Discussion on the overall power management technology and solutions for mobile phones

As mobile phones have more and more functions, users' energy demands for mobile phone batteries are also increasing. Existing lithium-ion batteries are increasingly unable to meet consumers' requirements for normal usage time. In response to this, the industry mainly adopts two methods: one is to develop new battery technologies with higher energy density, such as fuel cells; the other is to work hard on the energy conversion efficiency and energy saving of batteries.

Although there have been many innovations and developments in the technology for providing power to mobile phones in recent years, it is still far from meeting the needs of the development of mobile phone functions. Therefore, how to improve power management technology and extend battery life has become one of the main challenges in mobile phone development and design.

The time distribution ratio of a fully functional mobile phone during normal operation is roughly as shown in Figure 1:

In addition, there are many other functions that cause power consumption. Such a large amount of power consumption is undoubtedly huge for a mere 900mAH battery. Therefore, in the case that the current new battery technology is not mature enough, if you want to extend the working time of the mobile phone as much as possible, you can only do something about power management.

However, designers must first understand consumers' requirements for mobile phones, which are mainly reflected in the following aspects: First, small size. This requires improving the system's integration, reducing the packaging volume of components, and reducing the area of ​​PCB boards, which may increase the difficulty of solving electromagnetic interference (EMI) in the design. Second, light weight. It is required to use high-efficiency batteries to increase the energy density of batteries under limited volume and weight. At present, most mobile phones use single-cell lithium-ion or lithium-polymer batteries with a capacity of 850~1000mAH. Third, long talk time. It is required to improve the conversion efficiency of the battery's electrical energy during work, reduce the leakage current during standby, and improve the use efficiency. Fourth, low price. The product solution is required to have high integration, few discrete devices and low cost. Fifth, fast product updates. It is required that the components are simple and easy to use, easy to design and use, and the hardware and software platform is unified to facilitate the addition of new functions and features.

Therefore, the power management of mobile phones should be comprehensively considered when designing the mobile phone system solution, balancing multiple factors such as power saving, cost, volume and development time to make the best choice. In general, the overall power management of mobile phones can be carried out from two aspects: improving the conversion efficiency of electric energy and improving the efficiency of electric energy use.

1. Improve the conversion efficiency of electric energy

With the increasing requirements for power management, the power conversion in handheld devices has gradually shifted from the previous linear power supply to the switching power supply. However, switching power supplies cannot replace everything. Both have their own advantages and disadvantages and are suitable for different occasions.

Linear Power Supply - LDO (Low Dropout Regulator)

LDO has the characteristics of low cost, small package, few peripheral devices and low noise. When the output current is small, the cost of LDO is only a fraction of that of switching power supply. The package of LDO ranges from SOT23 to SC70, QFN, and even WCSP (wafer chip level package), which is very suitable for use in handheld devices. For the use of fixed voltage output, only 2 to 3 small capacitors are needed to form the whole solution.

Ultra-low output voltage noise is the biggest advantage of LDO. TI's TPS793285 has an output voltage ripple of less than 35μVrms and a very high signal-to-noise rejection ratio (PSRR=70dB at 10kHz), making it very suitable for use as a power supply circuit for noise-sensitive RF and audio circuits. At the same time, in linear power supplies, there is no electromagnetic interference (EMI) caused by large current changes during switching, so it is easy to design.

However, the disadvantage of LDO is its low efficiency and it can only be used for voltage reduction. The efficiency of LDO depends on the ratio of output voltage to input voltage: η=Vout/Vin. When the input voltage is 3.6V (single lithium battery), the efficiency is 90.9% when the output voltage is 3V, and when the output voltage is 1.5V, the efficiency drops to 41.7%. Such low efficiency will not only waste a lot of energy when the output current is large, but also cause the chip to heat up and affect the stability of the system.

Switching power supply - divided into inductive switching power supply and capacitive switching power supply

1. Inductive switching power supply

Inductive switching power supply uses inductance as the main energy storage element to provide continuous current to the load. Through different topological structures, this power supply can complete the functions of voltage reduction, voltage increase and voltage reversal.

Inductive switching power supplies have very high conversion efficiency. The main power losses when the product is working include: 1) the conduction loss of the built-in or external MOSFET, which is mainly related to the duty cycle and the on-resistance of the MOSFET; 2) dynamic loss, including the switching loss when the high-side and low-side MOSFETs are turned on at the same time and the power loss of the switch capacitor driving the MOSFET, which is mainly related to the input voltage and switching frequency; 3) static loss, which is mainly related to the leakage current inside the IC.

