Making the Most of Every Coulomb: The Art of Power Management in the Portable Era

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Making the Most of Every Coulomb: The Art of Power Management in the Portable Era [Copy link]

The rich functions of portable products give people a new way to entertain themselves. Things that originally needed to be done in a specific place, such as playing games, watching movies, listening to music, browsing the news, sending emails, and making reports, can now be done anytime and anywhere. However, it is very annoying that almost every time you are in high spirits, the reminder of "low battery, please charge in time" often comes unexpectedly... With the explosive growth of the portable device market and the continuous emergence of new "energy-hungry" functions, the power supply and power management semiconductor industry is experiencing an unprecedented technical challenge.

For any new generation of portable products, consumers want them to work longer between charges. New features such as enhanced multimedia or high-resolution color graphics require more power consumption, strong dynamic compression, codecs, high-performance audio and video implementation, and continuous communication connections, which require the computing power of the entire system to be several times higher than that of previous simple applications. At present, it is not realistic for new battery technologies such as fuel cells that can significantly extend the service life to enter large-scale commercial use in the foreseeable few years, and the energy density of lithium-ion batteries is also very limited.

Jeff Ju, Technical Affairs Engineer at Fairchild Semiconductor.

To avoid the product size increasing as the battery increases, engineers often have to squeeze more efficiency out of the battery-driven system and further miniaturize the product through more integrated designs. Jeff Ju, technical marketing engineer at Fairchild Semiconductor, well explains the current situation we are in: "The continuous enrichment of product functions will require more power consumption than the battery itself can provide. The advancement of battery technology is relatively slow compared to the increase in portable product functions and user needs, so product design must become smarter and more skilled to save power effectively. The most effective design countermeasure is for semiconductor suppliers to provide more sophisticated power management technology solutions."

There is a slightly exaggerated but not unreasonable saying in the industry: "The design of portable products is the design of power supply". This shows the importance of power management in portable applications. Liang Weiquan, the Asia Pacific RF product technology marketing and application manager of Freescale Semiconductor's Wireless and Mobile Systems Division, pointed out that considering the specific details of power management early is the prerequisite for the successful design of portable products, which helps to establish the hardware and software foundation for power saving, so as to obtain the best battery life.

Two keys to improving battery life

Generally speaking, the two keys to improving battery life are to improve the efficiency of power supply and reduce the power consumption of load. To improve the efficiency of power supply is to choose a suitable voltage conversion circuit. To reduce system power consumption, two indicators are mainly considered, namely working current and quiescent current. For working current, generally, chips or components with lower power consumption are used in design, and the load of circuit board is minimized as much as possible during wiring. Many power chips can be controlled by microprocessors, and different current outputs are selected according to current consumption. For quiescent current, chips with lower quiescent current are generally selected, all unused chips are put into sleep mode, and all unused IO ports of microprocessors are turned off. For example, when there is sufficient light, the display backlight is automatically turned off, or the audio chip is put into sleep standby mode when the headphones are not plugged in, etc.

Tony Armstong , Power Products Sales Manager at Linear Technology .

Improving power efficiency is a specialized technology, mostly composed of hardware circuits, such as efficient switching power supplies (DC-DC conversion), but it is only a part of power management, not all of it. At present, in order to make it easier for designers to manage power, some manufacturers have also developed power management software for embedded operating systems. Using such operating systems can effectively reduce the workload in software compilation and optimize the power management of the system. There are various options for reducing the power consumption of the load. In addition to reducing the system voltage and selecting low-power devices, there are also measures such as adjusting the working state according to the system operation, such as controlling the device to enter standby or sleep state during idle periods, and using a lower system operating frequency as much as possible while ensuring the working speed. Therefore, power management includes both the power conversion efficiency issue and the load power consumption reduction issue.

