Summary:
The ever-increasing number of features puts handset design in the face of more severe power management challenges. This paper analyzes the power management design trends and various corresponding solutions for mobile phones in the future from voltage conversion, voltage regulation, flash circuit, and battery charging.
3G phones can not only browse the web, send e-mails, take digital photos, or even play video streams. Mobile phone manufacturers are facing increasing pressure because they must integrate these functions into a shrinking volume while keeping their phones long working hours.
As can be seen from Figure 1, the increasing number of features has prompted mobile phones to require more low voltage output levels of different power levels. One example is an application processor for image processing that requires up to 360 mw of power during video capture. At full load, the peak power required for the load on the internal system of the phone will typically exceed 4W. This high power will quickly drain the battery's energy. Another important factor affecting battery runtime is power efficiency and system power management.
Low power conversion efficiency will cause heat. This heat is generated by the power loss of the voltage regulator during the energy conversion process. But there are no fans or heat sinks for cooling in the phone, only densely packed printed circuit boards. Therefore, there is no channel to allow heat to escape. This heat reduces battery life and reduces product reliability.
Because of the heat generated during voltage conversion, the industry needs to reconsider which regulator should be used. Manufacturers are now using switching regulators to replace simple but inefficient linear low-dropout (LDO) regulators because of the higher efficiency of switching regulators.
For different voltage regulators that meet the internal power conversion needs of mobile phones, we must carefully consider their advantages and disadvantages (see Table 1). There are currently three options: linear LDO regulators, non-inductive switching regulators (also known as charge pumps), and traditional switching regulators (based on inductors).
The linear LDO regulator is considered to be the simplest solution, it can only convert the input voltage to a lower voltage. Its most significant drawback is thermal management because its conversion efficiency is close to the ratio of the output voltage to the input voltage. For example, the input to an LDO is a single-cell Li-Ion battery, nominally 3.6V, which provides an output voltage of 1.8V at an output current of 200mA to drive the image processor. Then, its conversion efficiency is only 50%, so it will generate hot spots inside the phone, and will shorten the battery life.
Switching regulators avoid the efficiency disadvantages of all linear regulators. By using low-impedance switches and magnetic memory components, switching regulators can achieve 96% efficiency, significantly reducing power loss during the conversion process. This can reduce the size of external inductors and capacitors due to the very high switching frequency (greater than 2MHz). Switching regulators have fewer disadvantages and can be overcome with good design techniques.
Between the linear regulator and the traditional switching regulator is a charge pump. In a charge pump, the external energy storage component is a capacitor rather than an inductor. Since there is no inductance, it can mitigate potential electromagnetic interference problems, so as not to affect sensitive RF receivers or Bluetooth chipsets. Disadvantages of charge pumps are limited input-to-output voltage ratios and limited output current capability.
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