When the current load is large, these losses are relatively small, so the inductive switching power supply can achieve 95% efficiency. However, when the load is small, these losses will become relatively large, affecting the efficiency. At this time, there are generally two ways to reduce the conduction loss and dynamic loss. One is the PWM mode: the switching frequency remains unchanged and the duty cycle is adjusted. The other is the PFM mode: the duty cycle is relatively fixed and the switching frequency is adjusted.

The disadvantages of inductive switching power supplies are that the overall area of ​​the power supply solution is large (mainly inductors and capacitors), the output voltage ripple is large, and special care must be taken when laying out the PCB to avoid electromagnetic interference (EMI).

In order to reduce the need for large inductors and capacitors and reduce ripple, increasing the switching frequency is a very effective way. TI's TPS62040 has a switching frequency of 1.2MHz. When the output current is 1.2A, the external inductor only needs 6.2μH. In the future, TI will also launch products with higher switching frequencies.

2. Capacitive switching power supply - charge pump

The charge pump uses capacitors as energy storage elements, and the switch array inside controls the charging and discharging of the capacitors. In order to reduce EMI and voltage ripple caused by switching, many ICs use a dual charge pump structure. The charge pump can also perform the functions of boosting, bucking, and reversing voltage.

Due to the internal structure of the charge pump, when the output voltage is a certain multiple of the input voltage, such as 2 or 1.5 times, the highest efficiency can reach more than 90%. However, the efficiency will change with the proportional relationship between the two, and sometimes the efficiency can be as low as less than 70%. Therefore, the designer should try to use the best conversion working conditions of the charge pump.

Due to the limitation of energy storage capacitor, the output voltage generally does not exceed 3 times of the input voltage, and the output current does not exceed 300mA.

The charge pump characteristics are between LDO and inductive switching power supply, with higher efficiency and relatively simple peripheral circuit design. The EMI and ripple characteristics are in the middle, but there are limitations on output voltage and output current.

2. Improve the efficiency of electricity use

In mobile phones, reducing energy waste and using as much available power as possible where it is actually needed is the key to saving power.

Signal processing system

The signal processing system (mainly the signal processor) is the core part of the mobile phone. It is like the human heart and will always work. Therefore, it is also a major source of power consumption in the mobile phone. So how can we improve its efficiency? Generally speaking, the following two methods can be used.

Method 1: Partition management. Turn off the functional units that are not needed when processing a certain task. For example, when performing internal calculations, turn off the interface for external communication or put it into sleep mode. To achieve this goal, the signal processor in the mobile phone often involves many internal clocks to control the working status of different functional units. In addition, the power supply circuits that power different functional blocks can be turned off.

Method 2: Change the operating frequency and voltage of the signal processor. Currently, most signal processors are manufactured using CMOS technology. In CMOS circuits, the largest power loss is the loss caused by driving the MOSFET gate, and its magnitude is Ploss=Cgf(Iout)Vin2, where Cg is the gate capacitance and f is the frequency. It can be seen that the power loss is proportional to the frequency and input voltage, that is, the square of the IC's power supply voltage. Therefore, for different operations and tasks, reducing the frequency and power supply voltage to appropriate values ​​can effectively reduce power loss.

TI's DVS (Dynamic Voltage Scaling) technology effectively connects the processor (such as OMAP) and the power converter into a closed-loop system, dynamically adjusting the supply voltage through buses such as I2C, while adjusting its own frequency. The TPS65010 integrates charging circuits, inductive DCDC and LDO. At the same time, it can also adjust the output voltage of each channel through the I2C bus, which is very suitable for powering OMAP and similar processors.

Audio power amplifier

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? Traditional technology uses class AB linear amplifiers, whose efficiency varies with output power and is only 70% at best. Using class D power amplifiers and PWM can increase efficiency to 85-90%. For example, TPA2010D1 can output 2W of power with an efficiency of 90%.

At present, in order to make it easier for designers to manage power, some manufacturers have developed power management software for embedded operating systems. Using this type of operating system can effectively reduce the workload in software compilation and optimize the power management of the system.

Power management is becoming increasingly important for handheld devices. An efficient system is to integrate the concept of power management into every aspect of the design and balance the various factors of the system. With the development of semiconductor technology and circuit design technology, more and more energy-saving technologies will emerge to help the continuous development of handheld products.

