery voltage itself drops to 3 or 2.8 V.

While buck and boost regulators can offer high conversion efficiency, the type of regulator that starts out as a buck and then converts to a boost-the "buck-boost"-might be too inefficient to sensibly extract a little extra current from a lithium ion battery that's nearly depleted. A handset might just sacrifice a small amount of talk time and turn the cell phone off.

Indeed, European cell phone makers still have a tendency to turn off the phone when the battery voltage drops below 3 or 3.1 V, said Tony Armstrong, portable-product-marketing manager with Linear Technology Corp. (Milpitas, Calif.). But South Korean cell phone makers will let the voltage go down as far as 2.8 or 2.7 V, he said, although he cautioned that "you don't want to pull so much current at lower voltages that you damage the battery."

But the choice to shut off the handheld at 3 V is no longer an option for designers of disk-based handheld music players. While the internal logic of the player-even the peripherals and I/O circuits-now runs on far lower voltages, the HDD still needs 3.3 V. Thus, voltage regulator manufacturers like Linear Technology have come up with new buck-boost topologies-even devices that will supply the burst-mode currents typically demanded by a disk drive seek operation.

One example is Linear's LTC3442, which delivers up to 1.2 amps at 3.3 V and offers up to 95 percent efficiency. The device takes its input voltage from a lithium ion battery and maintains a 3.3-V output, even as the input degrades from 4.2 to 2.5 V. The switching frequency is adjustable from 300 kHz to 2 MHz, and the LTC3442 has the necessary MOSFET drivers integrated on-chip.

Having acquired additional sophistication with battery fuel gauging, Texas Instruments Inc. is now reconsidering the actual power available in a nearly depleted Li-ion battery. "Portable power management is no more a function of transient response than auto fuel economy is, strictly, a function of spark plug ignition," said Dave Heacock, general manager of Texas Instruments' portable power management business unit. "You have to consider the entire power delivery chain." New battery chemistries are allowing more energy to be extracted at low voltages, Heacock said.

On the horizon, though, are 4.2- and 4.4-V Li-ion batteries that offer as much as 30 percent more capacity. "Sanyo is off to the races," said LTC's Armstrong.

Such higher-voltage sources might eliminate the need for boost conversion, allowing a hard drive to be powered entirely by a stepdown regulator. Micrel Inc. (San Jose, Calif.) has given considerable thought to the issue of driving microminiature HDDs and is proposing two solutions. One is a family of synchronous buck converters (the MIC2202 and MIC2205) that transform themselves from switchers to LDOs under light current loads. The other is under development and powers HDD-based devices through their USB interface. Although USB limits current to 500 mA or below, said Thurston Awalt, product-marketing manager, "a kick-start feature on USB controllers offers a 125-millisecond window to start the drive in motion [before current limiting sets in]." The 40- to 80-mA spin cycle could then be accommodated through the USB interface, he said.

"It takes three to four minutes to read a song out to the flash," confirmed iPod fan Bernie Weir, engineering manager for ON Semiconductor (Phoenix). The bursty nature of the drive activity, combined with the 3.3-V requirement, may necessitate a buck-boost, he agreed. The closest that ON Semi comes to this are the NCP1510A and NCP1511 buck converters, designed to deliver up to 500 mA at 3.3 V from a lithium ion battery.

Getting inside the drive

Much of what is known about driving disk drive motors might be extrapolated from experience in driving shutters and autofocus mechanisms in digital still cameras, suggested Nazzareno (Reno) Rossetti, director of IC strategy at Fairchild Semiconductor Corp. (San Jose). Fairchild offers a series of miniature H-bridge drivers with bipolar transistors. Designers of music players can similarly come up with efficient schemes for driving motors inside disk drives-if indeed the drive mechanisms are accessible.

How efficiently a miniature HDD can be powered is somewhat contingent on how much access designers have to the innards of the drive. The simple process of finding and assembling digital music fragments could be streamlined-and use less power-if the music were parked in contiguous tracks and sectors on the drive. "You can save energy by avoiding track changes," said Dan Fisher, vice president for system-on-chip R&D at STMicro-electronics' data storage group.

Fisher has looked for object-oriented software to help control the head-cylinder interaction in the drive, particularly the head boundary and the track boundary. For MP3 music players, there is a trade-off between "dumb" and "intelligent" storage. A true consumer operating system would lobby for "dumb storage," Fisher said, in order to get greater control over the head-cylinder interface.

It turns out the same ARM9 device that is used as an applications processor in the Apple iPod, as well as in the nearly ubiquitous MP3 system-on-chip codecs from SigmaTel Inc. (Austin, Texas), is also the hard-disk controller (HDC) processor that runs the seek-and-play functions on a variety of HDDs.

"Managing power-what's turned on and what isn't-would be much easier if the applications processor and the HDC were in direct communication with each other," said Neil Robinson, stateside marketing director for the ARM Consortium (Los Gatos, Calif.). "The ATA interface requires a lot of pins and sucks a lot of power." While moving HDC control functions toward the applications processor gives the music player folks more control, it is not a power saver, Robinson believes. It simplifies the drive but complicates the applications-processing tasks. "It should be the other way around," in his view.

But the drive makers are not likely to allow it, Robinson said. A dumb drive offers more versatility. "The Apples of this world will still need to pick and choose."