Buck Converter Applications in Portable Products

Portable consumer electronic products are integrating more and more new features. As users expect smaller size and longer battery life, system designers face greater challenges. Each additional feature requires additional space and additional power, which leaves less room for batteries and requires more supply current with higher efficiency in a smaller space. This article compares different types of regulators based on the current requirements in power supply designs and discusses their applications in portable products.

Overview

The new generation of portable consumer electronic products integrates more and more functions, making it difficult to categorize these specific products. The increase in functions helps to increase sales, but in order to meet users' demands for smaller size and longer battery life, designers face greater challenges. Each additional function requires additional space and additional power, and since the space left for the battery is extremely limited, it is necessary to provide more supply current with higher efficiency in a smaller space.

Higher power requirements change design criteria

In the past two years, most handheld products use a buck converter and multiple low dropout (LDO) linear regulators, and some designs only use linear regulators. This solution can provide good working performance. Since most processors are powered by 3.0V or 3.3V, LDO can provide appropriate conversion efficiency when powered by a single Li+ battery. However, with the requirements for processor power consumption and the development of IC processes to smaller sub-micron technology, the core voltage of microprocessors has dropped to 1.8V, 1.5V, 1.3V or even 0.9V. In addition, the typical I/O voltage has also dropped from 3.3V to 2.5V or 1.8V. The reduction in voltage greatly reduces the efficiency of LDO, and the heat it generates offsets the benefits of low core voltage and low I/O port voltage. Therefore, in order to maintain high efficiency, designers must consider using a buck converter.

To meet the need for low supply voltages, many systems use multiple processors. For example, a mobile phone + PDA combination is a good example, where the system contains a baseband processor and an application processor, each of which requires a separate power supply. Camera modules used in mobile phones and PDAs still tend to be powered by LDOs, but the associated graphics processors usually require a lower supply voltage. Therefore, in multi-function designs, multiple buck supplies are often used. It is common to install three buck supplies on a PC board.

New custom power management ICs (PMICs) integrate one or more buck converters, but these products are often not enough to meet user requirements. Each new function may require an additional buck power supply or the need to increase the drive capability of the buck power supply. Obviously, only those manufacturers who can meet new design requirements in the shortest time with discrete power ICs can keep pace with the growth of market demand.

Size Challenge

At the current design level, adding a buck converter to the system not only increases the cost, but also takes up a certain amount of circuit board area. Three years ago, the typical buck converter used an MSOP package with a size of 15mm2, which could operate at a switching frequency of 1MHZ or lower, requiring a larger external inductor and tantalum capacitor . As shown in Figure 1b, the current 1MHZ buck converter uses a TDFN package, the size has been reduced to 9mm2, and ceramic capacitors and small-size inductors can be used externally, but its size is still much larger than that of an LDO, as shown in Figure 1a.

Advanced submicron BiCMOS mixed-signal processing technology is the key technology to solve the size problem. It can further reduce the size of the power IC, increase its operating frequency, and select smaller external components. Many IC manufacturers have been able to provide power ICs with 2MHz or even higher frequencies, and also use smaller packages. As shown in Figure 1c, Maxim's 4MHz step-down converter MAX8560 is almost as small as an LDO! Higher operating frequencies can be used, therefore, allowing the use of micro inductors, such as Taiyo Yuden's CB2012 series, 0805 package.

Although the manufacturing process of 1MHZ and 4MHz buck converters has improved, the efficiency of 4MHz buck converter is lower than that of 1MHZ buck converter, as shown in Figure 2. This is because the higher switching frequency will produce larger switching losses, and the micro inductor also has larger core losses. Fortunately, the efficiency difference between 1MHZ and 4MHz buck converters is not much, and their efficiency is much higher than that of LDO (41%).

in conclusion

From the above analysis, it can be seen that there are three options for system power design: a) small size, b) high efficiency, or c) small size and high efficiency. A trade-off needs to be made between battery life and system physical size. Since the high-frequency buck converter in solution c can greatly improve the power conversion efficiency without significantly increasing the size of the circuit board, it is naturally the preferred solution for multi-functional, portable consumer products. In addition, considering that the heat generated by the buck converter is lower than that of the LDO, it is possible to replace the LDO with a smaller size.