Although the increase in the functions of portable products is not proportional to the increase in battery power density, it seems to be an "impossible task" to make the battery playback time meet the requirements of consumers. However, facing the challenge, Zhang Hongwei, a senior product marketing engineer of TI's high-performance analog product marketing department, showed rare optimism. Based on TI's power management technology, he introduced five "magic tricks" to solve the "power gap" problem. The first is NMOS technology. He said that P-channel MOSFET is widely used because of its easy driving, but the problem of large on-resistance is also widely criticized. The company uses a built-in charge pump in a variety of DC-DCs and LDOs to drive NMOS and improve efficiency. The second is to equip some power products with a bus interface so that the system can dynamically increase or decrease the voltage according to performance requirements to maximize energy savings.

The third "magic" is the patented drive technology. The predictive gate switching technology realizes the dead time of the half-bridge arm MOSFET to swing between 0 and 5 ns, and the true drive technology realizes the maximum drive current at the conversion moment. These two technologies, combined with synchronous rectification, can greatly improve the efficiency of the PWM conversion circuit. Charging management is the so-called fourth "magic". Zhang Hongwei introduced that the two major features of 0.5% termination voltage accuracy and "gradual stop counter" enable TI's charging products to charge 6 to 8% more power. The last magic is power metering and buck-boost variable topology. He said that the judgment of battery exhaustion is no longer based on battery voltage but on coulomb counters, which can gain up to 20% more power in situations with large load changes. And with the latest buck-boost variable topology, the battery voltage can work to 2.7V, which will gain an additional 10% of power compared to the usual 3.0V cutoff.

In short, there are two main focuses for extending battery life: improving power conversion efficiency at all levels and reducing load power consumption. In terms of specific solution implementation, leading power semiconductor suppliers have their own strengths and focus areas.

Improve power conversion efficiency

In the power management system of new battery-powered products with rich functions, it is often necessary to perform multiple step-up and step-down conversions on the input voltage to meet the needs of various functional blocks. For mobile phones, the display backlight, processor, memory, flash, etc. require different power supply voltage inputs. How to reduce the waste in the input-output conversion process, that is, to further improve the conversion efficiency, is the most important focus of portable device power management. For example, the all-round energy-saving technology of National Semiconductor's PowerWise Interface (PWI) connects the power feeding system with the power consumption system so that the two systems can communicate with each other, thereby greatly improving power efficiency.

Boxun Chen, Product Manager of Analog Products Division at ON Semiconductor .

Another increasingly prominent development trend is to use switching power supplies to replace linear regulators, because this can greatly improve conversion efficiency, thereby extending the life of the battery. In the view of Chen Boxun, product manager of the analog products department of ON Semiconductor, improving power conversion efficiency is still the direction of further efforts. According to him, the company has made preparations in this regard several years ago. For the recently launched series of DC-DC products, conversion efficiency above 90% is already an established standard. For example, the NCP1508/09/10A/11 series of step-down DC-DCs have all reached 93% efficiency.

"On the other hand, from the perspective of battery power supply, ON Semiconductor also aims at its discharge characteristics and tries to squeeze out the last electron in the battery." Chen Boxun said that for lithium batteries, the lowest reliable voltage can be as low as 2.7V, but ON Semiconductor's IC can work normally at a voltage as low as 2.5V. In view of the discharge characteristics of alkaline batteries and nickel-metal hydride rechargeable batteries, ON Semiconductor has also developed many boost chips, and the operating voltage can be lower than 1V. The latest new chip NCP1423 can work as low as 0.8V, and even with a 150mA load, it can still start at less than 1V. These characteristics are just right for single-battery powered products, such as MP3, wireless optical mouse, etc.

One solution from Fairchild Semiconductor is to provide 95% efficiency and 1.5A continuous load current through a 1.5A DC-DC step-down device. These synchronous PWM current-mode DC-DC converter devices have integrated MOSFETs and are packaged in a tiny six-pin 3×3 mm MLP package. These converter devices have adjustable output voltage (from 0.8 volts to the input amplitude), providing flexibility for a wide range of DC/DC applications. The higher fixed operating frequency (1.3MHz) helps to use smaller external components. The high operating frequency and ultra-small package can produce space-saving and cost-effective designs. The company's Jeff Ju pointed out, "Looking to the future, system designers must prioritize power management technology and products that help save power consumption in their design thinking."

