Power management division of 3G mobile phones

Third generation (3G) mobile phones offer a wide range of features with more functionality. As consumers gain new and better functionality from their communications devices, they continue to demand longer operating time from a single battery and smaller form factors. While IC integration can help address device size issues, it also increases design complexity and limits design flexibility. Today's mobile phone designers must consider a variety of factors to extend battery operating time by effectively optimizing battery power usage. Therefore, a combination of highly integrated power management units and high-performance discrete components must be used to address battery management, power conservation, and system management issues.

Dilemma: Functionality vs. battery power

When designing an advanced wireless device, engineers face a fundamental dilemma. They need to pack a large amount of functionality into a given form factor that is often dictated by the battery and display size, as well as user interface complexity and design ergonomics. In addition, the battery's available energy is determined by chemical characteristics that determine its energy density and physical size. These changing parameters often force designers to more effectively utilize battery power management techniques to meet consumer expectations for device standby and operating time.

Today's 3G feature phones support several air interfaces and can provide multi-band modem connections such as GSM and WCDMA. Other connections can be made through Bluetooth, wireless LAN, infrared and USB interfaces. Digital cameras have become standard equipment in many mobile phones, which require a sophisticated camera engine and a high-luminance flash to take high-quality photos. With higher data transmission speeds, video calling functions can also be realized. In addition, high-speed application processors can provide audio/video processing capabilities to perform digital television (DTV) signals and MPEG audio codecs. Newer phones are also planned to add FM radio and digital TV tuners to increase the entertainment value of the phone. Higher data throughput ultimately requires high-density storage capabilities, which can be achieved through memory expansion slots or (even) micro hard drives. It is not difficult to imagine that these wireless phones also function as handheld gaming devices.

The battery, as an energy source, occupies a central position in the system. Today, almost 100% of 3G mobile phones use lithium-ion batteries because it is the rechargeable battery chemistry that provides the highest energy density. From a size perspective, most batteries have dimensions of approximately 50 x 40 x 5 mm and provide a capacity of 900-1200 mAh. Although fuel cell technology promises to provide higher energy density than lithium-ion batteries in the future, its widespread use is estimated to take several years due to technical and regulatory issues. In addition, gradual improvements in lithium-ion battery technology are expected to increase its capacity by 30%. Therefore, system engineers will basically continue to use lithium-ion batteries that provide approximately 1500-1800 mAh for the time being.

This dilemma will ultimately force digital and analog semiconductor technologies to the next low-power node and drive the development of ultra-efficient battery-use technologies.

Integration and layout issues

Clearly, with all the functionality packed into a relatively small package, a set of high-performance analog and digital components needs to be integrated. To highlight the complexity, Figure 1 shows the main system architecture of a 3G mobile phone.

Figure 1: System block diagram of 3G mobile phone

Figures and words (up, down, left, and right): power management, processing unit, PA and WCDMA transceiver, PA and GSM transceiver, FM receiver, Bluetooth processor, high PSRR LDO, camera sensor, camera flash, camera engine, boost DC/DC, 3MHz buck DC/DC, vibrator-microphone-earphone-earpiece-SIM card, analog baseband processor, baseband Power /audio codec and driver, digital baseband processor, application processor, memory, audio synthesizer, high PSRR LDO, keyboard, display 1, display 1, 3MHz buck DC/DC, white LED driver, white LED driver, battery capacity meter, lithium-ion battery.

But the question is, “What components need to be integrated, and how do you address the issue of where they can be placed in the phone form factor?” The answer is to integrate standard power supplies for the baseband processor, audio subsystem, and interface components, because different phone platforms use similar subsystems and components, and the phone power supply uses the same basic chipset . But there are two major inherent problems. First, industrial design considerations allow phones to be designed in a variety of different ways depending on the required functionality and ergonomics. Today, electrical design needs to consider whether the phone can be designed in the form of a candy bar, a clamshell, or a slider, all with different display, keypad, and speaker configurations. These design differences have a significant impact on the placement of the phone display, camera module, and other subsystems, and to some extent, limit the integration of these components. In some cases, the integration of power or audio functions may mean longer traces, complex PCB layout, or electrical design challenges caused by noise. Second, one should not forget the cost-effective management of phone model families required by phone manufacturers. In order to meet market demand with different phone models, phone manufacturers must provide a variety of features and performance levels at different costs. To achieve the highest margins in a competitive market, the cost of these models must scale with the functionality, which in turn limits the integration of every function onto a large IC. If features are not required for a given model family, certain specific functions and their power supplies should be removed from the board to reduce costs. In addition, mobile phone manufacturers using the same basic chipset also need to differentiate their products from those of other competitors, which in turn drives the reintegration of various different features. A typical example of product differentiation may include (but is not limited to) a brighter camera flash, a more powerful blowtorch mode, Class D stereo audio performance, special display and keypad backlighting effects, MP3 audio playback capabilities, FM radio reception, and accurate battery capacity measurement.

