The first of these market drivers is the availability of new cathode-chemistry Li-Ion cells with both increased capacity and an extended operating-voltage range. The second market driver is the recent introduction of sub-1" miniature HDDs with storage capacities up to 20Gbytes. Lastly, the third driver is the availability of highly integrated low-current buck-boost converters that can handle an input voltage above, below, or equal to the output voltage and provide high-efficiency conversion with enough peak-current capability to power a miniature HDD during spin-up.
Extended Li-Ion performance
One of the most popular Li-Ion battery formats is the 18650 cylindrical, which was first introduced in the early 1990s. At the time of its introduction, the 18650 cell capacity was only 960mAh. However, cell markers were limited in their ability to increase cell capacity due to the utilisation of existing electrode systems. As a result of these capacity limits with existing cathode and anode materials, they have started to introduce cells that incorporate new cathodes, and in some instances, new anodes. Nevertheless, even as these new materials have enabled increased cell capacities, they have simultaneously changed the battery's charge- and discharge-voltage range. These extended voltages will pose some new challenges for equipment designers as they try to use as much of the available voltage range - and energy - as possible. As recently as 2004, cell makers were producing Li-Ion cylindricals in the 18650 format with capacities as high as 2.6Ah. These cells used the same lithium-cobalt-oxide cathode materials and graphite anode materials as used in previous generations. Their designers attained this higher capacity by packing more active materials into the same-size package. While this capacity is fairly good, it has taken 13 years to attain. The next-generation Li-Ion cells have already taken cell capacities up to 2.8Ah as early as 2005, with 3Ah cell capacity on the short-term horizon. An example of a battery with a new cathode material is one developed by Matsushita. Instead of using a conventional lithium-cobalt-oxide cathode, they used a nickel-cobalt-aluminum oxide. This new cathode material enables higher capacity without affecting the current-charge voltage and is claimed to have excellent storage characteristics. Another Japanese battery manufacturer, Sony, introduced its Nexelion family of hybrid Li-Ion batteries in early 2005 with changes to both cathode and anode materials. This battery replaces the graphite-based anode with a tin-based amorphous anode in order to achieve a 50% increase in storage capacity per volume when compared to conventional cells, but discharges down to luch lower voltages.
Miniature HDDs
Over the last couple of years, the majority of the HDDs used in battery-powered hand-held devices had platters that were less than 2" in diameter. These disk drives need about 300mA at 3.3V for normal operation, but during spin-up, peak currents could be as high as 1.2A. However, recent 1"-diameter HDD introductions have made significant inroads in terms of storage capacity and power consumption. As an example, a 1" single-platter HDD can provide 20Gbytes of storage and only require a nominal 300mA of current at 3.3V for normal operation. Furthermore, during initial spin-up of the platter, a mere 500mA of peak current is required. These features make it very appealing for use as the main storage medium in a Li-Ion-powered portable device.
A common DC/DC converter problem is to generate a regulated output voltage that lies somewhere between a wide range of input voltages. To illustrate this problem, it is useful to think of an everyday application: consider a single-cell-Li-Ion-battery-powered media player with a 3.3V power rail. Having a conventional lithium-cobalt-oxide cathode cell means that its discharge profile is from a high of 4.2V down to 3.0V. Nevertheless, the system power rail requires a fixed 3.3V output. The output voltage is at times above, below and also equal to the input-voltage range for the battery. The traditional approach taken to solving this type of problem has been to use either a SEPIC (single-ended primary inductance converter) or a buck/boost converter. These types of converters will deliver a fixed output voltage whether the input voltage is above, below, or equal to the output voltage. However, there are some significant drawbacks when using SEPIC converters; these include a complicated design due to the multiple inductors or bulky transformer required, a large footprint and low conversion efficiency. A more effective approach would be to use a single inductor-based converter that can control four internal switches to perform the step-down, step-up and 100%-duty-cycle modes. A four-switch buck-boost converter would have the advantage of being easy to design, having a high power density and providing high-efficiency operation.
Convergence of three market drivers
PMP manufacturers are under ever increasing pressure to pack many features into an already constrained form factor while simultaneously gaining longer run-times. A key driver for the adoption of an HDD inside a PMP has been the need for large and easily read/writeable compact storage. PMPs can usually be powered from an AC adapter, a USB cable, or the Li-Ion battery. Most PMPs will use HDDs with platters of 1" in diameter; 0.8'' platters are on the horizon. These future 0.8" models will need less nominal current and have peak currents less than 400mA. If a newer cathode chemistry Li-Ion battery is used, then its discharge profile will be 4.2 down to 2.5V. Thus, designing a DC/DC converter to deal with this wide discharge-voltage range and delivering a fixed 3.3V output will be quite challenging. It is clear that designers of portable media players have a number of options available to ensure that battery life is optimised for their particular configuration. A combination of multifunction ASSPs and VLDO regulators can provide the necessary voltages and power levels to provide optimum system performance while ensuring that the power drain on the battery is minimised during normal operation. However, an ASSP might not be able to utilise the complete energy density of the newer cathode-chemistry Li-Ion batteries, such as the Nexelion product family. The reason is that once the battery's voltage is below the required 3.3V output level, the ASSP cannot boost this battery voltage to the required output level. Since the Nexelion range goes down to 2.5V, this would leave over 30% of the battery's energy unused. In this instance, a more simplified building-block approach - such as a monolithic synchronous buck/boost converter that can deliver a fixed 3.3V output regardless of whether the input voltage was above, equal to, or below the output voltage - would be the best solution. The LTC3532 is a high-efficiency, fixed-frequency power converter that can regulate an output voltage above, below, or equal to the input-source voltage with a single inductor. Its peak-output-current capability is 500mA at an output voltage of 3.3V. Its architecture provides a continuous transfer function through all operating modes (step-down, pass-through and step-up). This means it can extend battery run-time in single-cell Li-Ion, multi-cell alkaline or NiMH applications where input voltage decreases as the battery discharges; Figure 1 shows a compete schematic of the device.
Figure1: Miniature HDD power supply using the LTC3532.
The LTC3532 incorporates a constant-frequency, synchronous topology to obtain efficiencies as high as 95%. Switching frequencies up to 2MHz are programmed with an external timing resistor, and the oscillator can be synchronised to an external clock. Its burst-mode operation allows the internal MOSFETs to operate intermittently based on load demand.
