The electronics industry is entering another exciting phase in its short history. The last century had experienced invention and technological growth in semiconductors, communications and computing, enabling the 21st century with compelling new applications in the portable arena. The integration of technological advances in computing, communications and batteries provide for newer and greater ways of extending use of electronics beyond mere calculators and watches for the ever-mobile individual. Today's electronics are in cellular phones that fit snugly into the palm of a hand, notebook computers less than 1 inch thick, personal desktop assistants/organizers,allowing information to be more readily available. No longer limited to the military, portable applications allow anybody to be connected anytime and anywhere. For the average person, from the student who has to turn in an assignment over the Internet to the medical doctor accessing a patient's files from a remote location through wireless means, portable electronics has found new and challenging grounds.
A portable system is self-contained. Every phase of the design must preserve total integrity of the system. From the battery source to the load, software to the hardware interface, compatibility must be preserved along with form factor packaging optimization.
On an electronic level, mechanisms link the various functions together as well as the source to the loads. The battery source gets replenished either by replacement or through charging from an external line source. Such power transfer is managed from an operating system level where energy states are monitored and appropriate measures taken to route prioritized critical paths to maximize useful battery life.
With a software operating system, in a Windows CE for example, the OS suspends power to the processor when all threads are waiting for events. Even when the portable device is turned on, the processor is suspended by the OS 95 percent of the time under normal use conditions. When it is turned off, all hardware except the real-time clock is suspended. Automatic database synchronization occurs when a system event occurs.
System event interrupts can occur through a human interface device like a touch pad, keyboard controller, etc. The human interface device can also be programmed to determined the battery states in real-time and compare with pending loads. Depending on the load vs. source availability, smart algorithms and subsequent flags can be set to the polling interrupts, which in turn deactivates the necessary loads in order of critical priority.
When the load is enabled, efficient power delivery is essential for rated operation. Each load requires certain dynamic and steady-state parameters to operate. For example, in a mobile computer's CPU, currents in excess of 10 A with slew rates in excess of 1 A/ ns is required for the CPU to run at its rated speed. With an increase in clock speed, currents increase accordingly. The supply voltage to the CPU is throttled depending on the speed/ input state environment. For Intel's SpeedStep Technology, the supply voltage is reduced for increased load current at full speed while supplied from the line. When it runs in battery mode, the speed reduces and the voltage input is throttled down with respect to a predetermined VID code. Techniques are continually accessed to optimize performance vs. minimum power draw.
While other loads like memory require typically 100 mA to supply, the switch connecting to the source needs to have as low a voltage drop when conducting. Upon disruption from a host, design attention must be observed to prevent excessive voltage spikes from switching too fast and switching losses from turning off too slowly. In addition, while in conduction, the switch must be able to filter out high frequency noise, which may interfere with the operation of all loads. The voltage across the switch in conduction is known as the drop-out voltage. Ultra low drop-out voltage regulators approaching 10 mV is available today.
Another voltage level-shifter is in the form of a charge pump. In portable applications where real estate is indeed a premium, minimizing passive components remains one of the greatest challenges. Charge pumps make use of storage capacitors to deliver energy. Charge pumps integrated with capacitors are also available today. The various switches described-ULDOs, CaplessTM charge pumps and VID switching regulators form basic power delivery building blocks for the portable electronics market.
A combination regulator illustrates a power management system-on-a-chip. This IC provides a centralized power control center, which includes the intelligence to determine various powered states and sleep modes.
This integrated power management chip is popular in higher-end cellular phones. With the advent of 3G applications, power consumption in terms of increased computing demands and data reception will see the need for dedicated DSP and amplifier power management.
With much consideration given to present-day limitations, reshaping the power management strategy comes naturally. Power to the portable system is managed through a smart control protocol. That algorithm will take into account critical vs. noncritical loads, power consumption of the individual pass elements and battery energy dynamic capacity. Thoroughly understanding the individual characteristics of the components is necessary in order to optimize system performance. Multidisciplinary teams will need to work to minimize interface issues.
Cutting the losses
The amplifier driving the RF antenna in a wireless application easily consumes the most power and yet is the most frequently used. Present-day RF amplifiers operate at a maximum efficiency of 35 to 40 percent at a full load of about 28 dBm, while light-load (about 10-dBm) efficiencies are in the single digits. Beyond the efficiency number, the absolute power dissipation needs to be managed for thermal and battery operational lifetime. During the transmission of data or voice, the RF amplifier is the single most power-inefficient component. Techniques to reduce the losses include closed-loop biasing, dynamic Vcc control and newer semiconductor technology in the areas of SiGe and GaAs structures. During standby time, the clock is analyzed as a major source of current drain. Techniques have been proposed and developed to minimize that loss.
For an integrated power management solution, the algorithm linking all the loss reduction techniques for power-critical applications must be embedded, and hardware optimization must continue to improve. Trade-offs will have to be made in terms of dynamic performance and battery life.
The best way to preserve battery life, of course, is never to turn on the device, but that's not practical. Normal consumer usage of the portable device must be statistically analyzed. Power design centered on that curve will provide the maximum perceived battery life and maximum consumer satisfaction.
Systems-on-chip may be the ideal, but the long development cycle limits their flexibility and their ability to pace market needs. The best approach is still to integrate functions on the board wherever practical, preserving flexibility for forward compatibility. With the explosive pace of the portable market, that compromise is a viable way to align power management optimization with market conditions.
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