Power Management Design for Multimedia Smartphones

Mobile phones are integrating more and more functions, but the growth of battery capacity has never kept up with the pace of functional changes. How to increase the usage time of mobile phones within limited capacity? Good power management and improved process technology are both effective ways to reduce mobile phone power consumption.

Traditional mobile phones that are only used for communication functions can no longer meet the application needs of consumers. MP3 function phones have become necessary equipment, and multiple multimedia function designs have become the functional demands that industry players highlight in the 3G mobile phone era.

Mobile phones with camera functions require complex camera engines and high-brightness flashes. As the bandwidth of wireless communications increases, the application of high-speed processors can provide audio and video processing capabilities such as Bluetooth wireless transmission, satellite positioning, mobile Internet access, digital TV, voice signal encoding/decoding, etc.

As a result of the increasing number of functional requirements, the load on mobile phone batteries is bound to be heavier, and the internal power design of mobile phones has become more complicated, so more suitable power-saving technologies are needed to meet the various functional requirements of mobile phones. Obviously, how to integrate a large number of functions in a small space and integrate appropriate high-performance analog and digital components is the most fundamental approach.

Furthermore, at this stage, when it is impossible to increase the battery capacity of mobile phones, it becomes more important to effectively reduce the power consumption of digital signal processors. In addition to reducing the driving voltage of digital signal processors through advanced process technology, good power management has become the only way to save mobile phone power consumption and design mobile phones that meet consumers' needs for long-term use.

Multimedia mobile phones, in addition to communication functions, have various audio and video, video, and wireless transmission capabilities, which have become mobile phone features, but also increase battery power consumption, so more power management technology is needed to increase the use time of mobile phones.

Display screen and RF block are the top two power consumption areas

When a mobile phone is generally used, it is in standby mode for more than 90% of the time. However, the screen still needs to display the date, time, battery level, and reception status. The power consumption of the standby mode at this time requires the entire screen to support the display, so the display part becomes a major power consumption problem of the mobile phone.

Currently, most mobile phones use TFT LCD as a display. LCD is a passive display and needs a backlight to achieve the display effect. In addition to the On/Off switch, the backlight design must also adjust the drive current to change the brightness of the LED. The brightness can be adjusted by controlling the forward current to reduce the power consumption of the display. There are two main methods. One is to use a fixed current to drive the LED. The fixed current can eliminate the current change caused by the change in forward voltage and fix the LED brightness. The other method is to use a voltage power supply and a rectifier resistor to determine the voltage required to provide the LED to generate the expected forward current.

In addition to the screen being a major killer of power consumption, another source of power consumption is the RF block. Among the RF components, the power amplifier consumes the most power. The amplifier circuit that emphasizes better output efficiency has been increasingly valued, and its power consumption has also been gradually greatly improved.

As the new generation of mobile phones begins to use lower battery voltages, the RF power amplifier must change its design direction. The current HBT RF power amplifier technology must be suitable for voltages above 3.0 volts, and to achieve high linear RF power output on low-voltage DC power supplies, it must rely on E-pHEMT technology, which can provide mobile phone manufacturers with better power plus efficiency (PAE), low-voltage operation and high reliability... and other unique advantages. Compared with HBT, E-pHEMT technology has better efficiency at high voltages and is more attractive at low bias voltages. Since E-pHEMT components can maintain good linearity and gain performance under low bias conditions below 2 volts, it can avoid the cost and power loss of buck/boost converters, and can also improve battery efficiency and extend mobile phone talk time without adding unnecessary components.

Figure 0.4mm thick ultra-thin integrated circuit packaging technology from National Semiconductor is suitable for thinner and lighter mobile phones, monitors, MP3 players, PDAs and other portable electronic products.

LDO voltage dropout regulator reduces audio power consumption

Various audio functions of mobile phones, such as MP3 playback, polyphonic ringtones, and FM radio, will increase the power consumption of the mobile phone. Therefore, how to optimize the audio circuit to achieve low power consumption design has become an important issue to extend battery life.

To improve audio power consumption, mobile audio components are required to have good noise suppression, low operating voltage, and high power performance. Currently, the most common method is to use a low dropout regulator (LDO) to suppress noise (PSRR). Linear voltage regulation is the most common and easiest to use voltage regulation method. Its advantages are small package size and few external components, which is beneficial to the design of handheld devices with limited circuit area.

LDO is a common DC conversion component in circuit board design. LDO converts high voltage to low voltage. Its working principle is similar to the voltage division principle. It has few pins and can filter power supply noise. Although LDO has low conversion voltage and very low power consumption, it still has some power loss. If the audio amplifier can be increased to more than 60dB, LDO does not need to be applied, which can reduce the power consumption of the audio circuit.

Figure Austrian Microelectronics launches 200mA miniature ultra-low dropout regulator AS1369, which only occupies 1mm2 PCB space and is suitable for space-limited devices such as mobile phones and PDAs. AS1369 LDO can achieve performance improvement and extend battery life.

DC/DC power converter products reduce LDO requirements

In the selection of power management ICs, the terminal market has increasingly stringent requirements on product size and power conversion efficiency. Therefore, LDO linear regulators, which are low-cost but have poor efficiency, are gradually threatened by switching regulators (Switching Regulators) with better power conversion efficiency but higher design and cost thresholds. For example, the impact and threat of the DC/DC Converter market has caused LDO volume to shrink and even lead to price drops. The price competition pressure has caused many power IC manufacturers to turn to the research and development of DC/DC power products.