Reduce load power consumption

Chen Yongxin, display product marketing business manager for National Semiconductor Asia Pacific .

In the case that the power density of existing batteries cannot be significantly improved, reducing the load power consumption of various functions of portable products is an important guarantee for increasing battery life. For example, in a fully functional mobile phone, the various functions are ranked by power consumption, and they are speakers, color displays, flash, cameras, calls, MP3, etc. Among them, the first three are the largest energy consumers, consuming more than 300 milliwatts of power, and the output power of the speaker with a hands-free function is even as high as 2 watts. Therefore, how to further transform and upgrade the power management of these high-power consumption functions has become one of the important directions for extending the battery life of portable systems.

1. Display system is a key area of energy saving. Color LCD display has become a standard configuration for many portable products. The display screen is getting clearer and clearer, and the picture color is brighter and more beautiful than before. In addition, the corresponding new functions require the display screen to be used for longer and longer periods of time. How to effectively support this demand has become a major issue for power management device manufacturers. The technical improvements made for energy saving involve almost every corner of the display system. The display backlight is the circuit with the highest power consumption in the system, and its power consumption is more than ten times that of the power supplied to the display. More power usually means that the color LCD of a portable device can emit stronger light, but due to increasingly stringent power restrictions, high-brightness output displays are not practical. Using energy-efficient white LED backlights to maximize the user's overall visibility remains the most popular choice.

LCDs require different backlighting methods, including electroluminescence, cold fluorescent, and light-emitting diodes. Among them, the LED method can significantly reduce power consumption due to the reduction of external components such as converters. In addition, a variety of LED drive schemes use pulse width modulation to reduce the average current of the LED, thereby reducing the power required by the drive circuit. Currently, manufacturers are developing more advanced LED charge pump products to provide higher backlight power to meet the color screen display requirements of larger and more complex portable/wireless devices. Tony Armstrong, product sales manager of the power products department of Linear Technology, said: "The market requires that current LED charge pump devices must provide backlight power for the main and secondary screen displays and at least one RGB display."

The latest backlight LED driver products include TI's REG710x series of switched capacitor buck-boost converters in a 3×3mm SOT-23 package that can provide 30mA of current to white LEDs in single-cell lithium-ion battery applications. This inductorless charge pump is ready to replace a boost converter with a separate LDO regulator that performs the same task, and can provide voltages of 2.5, 2.7, 3, 3.3, 5 or 5.5V. The LT3465 from Linear Technology has built-in Schottky diodes and can drive up to 6 white LEDs from a 3.6V supply. The SP6691 boost converter from Sipex can drive up to 8 white LEDs, uses a 5-pin SOT23 package, integrates a 30V power switch, and can manage currents up to 450mA.

Maxim's MAX6965 RGB and white LED driver has an SMB interface and provides nine output ports, each of which can reduce the current to 50mA. Users can use its 8-bit PWM brightness control to set up to 240 brightness levels. In addition, Intersil has launched a high-frequency pulse width modulation boost regulator EL7513, which can drive 4 series LEDs or 12 series/parallel LEDs. The charge pump regulator ADM8839 launched by Analog Devices is specially designed for micro TFT LCDs, generating the voltages (+5V, +15V and -15V) required for LCD controllers and panel transistor gate drivers from a 3V power supply.

In addition, efforts to save power consumption are also reflected in the driver chip of the display system, such as the preview system of a digital camera. According to the current design, even in the preview stage, all image data must be sent to the controller first and then sent back to the display. This back and forth data transmission is completely unnecessary and wastes energy because the image data will be deleted after the preview. Chen Yongxin, business manager of the display product market in the Asia-Pacific region of National Semiconductor, specifically pointed out that the LCD driver launched by the company can support a wider video bandwidth and has a built-in 2D scaler, so that the camera can directly establish a connection with the display module when executing the preview function to transmit data.