As mobile phones have more and more functions, users' energy demands for mobile phone batteries are also increasing. Existing lithium-ion batteries are increasingly unable to meet consumers' requirements for normal usage time. In response to this, the industry mainly adopts two methods: one is to develop new battery technologies with higher energy density, such as fuel cells; the other is to work hard on the energy conversion efficiency and energy saving of batteries.

Although there have been many innovations and developments in the technology for providing power to mobile phones in recent years, it is still far from meeting the needs of the development of mobile phone functions. Therefore, how to improve power management technology and extend battery life has become one of the main challenges in mobile phone development and design.

The time distribution ratio of a fully functional mobile phone during normal operation is roughly as shown in Figure 1:

In addition, there are many other functions that cause power consumption. Such a large amount of power consumption is undoubtedly huge for a mere 900mAH battery. Therefore, in the case that the current new battery technology is not mature enough, if you want to extend the working time of the mobile phone as much as possible, you can only do something about power management.

However, designers must first understand consumers' requirements for mobile phones, which are mainly reflected in the following aspects: First, small size. This requires improving the system's integration, reducing the packaging volume of components, and reducing the area of ​​PCB boards, which may increase the difficulty of solving electromagnetic interference (EMI) in the design. Second, light weight. It is required to use high-efficiency batteries to increase the energy density of batteries under limited volume and weight. At present, most mobile phones use single-cell lithium-ion or lithium-polymer batteries with a capacity of 850~1000mAH. Third, long talk time. It is required to improve the conversion efficiency of the battery's electrical energy during work, reduce the leakage current during standby, and improve the use efficiency. Fourth, low price. The product solution is required to have high integration, few discrete devices and low cost. Fifth, fast product updates. It is required that the components are simple and easy to use, easy to design and use, and the hardware and software platform is unified to facilitate the addition of new functions and features.

Therefore, the power management of mobile phones should be comprehensively considered when designing the mobile phone system solution, balancing multiple factors such as power saving, cost, volume and development time to make the best choice. In general, the overall power management of mobile phones can be carried out from two aspects: improving the conversion efficiency of electric energy and improving the efficiency of electric energy use.

1. Improve the conversion efficiency of electric energy

With the increasing requirements for power management, the power conversion in handheld devices has gradually shifted from the previous linear power supply to the switching power supply. However, switching power supplies cannot replace everything. Both have their own advantages and disadvantages and are suitable for different occasions.

Linear Power Supply - LDO (Low Dropout Regulator)

LDO has the characteristics of low cost, small package, few peripheral devices and low noise. When the output current is small, the cost of LDO is only a fraction of that of switching power supply. The package of LDO ranges from SOT23 to SC70, QFN, and even WCSP (wafer chip level package), which is very suitable for use in handheld devices. For the use of fixed voltage output, only 2 to 3 small capacitors are needed to form the whole solution.

Ultra-low output voltage noise is the biggest advantage of LDO. TI's TPS793285 has an output voltage ripple of less than 35μVrms and a very high signal-to-noise rejection ratio (PSRR=70dB at 10kHz), making it very suitable for use as a power supply circuit for noise-sensitive RF and audio circuits. At the same time, in linear power supplies, there is no electromagnetic interference (EMI) caused by large current changes during switching, so it is easy to design.

However, the disadvantage of LDO is its low efficiency and it can only be used for voltage reduction. The efficiency of LDO depends on the ratio of output voltage to input voltage: η=Vout/Vin. When the input voltage is 3.6V (single lithium battery), the efficiency is 90.9% when the output voltage is 3V, and when the output voltage is 1.5V, the efficiency drops to 41.7%. Such low efficiency will not only waste a lot of energy when the output current is large, but also cause the chip to heat up and affect the stability of the system.

Switching power supply - divided into inductive switching power supply and capacitive switching power supply

1. Inductive switching power supply

Inductive switching power supply uses inductance as the main energy storage element to provide continuous current to the load. Through different topological structures, this power supply can complete the functions of voltage reduction, voltage increase and voltage reversal.

Inductive switching power supplies have very high conversion efficiency. The main power losses when the product is working include: 1) the conduction loss of the built-in or external MOSFET, which is mainly related to the duty cycle and the on-resistance of the MOSFET; 2) dynamic loss, including the switching loss when the high-side and low-side MOSFETs are turned on at the same time and the power loss of the switch capacitor driving the MOSFET, which is mainly related to the input voltage and switching frequency; 3) static loss, which is mainly related to the leakage current inside the IC.