High-frequency DC/DC converters can only require small inductors or capacitors around the system, thereby reducing the area of ​​the circuit board. As mobile phones and digital cameras (DSCs) develop towards thinner designs, the system's demand for single LDOs will inevitably be reduced. Therefore, the probability of using a single set of LDOs is lower, and many manufacturers have turned to investing in the research and development of DC/DC converters, including Anmao Microelectronics, which mainly operates LDOs, and is also fully investing in the development of DC/DC converters.

Trend of Integrated Power Management Chip

1. Low-end mobile phone baseband chips integrate PMU

PMU is mainly used to solve the power management problem inside the mobile phone. The main goal of developing PMU is to effectively save power for the mobile phone to avoid efficiency conversion loss. Currently, the core spindle baseband chip of low-end mobile phone chips has developed towards the integrated PMU power management mode. Since the integration of PMU in baseband chip can save space in the mobile phone, PMU can have the advantages of not occupying space and high integration, so manufacturers are committed to developing baseband chip integrated PMU solutions.

2. High-end mobile phone application processors integrate PMU

As for high-end mobile phones, such as 3G smart phones equipped with multimedia imaging and wireless transmission functions, their application processors consume more power, so it is necessary to develop dedicated, integrated PMUs to reduce the power consumption of application processors. For example, STMicroelectronics has integrated USB OTG high speed and audio codec to meet the requirements of smart phones for audio sophistication, allowing mobile phones to have fast data transmission functions. National Semiconductor has also developed a highly integrated lighting management unit (LMU) for LEDs to solve the problem of fast power consumption of LEDs.

In addition, the development of power management chips towards customization will become a future trend. In order to improve the flexibility of PMU use and expand the application space, customized specifications should be developed for a certain type of market or processor platform to create market competitiveness for the industry. For example, Texas Instruments has launched a dedicated PMU solution for Samsung's application processors, hoping to increase its product market share by providing customized PMU product services and actively developing catalog PMUs.

STMicroelectronics' mass-produced integrated PMU adopts a modular strategy, integrating as many necessary functions of high-end mobile phones as possible. If a customer suggests that a certain function is inappropriate, it only needs to be turned off while retaining other functions, thus meeting the requirements of different customers for mobile phone positioning.

Figure 1: National Semiconductor's highly integrated light management unit (LMU), with built-in high-voltage boost converter and programmable constant current driver, can control up to 20 series-connected LEDs in the display backlight system, as well as drive the LEDs and red, green and blue (RGB) LEDs in the keypad and camera flash.

Separate power chip design has good flexibility

Since integrated power management chips still have functional design limitations and poor flexibility, they cannot integrate all the additional functions of mobile phones on the market. Once an integrated PMU is selected, if there are defects in the design and production process, it will affect the subsequent operation procedures; discrete power chips have more flexible designs and can be compatible in specifications. There are more choices in functionality and design solutions, which can avoid problems such as a single chip manufacturer affecting design, production, etc.

The various design limitations of integrated power management chips highlight the greater flexibility and variability of discrete power chips. The multi-chip package (MCM) model developed by Torex Semiconductor is a type of discrete power chip. It can perform power management design and function changes for mobile phones with slightly different functions, while integrated power management chips can only be used in a certain model of mobile phone.

The multi-chip package (MCM) model and discrete power management chips such as LDO, DC/DC converters, etc. have great design flexibility, allowing them to keep pace with the market position of integrated PMU. Many industry players have also proposed standardized and modular chip designs. Discrete power management chips can conduct technical research and development on a single function, in order to meet the customization needs of mobile phone manufacturers in the future industry trend of rapid replacement of old phones with new ones and seeking innovation and change.

Among the companies that have proposed separate power management chips, National Semiconductor's Mobile Pixel Link (MPL) physical layer technology manages the power consumption caused by image display pixels to further solve power supply, noise, and stability issues. This technology is mainly used in small displays and digital cameras. MPL can convert RGB data images from the original 24-bit transmission to 18-bit data for transmission, making the image signal transmission path larger, the image clearer, and achieving power saving effects.

Battery capacity management increases usage efficiency

Battery conversion technology continues to improve, but battery capacity still cannot keep up with the needs of mobile phone manufacturers and consumers' satisfaction with mobile phone usage time. Although in recent years, in order to increase the battery life of mobile phones, the gradual improvement of lithium-ion batteries is expected to only slightly increase battery capacity by 20% to 30%.

Battery capacity will not have a breakthrough development in the short term, so it is necessary to properly manage the battery capacity so that the battery can provide a longer use time, and proper battery management is particularly important. For example, battery detection, charging control, battery protection, etc. can effectively control the battery capacity status. Through the battery detection component, the remaining power of the battery can be checked. A simple charge meter is used, and the central processor is responsible for calculating the remaining power. Or through the measurement component of the microcontroller, the remaining power supply time, remaining power, battery voltage, temperature, current, etc. data can be provided, allowing users to more effectively grasp the current status of battery power supply.

Caption: Texas Instruments' battery charge monitoring chip can measure the impact of battery discharge rate, temperature, aging, and other factors on battery impedance, with an accuracy rate of up to 99% in measuring battery life.