The smartphone and tablet market segments are two of the fastest-growing ones in the electronics industry. New offerings are introduced every week with a large focus on display size, processing power, feature-set and fast wireless connectivity.
All these features result in very powerful portable devices; however, they also introduce significant constraints on system run time and battery life. While Li-ion batteries with increased capacities are being used, there is still a limit in terms of battery weight and volume to ensure appealing industrial designs. In addition, today’s systems usually stop operating at battery voltages as high as 3.5V, and do not use the remaining battery capacity, thereby significantly shortening usable battery life.
The problem is further compounded with the addition of new technologies, like Long Term Evolution (LTE), whose power amplifiers (PAs) may require higher operating voltages for maximum output power. LTE allows higher download and upload data rates, which in turn enable new applications such as video-streaming. Furthermore, this new wireless standard can co-exist and transfer with legacy standards (GSM/GPRS, UMTS) allowing seamless transition in areas where LTE is not available. Last but not least, high-quality data transmission is possible even at high speeds (350kb/sec and more).
While LTE provides a lot of benefits to the consumers, it comes with new challenges for portable system designs. In a typical design, all of its support components need to process higher data-rates and more channels, thereby consuming more power. For providing the highest power output, many of the LTE power amplifiers need to consume up to 600mA of current, a considerably higher battery drain than in the case of 2.5G and 3G systems.
Even during standby mode, current consumption may be as high as 150mA, which can significantly reduce usable battery life for a single charge. The PA collector voltage may be lower than 3.3V or 3.4V; however such lower-voltage operation may result in a sub-optimum performance. While in the long-run some new battery technologies may be able to address the power source constraints, most of the short- and medium-term challenges for achieving adequate system run times lie with the system architecture.
Traditionally, a system that has a cut-off voltage of 3.5V is simply turned off as the battery voltage drops to that level. This of course, reduces the runtime of the system, and the problem is further compounded as the battery ages, and its internal equivalent series resistance (ESR) increases. The only way to improve this is by ensuring that the resistance from the battery to the system is as small as possible, which will prevent large voltage drops and not further degrade the usable time of the battery.
This can sometimes be a very difficult task, due to the industrial design of these types of devices. Additionally, the fact that total system loads are increasing with LTE, AMOLED displays and multi-core processors, means that for a given resistance, the voltage drops are becoming larger. So it truly is a compounding problem.
Even if PA design is improving, thereby allowing for operation at lower battery voltages, there are still many voltage rails that require a higher voltage for operation. This can be achieved using a buck-boost regulator, but such solutions are often expensive, require a lot of area for the control IC and power FETs, and the inductor. The other drawback of the buck-boost topology is that the transition from buck-mode to boost-mode is rarely ever smooth, and can glitch, thus causing problems in operation.
Figure 1: Traditional buck-boost regulator
With battery capacities--and therefore charge currents--for the battery increasing, it has become necessary for many portable applications to use switch-mode battery chargers to ensure that thermals are not too high in a given design. The switch-mode architecture has many advantages, including faster, more-efficient charging; however, the main drawback is that cost can be slightly higher than that of a linear charger due to the addition of an inductor, and often-larger FETs for better efficiency.
Assuming that a switch-mode charger has a separate path for the system load, and the battery charging (also known as Current Path™) it is possible to run the buck regulator, which is normally used for charging the battery from a power input, in a reverse mode. Running the buck regulator in reverse can actually boost the voltage from the battery to the system, using the existing battery charger and inductor.
This operation works as follows, and is known as System Extend Mode™. When the battery is above the system cut-off voltage, a low Rdson FET is used to connect the battery to the system load. If the PA can be run from a lower voltage, say, 3V, it should be connected directly to the battery, to reduce the voltage drop during transmission.
As the battery discharges, and the voltage drops, when it approaches the system-cutoff voltage, the switch-mode charger should automatically begin to boost the battery voltage up to provide the system with a higher voltage. To minimize the extra current drain on the battery, the regulated (step-up) voltage to the system should not be too high; however, some margin is necessary to account for drops due to transient load conditions, and resistance-caused voltage drops. The ideal voltage used for this application is approximately 4.17V.
Figure 2: System Extension Mode ™ typical application
It is also very important that the efficiency for the boost regulator be high to also reduce the current consumed from the battery during boosting. In addition to efficiency, it is necessary to have a high switching frequency for the boost regulator. A frequency of 3MHz is a good choice, allowing the use a smaller value inductor, 1mH, for better transient response and lower DCR for efficiency, while also allowing the crossover frequency of the boost converter control loop to be higher to improve the transient response time.
Figure 3: Transition between normal operation and System Extension Mode ™
Another feature of importance is a voltage monitor to automatically prevent the battery from discharging too low. When the battery voltage falls below around 3.1V (depending on the battery characteristics) the boost regulator should be turned off to ensure the battery does not open-circuit itself. For existing battery technologies, this is very useful, but as the battery gets to a low state of charge, the voltage begins to drop more rapidly due to the ESR increasing.
Li-ion batteries with more exotic anodes and cathodes are now becoming available that will ensure the ESR does not increase at a low state of charge. What this ensures is that that the voltage will slowly drop when the voltage drops below 3.5V. System Extension Mode then allows you to take advantage of far more capacity that could not be realized with traditional implementations, and eliminate the need for high-cost buck/boost regulators.
Another very practical use of this mode is during emergency situations, in which a last-call needs to be made. In this case, the additional battery drain is a non-issue compared to the value of this functionality. Furthermore, with many of the portable devices using very similar hardware, operating systems and sales channels, functions like the one described here can provide the means for differentiation for many of the device manufacturers.
With the smartphones and tablets becoming minicomputers in terms of processing power and using displays, and with wireless technologies (such as LTE) that are very power hungry, system run time will become a very big challenge. On the one hand, the industry is trying to keep portable applications as thin and light-weight as possible; on the other hand, there is a “hidden” target of one-charge-per-day for the battery. New battery-charging technologies are now available which enable extended system operation without the addional cost and complexity of traditional implementations.
About the Authors
George Paparrizos is product marketing director at Summit Microelectronics. He holds a MSEE degree from the Technical University of Aachen, Germany (RWTH), and an MBA from the Haas School of Business at University of California at Berkeley.
Shadi Hawawini is a Senior Applications Engineer at Summit Microelectronics, responsible for customer support and product development. He holds a provisional patent in Advanced Battery Charging Algorithms and a BSEE degree from San Jose State University.