When the current load is large, these losses are relatively small, so the inductive switching power supply can achieve 95% efficiency. However, when the load is small, these losses will become relatively large, affecting the efficiency. At this time, there are generally two ways to reduce the conduction loss and dynamic loss. One is the PWM mode: the switching frequency remains unchanged and the duty cycle is adjusted. The other is the PFM mode: the duty cycle is relatively fixed and the switching frequency is adjusted.

The disadvantages of inductive switching power supplies are that the overall area of ​​the power supply solution is large (mainly inductors and capacitors), the output voltage ripple is large, and special care must be taken when laying out the PCB to avoid electromagnetic interference (EMI).

In order to reduce the need for large inductors and capacitors and reduce ripple, increasing the switching frequency is a very effective way. TI's TPS62040 has a switching frequency of 1.2MHz. When the output current is 1.2A, the external inductor only needs 6.2μH. In the future, TI will also launch products with higher switching frequencies.

2. Capacitive switching power supply - charge pump

The charge pump uses capacitors as energy storage elements, and the switch array inside controls the charging and discharging of the capacitors. In order to reduce EMI and voltage ripple caused by switching, many ICs use a dual charge pump structure. The charge pump can also perform the functions of boosting, bucking, and reversing voltage.

Due to the internal structure of the charge pump, when the output voltage is a certain multiple of the input voltage, such as 2 or 1.5 times, the highest efficiency can reach more than 90%. However, the efficiency will change with the proportional relationship between the two, and sometimes the efficiency can be as low as less than 70%. Therefore, the designer should try to use the best conversion working conditions of the charge pump.

Due to the limitation of energy storage capacitor, the output voltage generally does not exceed 3 times of the input voltage, and the output current does not exceed 300mA.

The charge pump characteristics are between LDO and inductive switching power supply, with higher efficiency and relatively simple peripheral circuit design. The EMI and ripple characteristics are in the middle, but there are limitations on output voltage and output current.

2. Improve the efficiency of electricity use

In mobile phones, reducing energy waste and using as much available power as possible where it is actually needed is the key to saving power.

Signal processing system

The signal processing system (mainly the signal processor) is the core part of the mobile phone. It is like the human heart and will always work. Therefore, it is also a major source of power consumption in the mobile phone. So how can we improve its efficiency? Generally speaking, the following two methods can be used.

Method 1: Partition management. Turn off the functional units that are not needed when processing a certain task. For example, when performing internal calculations, turn off the interface for external communication or put it into sleep mode. To achieve this goal, the signal processor in the mobile phone often involves many internal clocks to control the working status of different functional units. In addition, the power supply circuits that power different functional blocks can be turned off.

Method 2: Change the operating frequency and voltage of the signal processor. Currently, most signal processors are manufactured using CMOS technology. In CMOS circuits, the largest power loss is the loss caused by driving the MOSFET gate, and its magnitude is Ploss=Cgf(Iout)Vin2, where Cg is the gate capacitance and f is the frequency. It can be seen that the power loss is proportional to the frequency and input voltage, that is, the square of the IC's power supply voltage. Therefore, for different operations and tasks, reducing the frequency and power supply voltage to appropriate values ​​can effectively reduce power loss.

TI's DVS (Dynamic Voltage Scaling) technology effectively connects the processor (such as OMAP) and the power converter into a closed-loop system, dynamically adjusting the supply voltage through buses such as I2C, while adjusting its own frequency. The TPS65010 integrates charging circuits, inductive DCDC and LDO. At the same time, it can also adjust the output voltage of each channel through the I2C bus, which is very suitable for powering OMAP and similar processors.

Audio power amplifier

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? Traditional technology uses class AB linear amplifiers, whose efficiency varies with output power and is only 70% at best. Using class D power amplifiers and PWM can increase efficiency to 85-90%. For example, TPA2010D1 can output 2W of power with an efficiency of 90%.

At present, in order to make it easier for designers to manage power, some manufacturers have developed power management software for embedded operating systems. Using this type of operating system can effectively reduce the workload in software compilation and optimize the power management of the system.

Power management is becoming increasingly important for handheld devices. An efficient system is to integrate the concept of power management into every aspect of the design and balance the various factors of the system. With the development of semiconductor technology and circuit design technology, more and more energy-saving technologies will emerge to help the continuous development of handheld products.