Power Electronics

Optimizing Power Systems for Portable Electronic Devices

System architects can optimize power-system efficiency by selecting the best system voltages for the application, using the right power-supply elements, and designing all the power-system elements to work together to reduce battery drain.

Portable electronic devices are a huge part of our everyday lives, with most consumers owning at least one mobile phone and usually at least one other device. Throw in the ubiquitous notebook computer, which — according to iSuppli — will represent 40% of total PC unit shipments by 2010, and this becomes a vast number of portable products. As a result, several types of portable devices, including notebooks, mobile phones, MP3 players and personal digital assistants (PDAs), have become significant drivers of the power management market.

Portable-device design must be capable of meeting the demand for maximum processing power without generating excess heat. While the design of battery-powered power systems with multiple supply voltages has always been a challenge for even the most experienced power-supply engineer, even more challenging is optimizing and integrating power-management systems that also meet requirements for shrinking form factors, increased functionality and higher performance.

By leveraging highly-integrated power-management systems, designers can ensure that all the elements of the power system are optimized to work together and reduce drain on the battery. In portable-device designs, the power system can be optimized by selecting the best system voltages for the application and using the right power supply elements.

ACHIEVING AUTONOMY

Autonomy — the time that the portable device can operate from the battery before recharging is required — is determined by power-system elements such as battery capacity, power-system efficiency and software power management. Autonomy can be increased when all of these power-system elements are working together to reduce the drain on the battery.

Early cell phones used NiCd batteries and four low-dropout regulators (LDO s) to achieve about one hour of talk time. In newer mid- to high-end cell phones, however, the same type of battery system would provide only five to ten minutes of talk time. Today's portable applications require more power from the battery but are bound by stringent space budgets, making the power system design very challenging.

Highly integrated power-management systems with more than 20 power domains for audio, battery charging, communications, housekeeping, and lighting functions are being used in many portable applications today. This creates a challenge for the system architect to route each of the power supplies to components placed in various locations throughout the portable device. Too often, the loads are placed far from the power-management IC (PMIC), resulting in PCB routing issues, line-voltage variations, and increased noise on the power line.

Using a regulator at the point of load (POL) helps reduce the variations and noise problems, but power dissipation in the PMIC can also cause problems. As portable devices evolve, the one-large-PMIC-fits-all philosophy cannot keep up with new power-hungry applications such as Internet browsers.

Conversely, a large PMIC can be overkill for simple voice-only applications, adding unnecessary cost to the system as many of the power supplies would go unused. Peripheral power products augment the PMIC to improve system efficiency.

POWER CONSUMPTION

Power consumption has become the most important design challenge for portable devices. With the continued demand for computation power the use of high-speed, microprocessor-based portable systems continues to grow, and some of today's portable devices have the processing power of a circa-2000 desktop computer, posing performance requirements that impact battery life.

For instance, very high clock speeds are needed to run the complex application-specific software used in a variety of ultra-portable devices including smart phones, portable media players, digital still cameras, personal navigation systems, and portable medical devices. The power system for these devices is very similar to that of smart phones.

Medical devices, for example, use cellular or 802.11 data networks to relay data to the doctor, allowing 24/7 patient monitoring. The system architect has to design a system with long autonomy to meet the mission requirements.

These powerful applications require power-hungry processors which further reduce system autonomy. The balance between power consumption and battery life is a key consideration for portable system designs. Decreasing processor speed reduces power consumption, extending the battery runtime by decreasing the software performance. System architects must optimize the user experience by selecting the optimum processor speed for the application.

Portable systems include some or all of the following components: battery, battery charger, microprocessor, buttons or sliders for man-machine interface (MMI), memory (SDRAM, Flash, micro disk drives), display, LED lighting, and audio, as shown in Fig. 1. Product-specific teardowns available on the Internet verify the similarities between portable media players, portable navigation systems, smart phones, digital still cameras, and medical devices. Analog Devices' portable power devices — such as the ADP2108 step-down dc-dc converter (Fig. 2) — have been designed to work across all portable platforms and can be added as required to address peripheral-power POL requirements.

Lowering the processor speed reduces power consumption, extending the battery runtime while decreasing software performance. The system architect must minimize power consumption while meeting critical performance requirements. These challenges can be met by using high-efficiency dc-to-dc converters to drive LDOs to increase system efficiency and battery autonomy. Fig. 3 shows an ADP2108 driving an ADP170 LDO to extend battery life in applications where the output voltage varies over the discharge cycle. The system efficiency versus a simple LDO will be compared in the following two cases.

CASE 1

An LDO is attached directly to the battery. LDO efficiency is:

Where:

IQ = Quiescent current

VOUT = Output voltage of the LDO

VIN = Input voltage to the LDO

IOUT = LDO output current

The quiescent current, IQ, is the difference between the input current, IIN, and the load current, IOUT, measured at the specified load current. The lower the IQ, the higher the efficiency is at light loads.

IQ is very small compared to IOUT, so it will be ignored in these examples. LDO efficiency then becomes VO/VIN×100%. For a rechargeable Li-Ion battery, the usable output voltage ranges from 4.2 V to 3.0 V. The system will shut down at 3 V. At 4.2 V battery voltage, LDO efficiency = (2.3/4.2)×100% = 55%.

At 3.6 V battery voltage, LDO efficiency is 64%. At 3.0 V battery voltage, LDO efficiency is 77%. Note that the closer the input voltage is to the output voltage, the higher the efficiency. Over the discharge curve of the battery, the average efficiency will be approximately 55% for 20% of operating time (OT), 64% for 60% of OT, and 77% for 20% of OT. The overall time averaged efficiency for an LDO connected to a battery is 65%.

CASE 2

An ADP2108 buck regulator and ADP170 LDO perform a double voltage conversion (Fig. 3). The buck regulator's output voltage is set to 2.5 V regardless of the battery voltage. The LDO efficiency is therefore always (2.3/2.5)×100% = 92%. We can calculate the ADP2108 efficiency using the Analog Buck Designer tool which can be downloaded from the Analog Devices website http://www.analog.com/AnalogBuckDesignerTool.

As shown in Fig. 4, the efficiency of the ADP2108 averages 90% over the input voltage range at 300 mA load current. Thus, the system efficiency (ηeff) = (ηDC × ηLDO ) × 100% = (0.9×0.92) × 100% = 83%.

Using a buck and LDO increases the system efficiency from 65% to 83%, a 28% improvement over a simple LDO. If this type of power-saving technique is applied to a portable system with multiple power rails, an improvement in efficiency at each load will result in increased system efficiency and much longer autonomy.

Even if battery life was not a consideration, power dissipation in the LDO would still be an issue. Power not delivered to the load is dissipated as heat within the LDO. Power dissipated is estimated as: dissipated power (PD) = (VIN - VOUT) × IOUT. Using Case 1 and the worst case VIN = 4.2 V, (PD) = (4.2 - 2.3) × 0.300 = 570 mW.

In this case, when the battery is fully charged and connected to the LDO, over a half-watt of power is wasted as heat, which will cause the temperature of the portable device to rise. If the designer used this technique for all power supplies in the system, the device would end up with short autonomy, although it might make a very nice hand warmer in winter.

TAGS: Content PMICs
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