Key points and methods of power management design for mobile phones

With the increasing functionality of mobile phones, users' energy requirements for mobile phone batteries are also increasing, and existing lithium-ion batteries have become increasingly difficult to meet consumer demand for normal usage time. In this regard, 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 energy conversion efficiency and energy saving of batteries.

Although the technology of providing power for mobile phones has been innovated and developed in recent years, it is 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 the main development in mobile phone development and design. One of the challenges.

At the same time, the designer must also understand the consumer's requirements for the mobile phone, which is mainly reflected in the following aspects: First, the small size. This requires increasing the integration of the system, reducing the package size of the components, and reducing the area of ​​the PCB, which may increase the difficulty of solving electromagnetic interference (EMI) in the design. Second, the weight is light. A high-performance battery is required to increase the energy density of the battery in a limited volume and weight. Most mobile phones currently use a single-cell lithium-ion or lithium-polymer battery with a capacity of 850-1000 mAH. Third, the talk time is long. It is required to improve the conversion efficiency of electric energy in the battery during work, reduce the leakage current during standby, and improve the use efficiency. Fourth, the price is cheap. It requires high product integration, low discrete components and low cost. Fifth, the product update is fast. The components are required to be simple and easy to use, easy to design and use, and the hardware and software platforms are unified, which is convenient for adding new functions and features.

Therefore, the power management of the mobile phone should be comprehensively considered in the design of the mobile phone system, balancing the various factors such as power saving, cost, volume and development time, and making the best choice. In general, the overall power management of the mobile phone can be started from the aspects of improving the conversion efficiency of electric energy and improving the efficiency of using electric energy.

First, improve the conversion efficiency of electrical energy

With the ever-increasing demands on power management, the power conversion in handheld devices has gradually moved from the conventional linear power supply to the switching power supply. But not a switching power supply can replace everything, both have their own advantages and disadvantages, suitable for different occasions.

Linear power supply

LDOs feature low cost, small package size, low peripheral components and low noise. When the output current is small, the cost of the LDO is only a fraction of that of the switching power supply. LDO packages range from SOT23 to SC70, QFN, to WCSP wafer level chip packages, making them ideal for use in handheld devices. For fixed voltage output applications, only 2 to 3 small capacitors are required on the periphery to form the entire solution.

Ultra-low output voltage noise is the biggest advantage of LDO. However, the disadvantage of LDO is its low efficiency and can only be used in the case of buck. The efficiency of the LDO depends on the ratio of the output voltage to the input voltage: η = Vout / Vin. In the case of an input voltage of 3.6V (single-cell lithium battery), the efficiency is 90.9% when the output voltage is 3V, and the efficiency is reduced to 41.7% when the output voltage is 1.5V. Such low efficiency not only wastes a lot of power when the output current is large, but also causes the chip heat to affect the stability of the system.

Switching power supply

Inductive switching power supplies use inductors as the primary energy storage component to provide continuous current to the load. With a different topology, this power supply can perform buck, boost, and voltage reversal functions.

Inductive switching power supplies have very high conversion efficiencies. The main power loss during product operation includes: conduction loss of built-in or external MOSFET, mainly related to duty cycle and on-resistance of MOSFET; dynamic loss, including switching loss when high-side and low-side MOSFETs are simultaneously turned on And the power loss of driving the MOSFET switching capacitor is mainly related to the input voltage and the switching frequency; the static loss is mainly related to the leakage current inside the IC.

These losses are relatively small when the current load is large, so the inductive switching power supply can achieve 95% efficiency. However, when the load is small, these losses become relatively large, affecting efficiency. At this time, the conduction loss and the dynamic loss are generally reduced in two ways. One is the PWM mode: the switching frequency is constant, and the duty ratio is adjusted. The second is the PFM mode: the duty cycle is relatively fixed and the switching frequency is adjusted.

The disadvantage of the inductive switching power supply is that the overall area of ​​the power supply scheme is large (mainly inductance and capacitance), and the ripple of the output voltage is large. Care must be taken when laying out PCBs to avoid electromagnetic interference (EMI).

In order to reduce the need for large inductors and large capacitors and to reduce ripple, it is very effective to increase the switching frequency.

Capacitive switching power supply

The charge pump uses a capacitor as an energy storage component, and its internal switch transistor array controls the charge and discharge of the capacitor. In order to reduce the EMI and voltage ripple caused by the switch, many ICs use a dual charge pump structure. The charge pump also performs the functions of boost, buck, and reverse voltage.

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

Due to the limitation of the storage capacitor, the output voltage generally does not exceed 3 times the input voltage, and the output current does not exceed 300 mA. The charge pump characteristics are between the LDO and the inductive switching power supply. They have high efficiency and relatively simple peripheral circuit design. The EMI and ripple characteristics are centered, but there are limits on output voltage and output current.

Second, improve the efficiency of the use of electrical energy

In mobile phones, reducing the waste of energy and using as much of the available electrical energy as possible for the actual needs is the key to saving electricity.

Key points and methods of power management design for mobile phones

Handheld device power system general structure

Signal processing system

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

Method 1: Partition Management. Turn off the functional units that are not needed when processing a task, such as turning off the interface with external communication or putting it to sleep when performing internal calculations. In order to achieve this goal, the signal processor in the mobile phone often involves a lot of internal clocks, which control the working state of different functional units. In addition, power circuits that supply different functional blocks can be turned off.

Method 2: Change the operating frequency and operating voltage of the signal processor. Most of the current signal processors are manufactured in CMOS technology. In CMOS circuits, the largest power loss is the loss caused by driving the gate of the MOSFET. It can be seen that the power loss is proportional to the frequency and the input voltage, which is the square of the IC's supply voltage. Therefore, for different calculations and tasks, reducing the frequency and supply voltage to an appropriate value can effectively reduce the power loss.

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

Audio power amplification section

The audio power amplifier is another energy-consuming consumer in the mobile phone, with an output power of up to 750mW, and up to 2W for mobile phones with hands-free function. How to improve the efficiency of the amplifier? The traditional technology uses class AB linear amplifiers, and its efficiency varies with output power, preferably only 70%. Using a Class D power amplifier, PWM can be used to increase efficiency to 85-90%.

Currently, in order to make it easier for designers to manage power, some vendors have developed power management software for embedded operating systems. Using such an 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 to balance the system's various factors. With the development of semiconductor technology and circuit design technology, more and more energy-saving technologies will emerge, which will help the continuous development of handheld products.

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