Summit Microelectronics Technical Article

	Power management in consumer handheld products
	Hearst Electronics Group - Electronic Products, August, 2005
	Tom DeLurio, Director, Applications Engineering, George Paparrizos, Product Marketing Manager, Summit Microelectronics, Inc.
	Consumer products, especially handheld devices, are becoming increasingly complex as many different devices with myriad voltage levels are powered from a single-cell battery or a wall supply when the battery is recharging. The power supply must handle low as well as high voltage, promote long battery life and high battery efficiency, and accommodate more end product functions&emdash;all the while being housed in the smallest possible package. Voltage A power supply in portable products must convert a single voltage level to power multivoltage processors, DSPs and ASICs, SDRAM, memory sticks, flash memory, and LCDs with white LED backlighting. Furthermore, device voltage levels for multivoltage processors, DSPs, and FPGAs are down to 1.2 V and are approaching 0.9 V, making system tolerances tighter and necessitating a precise way to keep these voltage levels within specifications. At the other extreme, stacked white LED backlights require as much as 30 V with precise current control to power up to 10 white LEDs in series. To further complicate matters, all these devices need to be turned on or off at different times for both reliability reasons and to conserve battery life. If all these requirements are not followed, performance degradation, fault conditions&emdash;such as bus contention, poor battery life, or device latchup&emdash;can arise. Battery life Design challenges in portables that can enhance battery life must also keep up with system changes and performance enhancements. The trend is toward programmable multiple output dc/dc power management devices with digital control to allow simple software tailoring of output voltage levels and power-sequencing requirements. Since system supply requirements change rapidly, a new platform solution that can change to meet any type of system power supply requirement eases the designer's job. This can be achieved by specifying a power block that can be standardized over a wide variety of applications and then digitally configured to individual requirements (see Fig. 1).    Fig. 1. A digitally programmable power platform can use PWM and LDO regulators. Inductor-based dc/dc converters One of the major trends is the fast adoption of inductor-based dc/dc converters. Traditionally designers were opposing the use of switch-mode power supply ICs, especially in portable communications equipment. The main concerns are increased switching noise and higher complexity vs. designs based on linear regulators. However, the continuous thirst for higher performance and more features in handheld devices has not been able to be quenched by higher battery capacities (for a given form factor), thereby putting pressure in the power supply circuit to use the available power more efficiently. Typical switching regulators provide efficiencies of 85% to 90% plus, whereas linear regulators are efficient only when the input-to-output voltage differential or the required load current is small. Furthermore, most of the new digital chips (ASICs, DSPs, CPUs, etc.) are being manufactured at technologies that produce core voltage levels as low as 0.9 V and with requirements for higher current levels. This introduces additional stringent requirements for power conservation, which can only be addressed by using switching regulators or controllers for the most power hungry components. More functions With modern electronic devices integrating a lot of secondary functions, they find themselves in a gray area as far as the product type is concerned. This leads to ever-changing power requirements, hence no standardization is feasible. For example today's cell phones integrate camera as well as PDA functionality. This trend is expected to continue leading to a convergence of applications and functions. Integrating such functionality increases the number and type of power supply rails required in a typical design. Modules The addition of new functionality in portable devices often finds place through the addition of modules in the original design. This introduces an additional level of complexity, since different customer products might be using a variety of modules, thus having different power management requirements. In tight supply market situations or when companies decide to offer a wide range of products (with a diverse mix of functions), using one specific module can lead to ugly situations. Future ICs will need to provide a level of design flexibility that eliminates frequent board respins. Rather ideal solutions would allow a level of power adjustment via software control. Such products can assist companies reduce time-to-market, ensure stable production and save precious engineering resources (see Fig. 2). In addition, such power management topologies would allow the development of a power supply platform that can be reused with minor "one-shot" software modifications over several projects. Using the same power platform not only reduces development; but increases the reliability of designs. All these trends in modern portable electronic equipment have also created the need of integrating power delivery together with power control functionality. Just using conventional dc/dc converters does address most of the power management needs, however it does not address the need for better control of the power supply, which is ultimately how power can be conserved without performance degradation. Individual shutdown control for each channel is one way to reduce current consumption by providing power only to the components that are operating. Another way to accomplish longer battery life is by adjusting the voltage levels during sleep, standby, or low-activity modes.    Fig. 2. Software can provide design flexibility that eliminates frequent board respins. As mentioned above the mix of components in typical modern applications varies significantly, blurring the borders between a cellular phone, a PDA, a media player, and a portable gaming device. With every component having different voltage and current requirements, it is also important to adjust all the protection functions. With varying voltage levels, the undervoltage and overvoltage protection functionality and levels also need to be adjusted. Even the hardware-based logic level signals might need to be changed to match with other components on the board. Timing Multivoltage DSPs and CPUs have stringent requirements regarding which voltage needs to be powered up first, second, and so forth. It is not sufficient to take a passive role in the monitoring of the supply voltages. Many components have very strict requirements in terms of supply sequencing (that is, the core voltage must be valid before the I/O voltage is brought up), or differential tracking of the supplies (that is, the core and I/O supplies must be ramped simultaneously with a small differential voltage allowed). Additional constraints such as supply loading changes during system operation make a simple time-based sequencing implementation inadequate. Therefore, a supervisory function must be used to control many different functions from turning the supplies on to timing interval generation. Supply sequencing simply turns on the regulators or converters, one after another, at a set time interval. This is the easiest way to ensure that the supplies are turned on in a specific order. A limitation of time-based supply sequencing is that it only controls the time that each supply begins to turn on. Based upon such factors as slew rate and loading, this may not guarantee an optimum sequence of the supplies reaching their valid voltages. Cascade sequencing is needed to ensure that the supplies are enabled a variable period of time after the previous voltage has reached its minimum valid level. Each succeeding voltage must reach its minimum valid level before the timer in the next sequence position is started; this condition guarantees that the programmable variable interval for the next voltage will be adhered to. Not until the timer has elapsed is the next supply enabled. Size vs power consumption Trying to squeeze all the new features in continuously decreasing board spaces poses a tremendous challenge, especially with consumers demanding no compromises in user experience. Power management ICs are also increasingly integrating more functions in smaller packages, both in terms of dc/dc conversion channels and power control functionality. Naturally, the integration of more power conversion channels results in a significantly higher power dissipation in a single package, introducing additional design concerns. The latest packaging technology, such as leadless QFN, offers the ability to accommodate large die sizes and at the same time dissipate power very efficiently, especially when the bottom pad is soldered on the board. New digitally programmable power supply controllers provide programmable output voltages, power on and off sequencing, individual channel enable control, battery monitoring, UV and OV monitoring on PWM outputs, margining and slew rate control. The integration of active accuracy control, programmable features, and built-in flexibility allows the system designer to create a platform solution that can be easily modified via software without major hardware changes. Combined with reprogrammability, this facilitates rapid design cycles and the proliferation from a base design to future generations of the product.
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