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Architecting miniature HDDs for battery saving

Posted: 01 Nov 2005 ?? ?Print Version ?Bookmark and Share

Keywords:ce? portable? device? storage? power?

The growing popularity of portable consumer electronics products that use high-capacity storage from mobile phones to A/V players and digital cameras is challenging the disk-drive industry to develop solutions that meet rapidly evolving demands.

Power consumption is especially critical in handheld CE devices. Makers expect new small-form-factor drives (1.0-inch and 0.85-inch) to store more music and deliver fast read/write speeds that support video playback and other functions, placing greater strain on power consumption.

Portable CE devices continue to expand into more advanced, storage-intensive applications, with mobile phones representing the most logical point of convergence, given their widespread use and growth rates. New mobile-phone designs incorporating media-player functionality and other capabilities are allocating roughly 10 percent of their power budget to storage needs that, depending on the battery technology, translate to an average power consumption of about 110mW.

Li-ion batteries are currently preferred in handheld applications. Disk-drive operating voltages have been reduced to 2.7V minimum to approach the knee of the battery's operating point for maximum playtime.

Figure 1 shows a typical current duty cycle in the transfer of streaming music files. In general, the host will put the drive in a low-power state or power-down mode when no data is being transferred. When data transfer is required, the drive will spin up, transfer a lot of data at the fast hdd transfer rate into the host buffer and then return to the low-power state. The host will then pull data from the buffer at the slower application rate. This allows a very low duty cycle for the drive, resulting in low average power consumption and thus longer battery life. However, the drive will also use power-management techniques even during data transfer. For example, the highest power will occur during a sector read, but when the host is busy, the drive will wait and go into a lower power state.

A buffer is used as a host speed-matching device. The size of the host buffer will determine power dissipation; more buffer means less power-hungry spin up/down drive cycles, resulting in lower average power dissipation. Trade-offs in buffer size, power and cost help host manufacturers select a buffer that meets power-dissipation goals of the application while minimizing cost. Typical music-player buffers are in the range of 8MB to 16MB, which is very small, with resultant price pressure driving HDD manufacturers to decrease power.

Two supplies needed

A typical HDD will need two voltage suppliesdigital core voltage for the logic and an analog voltage for motor drive, ADC and interface. The analog voltage traditionally comes only from the host and is regulated to a maximum value. The core voltage has traditionally been regulated from the host to 1.2V nominal using a linear regulator in the drive's motor controller IC. Today's drives use 3.3V of native power and linear regulators convert this power to the 1.2V/200mA required for storage ICs. A linear regulator draws 200mA of current and converts at an efficiency rate of only 36 percent.

One of the largest potential improvements in power savings for SFF drives can be achieved by replacing the motor controllers' linear regulator with a switching regulator for core voltage generation. While these devices also convert 3.3V to 1.2V, they only draw about 90mA of current for an efficiency rate of 80 percent. The noise typically associated with switching regulators can be cost-effectively managed to maintain a level of signal integrity and the difference achieved using switching (300mW) over linear (660mW) regulators amounts to a total power savings of 55 percent.

Another target is lower analog voltage. Voltage from the host is typically controlled to 3.3V maximum, but opportunities exist to reduce power dissipation by targeting a nominal 2.5V operation. However, this needs to be addressed through either a lower host voltage or by onboard regulation in the HDD. When using the latter method, there is minimal power reduction due to efficiency losses in the regulator (even a switching regulator). This approach only makes sense in systems using lower host voltage supplies.

Host/drive partitioning

Another opportunity for power savings can be found in effective drive/host partitioning, where functions are migrated from the host to the drive or vice versa. As more multimedia features are added to portable devices like media players and mobile phones, functional integration through repartitioning offers the advantage of reducing overall power and increasing precious real estate in a handheld device. While many of today's devices contain standalone, removable drives, the use of embedded disk-drive designs will capitalize on power, size and cost savings.

The storage SoC typically has a multistate sequencer, allowing self-managed power capabilities through firmware control. These functions adjust the power level depending on the drive settings and include the ability to selectively deactivate circuitry, enable/disable clocks in all functional blocks and provide dividers for functional blocks where clock disabling is a problem. Other power-management techniques involve selecting the correct system clock frequency to optimize the drive, typically achieved by choosing the appropriate processor clock rate. These functions all serve to maximize overall power efficiency.

Architecting for durability

The active protection of disk drives becomes essential as these high-capacity products become popular in portable CEs.

Improving the ruggedness of these devices requires the ability to reliably detect a free-fall drop and park the drive heads prior to impact. This is no easy task, given that a simple free-fall from 1m to the ground can take less than half a second.

At the semiconductor level, SFF drives require a coordinated design of the motor controller chip, storage SoC (or read channel), preamplifier and sensors used to detect free-fall events. Miniature drives currently use one of two sensor typesshock sensors or accelerometersalong with the associated algorithms.

When a drop occurs, the motor controller IC receives an input signal from the sensor and, in drives using shock sensors, will amplify the analog signal and convert it into digital. The motor controller then compares the signal to a reference to determine if the signal is abnormal. Consumers will not tolerate a lot of false positives unnecessarily shutting the device down or false negatives failing to protect the device prior to a drop, so care is taken in conditioning the signal to ensure a proper reading against this window comparator. If the signal falls outside these references, the motor controller alerts the SoC that the drive is in free-fall.

The SoC will first check the drive status to see if it is on seek operation or in idle mode. In contrast to desktop PC disk drives that perpetually keep the heads positioned over tracks on the spinning platter when not reading or writing, miniature drives value battery savings over data transfer speed and thus shut down the spindle motor and park the HSA on its ramp when in idle mode. If the drive is in write mode, the SoC will have the preamplifier IC shut off the write current, while the motor controller shuts down the spindle motor and moves the HSA to its ramp.

New SFF drive designs will increasingly need to rely on chips to speed up reaction time.

- Duncan Furness

Sr. Technical Manager, Storage Division

Agere Systems Inc.

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