Why Batteries Don’t Last Long Enough

The hidden cost of analog: More on-chip real estate means mobile devices now need to support more wireless interfaces while still maintaining high signal-to-noise ratios.

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By Pallab Chatterjee
While there have been great strides in process scaling for power reduction on a per-gate level for mobile devices, a large part of the power is still consumed by the power amplifier, filter and analog mux arrangement from the systems.

Most of the logic systems have benefited from scaling to the sub-40nm technology range, which reduces standby and operating power by several orders of magnitude compared with traditional 130nm technologies. The small size allows very complex systems using small pin size and serialized interfaces to fit into the reduced form factor of today’s modern mobile devices. But that has proved to be both a benefit and a drawback. These devices are now designed to communicate with multiple wireless interface standards. For example, a standard North American smart phone can communicate on four different cellular bands, two different WiFi bands, and additional mesh and local networks such as Zigbee and Bluetooth. The WiFi band amplifiers need to work with the 2.5GHz and 5GHz channels and support multiple protocols. As a result, at least two power amplifiers (PA) are needed to drive antennas for these two bands.

Some of the physically larger systems (tablets and laptops) can support the MIMO capability of the new 5GHz standards (802.11n and 802.11ac/ad). In this case, a separate PA and antenna are needed for each data path. A quad-band cell phone requires four separate PAs, each handling one of the bands. These are usually found along with analog muxes and filters to support the specifics of the carrier network for which the phones are assigned.

These analog blocks need to maintain high levels of signal-to-noise ratio and also support high levels of sensitivity to support the longest wireless range. This combination forces the analog blocks to run at higher voltages and use more power. The cellular and WiFi analog operate between 3.5V and 7V. Higher frequencies above the 5GHz band and up to the 60GHz band operate with voltages between 12V and 48V. The biggest issue here is not the voltage. It’s the current. In mobile devices such as tablets and phones, the current per amplifier for 2.4GHz and below is between 50ma and 100ma. The higher frequencies can draw between 200 ma and 700ma per amplifier. For this reason, the higher frequencies have limited battery-only applications and are for devices that can be plugged into the wall.

These current levels have an impact on use models, as well. For an Ultrabook with a combined CPU/GPU core chip and dual-band WiFi connectivity, the target use is for creation of graphics, video playback and gaming. For gaming in particular, the system should work fine because the data going back and forth is small after the initial scene load, so the “pulsed” use of the analog for communication should not impact the battery. However, for video viewing, the streaming data interface will be running at peak data rate for the full duration of the video, which typically lasts anywhere between 10 minutes and 2 hours.

Conservatively, the analog for a dual-band 802.11n system (5GHz)—the minimum for HD video transfer—would use 2 x 200mA + 50% additional current for other analog muxes/filters/etc. That equals 750mA for the WiFi subsystem alone. For a typical two-hour streaming movie that equates to 1500mA per hour of power. This does not take into account the power used by the display, the CPU, memory, storage, and operation of the computer on the other side of the analog Front End Module (FEM). These other components account for another 1500mA per hour of power for this same period. For a normal 15-inch Ultrabook, the systems have an 800 to 900mA per hour battery. With this scenario, the power design cannot operate in full continuous mode, and must have very aggressive and through power cycling systems.

The design use is operating at one-third the available power, so just implementing a reference design without full firmware, operating system and application software control of the power management will not provide a usable result. This scenario is typical for mobile device designers today. While the increased bandwidth requirements for data can be processed at lower power in the logic sections, getting the data there is still a major challenge area.