This design ensures that the baseband processor and articulation position can be completely idle when the camera is performing the preview function, thereby further saving power. National Semiconductor has also launched a new TFT LCD display driver with intelligent backlight control function. Whenever the brightness contrast of the picture is large, the driver will automatically reduce the backlight brightness. This will significantly save the power consumption of the backlight system without adding additional cost to the system.

Doug Chaffee, director of engineering for Microchip's Standard MCU and Technology Division .

2. Reduce the power consumption of the audio part

Audio power amplifiers are another major energy consumer in mobile phones, with output power up to 750mW, and up to 2W for mobile phones with hands-free functions. How to improve the efficiency of the amplifier to save power consumption on audio output? A class D audio power amplifier based on digital technology came into being. The power output stage of the class D audio power amplifier works in a switching state, while the power output stage of the traditional class AB analog audio power amplifier works in an amplifying state, so the efficiency of the class D audio power amplifier is higher than that of the class AB audio power amplifier. The emergence of the class D audio amplifier marks the end of the decades of audio power amplification dominated by analog technology. The efficiency of the traditional class AB amplifier varies with the output power, and the efficiency is only 70% in the best case. The use of class D power amplifiers can increase the efficiency to 85-90%.

Chen Boxun of ON Semiconductor said, "In fact, there is not much room for improvement in power management, so ON Semiconductor has also made a lot of efforts in other areas, such as audio amplifiers." According to him, the company's NCP2820 class D power amplifier has increased efficiency to almost 90%, which is about double that of class AB. This is similar to a product launched by TI, whose TPA2010D1 can also achieve 90% efficiency and output power of 2 watts. According to market research company Forward Concepts, global sales of audio amplifier channels will exceed 1 billion between 2004 and 2006. Driven by such a huge market demand and the development trend of various terminal audio devices to be light, thin and small, class D amplifiers are gradually coming to the fore.

3. New processes and technologies

In the emerging field of technology development, the voltage of the core processor is reduced to below 1V by using deep submicron CMOS technology, thereby achieving the goal of reducing chip power consumption. This requires the power conversion chip to provide an output voltage below 1V and a suitable output current, high transmission efficiency, low noise and a small footprint. In this field, several leading semiconductor manufacturers have launched their own products. For example, the TPS62300 DC-DC converter launched by TI uses a 2mm×1mm chip-scale package. This new device has a power conversion efficiency of up to 93%, and the output voltage can be as low as 0.6V, which can support the most advanced digital signal processors and other processors.

"The use of deep submicron CMOS wafer manufacturing processes is reducing the CPU and DSP core voltage to below 1V, which will require a new type of power conversion chip that must accept low voltage input and provide an output voltage far below 1V, with reasonable output current, high conversion efficiency, low noise, flat shape and compact footprint." said Tony Armstrong, product marketing manager of Linear Power Business Unit. The company's LT3020 low-dropout regulator is one of them, which can accept an input voltage of 0.9V to 10V and provide an output current of 100mA and an output voltage as low as 0.2V.

There are also many new technologies that can play a role in reducing power consumption. According to Jeff Ju, Fairchild's IntelliMAXTM switch allows system designers to switch between different portable product loads to save power. At the same time, its operating range of 1.8~5.5 volts allows the above load switches to operate when the battery is at a low voltage. The above-mentioned load switches also have other functions such as low-voltage self-locking, overheating cutoff, less than 1uA standby current and automatic start-up functions. When continuous overcurrent causes overheating, the above load switches can provide overheat protection and shut down the switch to avoid damage to the product. When the current reaches the allowed extreme value, the load switch will operate in a constant current state to prohibit damage caused by overcurrent.

Efforts related to saving power consumption are also reflected in the development of other related analog products. Doug Chaffee, engineering director of Microchip's standard microcontroller and technology department, said that through the development of the PowerSmart series of products, Microchip is committed to solving comprehensive battery management issues. Its power measurement products can very accurately measure the available power of the battery at any stage of life, allowing users to maximize the use of the power in the battery, thereby extending the charging interval and the overall life of the battery. Secondly, the company's battery charging control chip can maximize the battery charging capacity while requiring less charging time.