Design Challenges and Solutions for PDA Power Systems

　　Today, battery-powered PDAs face severe challenges in power management as energy-hungry features are added to PDAs while keeping battery life reasonable and keeping batteries smaller and lighter. This requires efficient energy conversion, precise control of battery charge/discharge cycles, and as many power-saving modes as possible. Even with these measures, PDA manufacturers are still working on higher capacity batteries to meet the growing load requirements. Energy density is the primary factor in battery selection. Lithium-ion batteries have become the de facto choice for all PDAs, as they have twice the energy density of their closest competitors.

Discussion of power management technology in three key areas

　　The power management (PM) module is responsible for the power supply and management of the entire PDA system. Figure 1 outlines the framework of the PM module, which consists of three submodules: battery management, voltage management, and load management.

Battery management submodule

　　The battery management submodule is responsible for charging, protecting and detecting the battery. Its main function is to optimize battery utilization and extend system operation time. Its circuit structure depends on the selected battery chemistry, capacity and number of charge/discharge cycles. Once these factors are determined, the power architecture designer can choose the charger topology (linear, PWM or pulse mode) and battery protection scheme. When the input is switched from the AC power adapter to the current-limited USB bus voltage, a recent consideration includes reconfiguring the embedded IC charger to a lower charging rate (C-Rate).

　　Energy measurement was originally a battery management function, but as users are increasingly concerned about battery health, it is also becoming an integral part of load management. Accurate energy measurement requires precise measurement of the battery charge/discharge current that varies over time. The widely used voltage measurement technology based on the battery discharge curve, although low cost, has been proven to be inaccurate in reflecting the battery's health. For this reason, complex "fuel metering" ICs such as the Intersil ISL6295 are gaining more and more applications.

Voltage management submodule

　　The voltage management submodule mainly performs distribution and regulation functions. It is responsible for efficiently regulating the unregulated battery voltage (3-4.2V for lithium-ion batteries) to meet demanding load requirements. With the emergence of processors with operating voltages below 2V, linear regulators are no longer suitable for stabilizing the processor core voltage.

　　In their place, fully integrated switching regulators such as Intersil's EL7536 and ISL6271 have emerged, which have high conversion efficiency over a wide dynamic load range and can be used to regulate system and core processor voltages. Linear regulators are now mainly used for LDO (low dropout) conversion and are only used to provide power to noise-sensitive circuits in memory, audio, and clock systems.

Load management submodule

　　System load consumes at least 90% of the battery capacity (the remaining 10% is consumed in the energy conversion link), which is the key to extending battery life and helping to achieve advanced PDA performance. The CPU must switch these circuits on and off in sequence according to the activity and system operation mode of the clock circuit, Hi-Fi audio amplifier, RF antenna driver, memory, expansion card slot, processor core and display backlight. This load management method (commonly known as "load shedding") not only extends the battery operating time, but also ensures that the battery does not fall into a deep discharge state.

Energy Metering

　　One way to understand the energy consumption of a battery is to measure it, and this is the main function of an energy measurement IC (such as Intersil's ISL6295). By connecting a small sense resistor in series with the battery, the ISL6295 can accurately calculate the energy consumed or replenished by accumulating the battery current flow (mA-hours) over a period of time. The ISL6295 records voltage, residual capacity, current and temperature via the I2C serial bus. It can be used to monitor the power consumption of a single load and help implement advanced power management functions including load shedding and battery protection.

The backlight circuit is a high energy consumption load

　　While LCD backlighting improves the readability of the display, it also consumes a lot of battery power. The three commonly used LCD display backlight technologies are: EL (electroluminescence), LED (light emitting diode), and CCFL (cold cathode fluorescent lamp). EL and CCFL backlights require a high voltage DC/AC inverter, while LED backlights require a capacitive or inductive boost converter to drive a set of parallel or serial white LEDs. Considering a typical backlight configuration with 4 LEDs in series, each LED has a forward voltage drop of 4V and a forward current of 20mA, the minimum output power of the boost converter will be =320mW. If the boost converter is 85% efficient, the display backlight is equivalent to a 380mW load on the battery!

　　Wireless communications with PDAs are also power hungry. For example, a typical palm-sized PDA with an embedded WAN radio requires 0.1W to maintain an “always on” standby state for data communications, and typically 0.25W when communicating data. This is in line with the energy required by a state-of-the-art PDA processor to handle active video (not including backlight power consumption). In addition to embedded WAN wireless communications, LAN cards are also widely used for wireless 802.11b connections. In typical PDA usage patterns, you will find that for an average 1W peak power radio, receive and transmit modes account for only 2-3% and 1-2% of the total usage time, respectively. The rest of the time, the radio will be idle, but will still draw a modest 20mA from the 3.3V system regulator.

Adjust voltage and frequency to increase usage time

　　需要大量运算的应用(如多任务操作系统、流视频、MP3回放和无线通信等)使本已左支右绌的电池使用时间更加不够用了。幸运的是，来自英特尔、AMD和其它供应商的基于低功耗RISC架构的处理器正陆续应运而生，它们可以根据系统需求平衡计算吞吐量和优化功耗。由于处理器功耗与工作频率成线性关系并与处理器内核电压的平方成正比，因此通过动态控制这些工作参数我们能够显著降低功耗。

　　Intel's XScale technology is the successor to StrongARM, and was developed primarily for mobile Internet applications. This power-saving architecture represents the key to Intel's wireless application processors, which are based on an open architecture Intel calls the Personal Client Architecture (PCA). The 0.18-micron process used by the PXA250 and PXA210 enables them to operate at 400MHz, while the low-dielectric material used reduces power consumption. Intel's XScale technology is a highly scalable microarchitecture that has voltage and frequency adjustment capabilities to achieve optimal performance and power consumption (the highest operating frequency achievable with the 0.18-micron process is 1GHz).

　　PXA250 has millions of transistors operating at a switching frequency of 400MHz. In order to minimize the switching loss, Intel uses "automatic clock gating" technology to shut down the circuits in the non-working state. The maximum operating power consumption of PXA250 is 360mW. There are two low-power modes, standby and sleep, which consume 100mW and 50uW respectively. A smart programmer can flexibly switch between different modes according to the operating status to reduce power consumption.

in conclusion

　　With the advent of broadband wireless communications and the emergence of processors with "tunable" processing power, the PDA market will usher in a new round of growth. Future PDAs can provide users with an "always online" wireless data experience and rich multimedia features, which will make PDAs more dynamic in the enterprise market. However, the core of portable electronic products is battery life, and the effective management of this limited resource is related to the success or failure of any type of PDA. After manufacturers have integrated as many functions as possible into PDAs, the success of any portable device will still largely depend on the following factors: ease of use, weight, cost, size and battery operating time, four of which are directly related to batteries and battery management.