Posted on 01 March 2019

Trends in Battery Powered Portable Devices

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Power rail requires a fixed 3.3V output

The year of 2006 turned out to be a stellar year for the shipment of battery-powered portable electronic devices. During the course of 2006, consumers bought over 950 million cell phones, 220 million notebooks, 140 million MP3 players, 90 million DSCs and 10 million GPS systems.

By Tony Armstrong, Product Marketing Manager, Power Products Group, Linear Technology Corporation


However, another category of products not mentioned, are hybrid versions of two or even three of these product functions, such as portable media players (PMP) or digital media broadcast (DMB) products. These too, include a Lithium-Ion battery as their main power source, and are fast becoming a significant player in the area of consumer electronics.

The advantages of a PMP are that they can play both MP3 and MP4 formats. Therefore, a single device can be used to listen to music or watch a movie from a DVD-CD, or from a web site downloaded file. Many of these PMPs include a hard disk drive (HDD) for high capacity storage capability. Typically, this allows a device to store over 150 hours of video or 1,200 hours of music. However, manufacturers of these PMPs are under ever increasing pressure to pack all of these features into an already constrained form factor while simultaneously gaining longer runtimes.

Since most PMPs have the functionality of both a video player and a MP3 player, the internal electronics requires multiple low voltage output rails at varying power levels. The reason for this is clear; the majority of the digital large scale integrated (LSI) ICs have operating voltage of 1.5V or less. At the same time, memory and I/O voltage requirements can vary between 2.5V and 3.3V. Thus, it is becoming impractical to use multiple point-of-load (POL) DC/DC converters directly from the Li-Ion battery, and so system designers are adopting a more integrated approach.

Similarly, cellular phones do more than just allow people to talk with each other. Most ‘Smart’ cellular phones allow web browsing, wireless transfer of emails, photography, streaming video, TV reception and even GPS capability. An embryonic trend is to also include a micro hard disk drive for high capacity storage capability, allowing these Smart phones to also have high-density storage capability. Most of the HDDs used have platters of 1” in diameter, or less. There are10GB micro-drives on a single platter with only 1” in diameter which are already proven and available, with 0.8’’ diameter platters are on the near term horizon. The 1” disk drives only need about 300mA at 3.3V for normal operation. However, during spinup peak current demand can be as high as 500mA. Future 0.8” models will need less nominal current and will have peak currents less than 400mA.

Looking at the Smart phone block diagram in Figure 1, it is easy to see how the increasing number of features is also driving the need for more low voltage output rails at varying power levels.

Block Diagram of a Smart Cellular Phone

Figure 1 caption

The Use of a HDD in a PMP or a Smart Phone

A key driver for the adoption of a HDD inside a PMP or Smart phone has been the need for large and easily read/writeable compact storage. Consider the case of the PMP, It can usually be powered from an AC adapter, a Universal Serial Bus (USB) cable, or the Li-ion battery. However, managing the power-path control between these power sources presents a significant technical challenge.

Another common DC/DC converter problem is to generate a regulated output voltage which lies somewhere between a wide range of input voltages. To help put this problem into a clearer perspective, it is useful to consider a single cell Li-Ion battery PMP 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. As can be seen, the output voltage is at times above, below and also equal to the input voltage range for the Li-ion battery.

The traditional approach taken to solving this type of problem has been to use either a single- ended primary inductance converter (SEPIC) 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.

Complicated design due to the multiple inductors or bulky transformer required. The control loop is complex and difficult to stabilize under a wide input voltage range typical with Li-Ion battery applications.

  • The solution footprint is large and also has a high height profile.
  • The efficiency of conversion is low – usually mid-70s to low 80s percent.
  • Thermal problems can arise at higher output power levels.

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. This type of 4-switch buck-boost converter would have the advantage of being easy to design, have high power density and also provide high efficiency operation due to its synchronous drive capability.

Supplying 3.3V at 500mA for HDD Spin- Up

It is clear that designers of PMPs have a number of options available to ensure that battery life is optimized for their particular configuration. A combination of multifunction ASSPs can provide the necessary voltages and power levels to provide optimum system performance while ensuring that the power drain on the battery is minimized during normal operation.

However, an ASSP might not be able to utilize the complete energy density of the newer cathode chemistry Li-Ion batteries such as Sony’s Nexelion product family. The reason being 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 battery range is 4.2V down to 2.5V, this would leave over 30% of the battery’s energy un-used. In this instance, a more simplified building block approach is a good alternative to supplying a HDD with the necessary power during the natural discharge cycle of this newer type of Li-Ion battery. In this case, a monolithic synchronous buckboost 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 ideal solution. Fortunately, Linear Technology has recently released a new buck-boost converter to address this specific need; it is the LTC3532.

The LTC3532 is a high efficiency, fixed frequency, buck-boost DC/DC converter that can regulate an output voltage above, below, or equal to the input source voltage with a single inductor. The input voltage range is 2.4V to 5.5V and its output voltage range is 2.4V to 5.25V. Its peak output current capability is 500mA at an output voltage of 3.3V. The LTC3532’s architecture provides a continuous transfer function through all operating modes, namely step-down, pass through and step-up. This makes the LTC3532 ideal for extending battery run-time in single cell Li-Ion, multi-cell alkaline or NiMH applications where input voltage decreases as the battery discharges. See Figure 2 below for compete schematic of the LTC3532.

Miniature Hard Disk Drive Power Supply using the LTC3532

Thus, in a single cell Li-Ion battery powered PMP, a 1” miniature HDD needs a constant 3.3V at a nominal 300mA, with 500mA peak currents. The output voltage from the Sony’s Nexelion battery varies from 4.2V down to as low as 2.5V. The LTC3532 would operate in step-down mode while the battery voltage declines from 4.2V to a nominal 3.3V. At 3.3V, the LTC3532 will operate in 4-switch buck-boost mode. This 4-switch operation ensures a smooth switchover between buck and boost modes, providing no jitter, ripple or erratic noise generation. Once the battery voltage drops below 3.3V and continues down to 2.5V, the LTC3532 operates in stepup mode. Compared to a conventional stepdown converter, using the LTC3532 can extend the useful life of the battery by over 30%.

A Multiple Output DC/DC Converter for 4 Low Power Rails

It is clear from the above discussions that there is also a growing demand for a moderately integrated switching DC/DC converter that can supply multiple outputs with high efficiency operation at moderate current levels. Such a converter would easily find a home in either a PMP or Smart phone.

Linear Technology has recently introduced the LTC3544 to bring an optimized, compact solution for just this purpose. The LTC3544 is a quad synchronous step-down DC/DC converter which operates at 2.25MHz and has four independent outputs with continuous current output capability of 300mA, two times 200mA and 100mA, respectively. Its 2.25V to 5.5V input voltage range makes is well suited for Li-Ion/polymer battery-powered devices. The high switching frequency operation, combined with a 3mm x 3mm surface mount package allows for the use of small surface mount inductors and capacitors while providing a compact solution footprint. Finally, it’s internal synchronous switches increase efficiency and eliminate the need for external Schottky diodes.


Designers of battery powered portable devices have a number of options available to provide for the adoption of a HDD within their product or provide for a small compact DC/DC converter for their low power rails. Fortunately, manufactures like Linear Technology are offering a range of products in this area to simplify the designer’s task.


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