Discrete Power Device Selection

As shown in Figure 1, typical non-integrated power devices that power differentiated sub-devices may be a battery capacity gauge as part of a mobile phone battery case, a high-efficiency but small high-frequency DC/DC core power supply, a high-performance DC/DC boost driver for the camera white flash LED, a white LED backlight driver with organic LED power, a sub-display, and a linear regulator with ultra-low power supply rejection ratio (PSRR). When integrating, first integrate the known features that consumers are familiar with. Leading analog semiconductor technologies with higher performance and efficiency, including optimized discrete power management devices, will be increasingly integrated as shipments increase and functions are standardized. To further optimize power management and extend battery operating time, the following three aspects must be considered. First, battery management must handle battery charging and capacity measurement. Second, power conversion must be able to convert battery power to system usable power as efficiently as possible. Third, system power management, which analyzes the actual power consumption of the processor and controls the power supply, must optimize the efficiency of battery power usage. The first and second aspects can be achieved by selecting the right power management device, while the third aspect is related to major software development on the processing side.

Battery capacity meters are becoming increasingly popular in battery management. Traditionally, battery capacity is measured by measuring the voltage of a lithium-ion battery and then using a capacity lookup table stored in memory to derive a result about the available battery power. However, this approach is not practical due to the complex power consumption characteristics of 3G mobile phones and the changes in lithium-ion battery performance over time, temperature, and load conditions. To accurately measure the remaining battery capacity so that the processor can better manage the power consumption of the mobile phone, high-performance coulomb counters with impedance tracking capabilities and the ability to measure the actual amount of power entering and leaving the battery are used. This allows the processor to effectively deploy battery saving modes, accurately determine when the battery is depleted, and warn the end user when charging is required. Figure 2 shows a coulomb counter integrated in the battery pack and sending parameters to the host processor through the I2C communication interface.

Figure 2: Battery capacity gauge for accurate battery capacity measurement

Figure and text (up and down, left and right): ESD protection, lithium-ion battery protector.

In the area of ​​power conversion, DC/DC converters are playing an increasingly important role in providing efficient solutions for LED driving and processor core power. The resolution of CMOS and CCD sensors continues to increase to improve the performance of digital cameras and video conferencing. Simple physics dictates that with increasing sensor resolution, higher light levels are required to take high-quality photos, which in turn requires solutions with brighter camera flash capabilities. Many of today's mobile phone camera flashes provide much less brightness than toy flashes that drive white LEDs at less than 100 mA . This design does not actually improve the quality of the photos taken. To truly differentiate the product, high-power white LEDs need to be driven at currents close to 1A. This is a current that is difficult to achieve with a charge pump because the corresponding 2A battery current required would exceed any battery power budget reserved by the system for this function (i.e., camera function). To solve this battery current problem, Figure 3 shows a high-efficiency DC/DC step-up converter that can drive 700 mA into a white LED for camera flash applications.

Figure 3: High-efficiency DC/DC boost block for high-brightness camera flash LEDs

Several subsystems in a cell phone may require a precise core supply voltage. Linear regulators are generally considered a small, low-cost solution for voltage regulation. But at currents above 200mA, they begin to require space-consuming and expensive heat sinks because of the high power dissipation. The power dissipation is due to the large input-output voltage difference multiplied by the output current when supplying power, such as a 1.2V/500mA core voltage from a 3.6-V lithium-ion battery. While linear regulators can perform this regulation with 33% efficiency and thus become the main source of battery power burn and heat generation, DC/DC converters can operate well with efficiencies above 90% and consume only a small fraction of the power wasted by LDOs. The latest generation of DC/DC converters, using the most advanced analog process and design techniques, have several space-saving features. Figure 4 shows an ultra-small and high-precision DC/DC step-down converter for up to 500-mA core current. Because both switching transistors are integrated, the circuit requires only one inductor and two small capacitors . The unique control architecture enables the power supply to react very quickly to load transients while retaining high voltage regulation accuracy of +/-1% - as required by today's high-performance processing cores. The 3MHz switching frequency reduces the inductor size to only 1mH, allowing the use of low-profile chip inductors with a height of less than 1mm. The device is also available in chip-scale packaging to reduce the IC size to 2mm x 1mm. The entire solution can be built to fit into a 5 x 5mm2 space. To further optimize power consumption, the advanced DC/DC regulator also features automatic PFM/PWM mode switching capability to improve conversion efficiency over a wide load range. Under light load conditions, the converter enters pulse frequency modulation (PFM) mode, and when the load current is above 50-mA, a pulse width modulation (PWM) control scheme is used, providing a 1.8-V and 500-mA core supply with up to 80%-90% efficiency.

Figure 4: High-frequency 3-MHz DC/DC converter with small components and packaging

in conclusion

Integration of power and other analog components is inevitable, and the key to integration is to select features that have become standards and adopted by various mobile phone platforms. Leading technologies that drive functional differentiation generally first adopt discrete forms with customizable features, which is very important for consumers and mobile phone model family management, which is critical for service providers and mobile phone manufacturers. Power Management Devices are continuing to push size, efficiency and power consumption to the extreme and increasingly play a key role in reducing the size and weight of mobile phones.