by jeff falin, texas instruments

introduction

third generation "smart" cell phones combine the traditional 2g cellular phone with pda-like features as well as digital still cameras (dscs) and music players (mp3s). figure 1 shows a simplified block diagram of a 3g phone with its major subsystems and their respective voltage rails.

figure 1 " 3g smart

such diversity in functionality requires numerous components, most of which have different power rail voltages, with each rail having growing power demands and application-specific requirements. at the same time, consumers want smaller phones with maximum battery life and minimal battery charge time. all of these requirements have driven development of various high performance and/or highly specialized power management integrated circuits (ics). this article discusses how power management ics have evolved to meet many of the requirements of the new smart phones.

the battery

the input power source strongly influences power ic design. at present and into the near future, the li-ion rechargeable battery will be the power source for portable electronics. its high volumetric and gravimetric energy densities (270-300 wh/l and 110-130 wh/kg, respectively) mean it provides more power for its size and weight than other battery types. second, its 2.7 v to 4.2 v operating voltage is higher than the primary high current power rails. this means that high dc/dc conversion efficiencies, and therefore long battery life, are possible because stepping down (or bucking) a higher voltage rail to a lower voltage rail is almost always more efficient than stepping up (or boosting) to a higher voltage rail.

proper management and control of a rechargeable battery is critical for maximizing battery life. battery management consists of three parts: battery monitoring, battery protection and charge control. battery monitoring and protection ics are typically packaged with the battery itself. however, charge control is part of the portable device. charge control ics have evolved significantly from simple linear controllers with only basic protection and control circuitry to switching dc/dc converters with integrated switches and features to meet user demands, such as reduced charge time. early linear charger ics had lower efficiency, computed as output voltage over input voltage, and therefore either their input voltage or their charging current was limited by the power dissipation capabilities of the package.

recently, semiconductor manufacturers have been developing not only processes with lower minimum gate lengths for higher-density and higher-speed digital circuitry, but also drain-extended devices capable of higher voltages for more analog and power applications. ti's bq24100 switch-mode battery charging ic with internal power fets, for example, has the digital circuitry to provide the protection and control required of a battery charger. its wide input voltage range, up to 16 v, makes it useful for charging one to three li-ion cells in series. it is capable of supplying up to 2 a of charge current for fast battery charging. although it is packaged in a small 3.5 mm x 4.5 mm qfn package, power dissipation concerns are minimized due to its highly efficient pwm switching converter, which has peak efficiency above 92 percent.

falling voltage rails with "tighter" tolerances

as demands increase for improved ic functionality at lower operating voltages and smaller packages, there are more stringent tolerance and efficiency requirements on the power management blocks. for example, imposing ±3 percent tolerance on a 1.2-v dsp power rail requires that the output not vary more than ±36mv versus a 3.3-v i/o rail with a ±3 percent tolerance being allowed ±99 mv of variance. load transient responses on the order of 50 mv to 100 mv are not uncommon from traditional