Power management

Beat power management challenges in advanced LTE smartphones

David McTernan of Renesas Mobile takes a look at how to meet power management challenges faced by advanced LTE smartphones.

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Ask any group of people about their phone, and while several may comment on the cool new features and apps, you are also sure to find several who complain about smartphone battery life. Mobile applications are the latest phenomenon causing a larger than normal spike in power demand. Multiple power-hungry functions, including high-speed graphics processing and mobile broadband connections, in addition to other radio connections that manage functions such as location, are all running simultaneously.

This power spike will have a significant impact on the mobile industry as handsets and user expectations evolve and more advanced air interfaces such as LTE become common. Whether they are mobile business warriors conducting work via the cloud on a tablet or smartphone, the geekiest gamers or somewhere in between, mobile devices users share two things in common. First, they expect their technology to work whenever and wherever they use it. Second, they are frustrated by battery issues.

There are, of course, short-term workarounds. For instance, some iPhone cases double as a battery, which extends the device battery life but trades that convenience for a significantly bulkier form factor. It works, however, it doesn't address the central problem: These devices require a lot of juice and frequent charging, which is, in most cases, inconvenient for users.

Factors

A number of factors affect power consumption and the subsequent impact on device battery life. In the case of smaller mobile devices, like smartphones, thermal management is one of the key challenges. Efficient thermal management will help increase battery life, as well as stop the waste heat from leaching out. Reduction of temperature can be achieved through several other methods, including the casing design.

Reducing the temperature not only conserves the power in the device, but also preserves the device itself. Mobile devices typically have few moving parts, so that element of wear and tear is not an issue; wear and damage due to heat, however, is a real concern. Overheating will not only damage the short-term performance of a device as it slows down to avoid thermal runaway, but it may also affect components within it, causing longer term issues with performance.

Figure 1: Handset head dissipation. Cover on and off. Source: Renesas Mobile Corporation.

Other drawbacks

The power consumption issue is compounded by the need for higher data rates. Essentially, the more data that needs to be processed at speed, the worse the power consumption problem is going to become. Consider the nature of smartphones: These devices are more than just phones; they're expected to play music and video files, as well as streaming video. They often also serve as photo storage devices. This multitasking requires immersive graphics and sound, memory access and a high-speed broadband network connectivity through HSPA+ and LTE - all of which are power hungry. To complicate the issue, very often the user has applications like email and Facebook running in the background. This means a peak maximum throughput of data is often needed, which leads to higher clock frequencies, all of which produce greater heat--and so the cycle starts again. In the meantime, end-user customers are asking for more multitasking and better graphics, not less. This contributes to the heat issue, which occasionally results in a complete, unexpected shutdown of the device.

Figure 2: Dynamic module stop results. Source: Renesas Mobile Corporation.

The underlying software has done a lot to address the issues in these devices. Hardware modifications can achieve a great deal as well. The first thing a design architect can offer is a thermal simulation. This enables a manufacturer to assess the temperature limitations and emissions for each component and to test them against use in GSM, 3G and LTE environments. Ideally, these limits and any warning systems should be put in place in ways that are transparent to the end user, because nothing appears untoward in the device until they experience an actual problem. Thermal software management can be built to monitor for battery charge as well as temperature in compliance with safety regulations. All of this will be calibrated not only with safety but with performance in mind so the temperature that would cause radio data loss or decreased CPU performance is also set

There are other hardware-based options to manage heat dissipation. A printed circuit board, for instance, can be a key heat spreader in a smartphone, USB dongle or miniPCIe card, so thermal elements should be taken into when laying out the floor plan. Mechanical design rules can help avoid having hotspots under a component, and in larger devices such as laptops, air flow and heat distributors can ensure the cooling is as efficient as possible.

The right design can make a big impact. For instance, Renesas Mobile has developed a communications processor for smartphones that combines an applications processor and a Cat-4 triple-mode LTE modem on a single die inside its MP5232 platform. This features a number of techniques to minimize power consumption, including higher levels of integration and moving to advanced processes like the 28nm process. The smaller the technology, the smaller the dynamic power consumption when the device is being used. The platform uses advanced power domain partitioning, only turning on those parts that are needed for a particular use case, and a sophisticated clocking architecture to reduce power consumption levels. The Renesas Mobile design also uses a mix of autonomous low-level hardware control where it decides what power state it should be in and clever software to manage the system level power control.

Figure 3: Block diagram for a single-chip, scalable, high-performance platform. Source: Renesas Mobile Corporation.

The smaller the consumer device, the more difficult it is likely to be to route the heat somewhere productive or least damaging as it has fewer places it can go. This is where ASIC and particularly digital baseband design comes into its own, with the ability to switch off some power domains to minimize leakage. Clock controls inside the chips have an impact on the power consumption of the digital baseband, and if the clock is gated when the device is in idle mode then this reduces the overall power consumption. The use of lower and adaptive frequencies and voltages in chips will also help as the higher frequencies and voltages use more power.

Figure 4: Impact of clock gating, frequency and power gating control on overall power consumption. Source: Renesas Mobile Corporation.

It is also desirable to use an optimal silicon geometry and process together with low cut off battery technology to optimize the power consumption. For instance the leakage current varies considerably between silicon processes, which impacts standby current and hence the life time per charge. Choosing the right process is a trade-off between maximum performance and reduced leakage for maximal inactive battery life. The expected process corners, how the manufacturing tolerances vary, also needs to be considered in the overall system design.

Smartphone demands for more multitasking and a better overall mobile experience are here to stay, whether it's using mobile applications, streaming video or another yet-to-be-imagined application. Next-generation devices will face even more critical power considerations, and the conflict is going to keep device designs evolving for the foreseeable future. Renesas Mobile is at the forefront of meeting these challenges through innovative architecture, design and leading edge modem and processor technologies.

About the Author

David McTernan, Vice President, Strategic Marketing and Communications, Renesas Mobile Corporation. David currently oversees communications strategy and planning, trade shows & events, and analyst & media relations, as well as managing the company website and social media presence. Prior to joining Renesas Mobile, McTernan worked at Nokia, Motorola, TTPCom, among other companies, in various roles including product management and engineering. He has a BSc in Electronics with Computer science from UCL, an executive MBA from Cranfield and a Diploma in Marketing.

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