Power Management Solutions for Multimedia Mobile Phones

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Power Management Solutions for Multimedia Mobile Phones [Copy link]

Multimedia applications such as mobile TV, mobile gaming, and audio playback present a huge challenge to power management design in mobile phones. Mobile phone designers need to keep the phone small and maintain long battery life while adding new multimedia features. New application processors offer great new features, but at the cost of higher power consumption. New AV features mean longer music playback time, so audio amplification must be more efficient to extend playback time. And as AV features on mobile phones become more sophisticated, the requirements for audio quality and output power will also increase. In this article, we discuss some solutions that can help designers meet these challenges related to power and audio amplification in new multimedia mobile phones.

Power for new application processors

In order to make mobile phones small, the use of integrated power management units (PMUs) is very common. The advantage of PMUs is that they simplify the power design and reduce the overall solution size compared to using several discrete component power solutions. However, on the contrary, the rapid development of multimedia functions has increased the use of independent regulators. As new application processors add multimedia features, existing PMUs are not enough to meet their increasing power requirements. On the other hand, the time for mobile phone development is getting shorter and shorter, so designers cannot wait for the upgrade of PMUs. Therefore, independent regulators are used to provide the additional system power required by new mobile phones.

In addition to considering the cost of the solution, the three most important criteria for selecting a standalone regulator are (priorities vary for different designs): 1. Low noise: The regulator must not generate noise or EMI that interferes with surrounding RF systems. 2. Low power consumption: This means high efficiency when fully operational and low current at light loads and in standby. 3. Small solution size (size and height): The regulator must be in a low-profile package, and the external passive components must also be small.

Power Dissipation in a Synchronous Step-Down Converter

The ideal voltage regulator for the new processors is a DC/DC step-down converter, which has high power efficiency (over 90%) and therefore generates less heat on the board. Figure 1 shows a system block diagram of a mobile platform with an integrated MPEG-4 codec. The synchronous step-down converter NCP1521 is used to provide the core voltage of the chipset and the supply voltage for the I/O circuits. With built-in synchronous rectifiers, this step-down converter does not require an external Schottky diode and provides excellent efficiency, ranging from 90% to 96% when fully operational. It provides an adjustable voltage between 0.9V and 3.3V and an output current of up to 600mA.

Figure 1: Example of an MPEG-4 mobile phone platform.

In mobile phones, when the microprocessor is in standby mode for a long time, the converter enters the light load region and the efficiency will drop below 90%. In order to reduce the power consumption during long standby, NCP1521 provides a solution that can automatically switch from high-frequency PWM to pulse frequency mode (PFM), whose switching frequency is proportional to the load, so the power consumption under light load is smaller.

Small solution size

To reduce the overall solution size, the new buck converter uses a switching frequency between 1MHz and 2MHz. To demonstrate the effect of switching frequency, we use a 1MHz step-down converter on the market, which uses the optimized LC filter values: L=10uH, Cout=10uF. For the NCP1521 with a switching frequency of 1.5MHz, the required output filter is: L=2.2uH, Cout=10uF. Similarly, for the NCP1522 with an oscillation frequency of 3MHz, the optimized LC filter values are L=2.2uH, Cout=4.7uF. The results are summarized in the table.

Table 1: Comparison of LC filter values vs. converter switching frequency.

This comparison shows that the higher the switching frequency, the smaller the inductor and output capacitor required, thus reducing the overall solution size. In multimedia mobile phone designs where PCB space is highly limited, it is recommended to use the NCP1522 with a higher switching frequency to reduce the solution size and lower the cost of passive components.

The SOT23-5 with a pin size of 3x3 is currently the industry standard package for synchronous step-down converters. However, there are smaller package options (such as chip-scale and DFN packages) on the market to meet the needs of designers to further reduce the solution size.

Further integration: Buck and LDO in one package

From the above, we have seen that new application processors with low core voltage and high power requirements are best powered by DC-DC buck converters. On the other hand, RF-sensitive analog circuits with input voltages as high as 2.8V to 3.3V are still mostly powered by low-noise LDOs. The integrated power IC (NCP1526) with buck converter and low-noise LDO provides a flexible and compact solution for DVB-H or DMB-enabled mobile phones, which are the digital TV standards for the European market.

Audio playback challenges in multimedia phones

Portable multimedia features pose two major challenges to audio amplification in mobile phones. First, multimedia mobile phones need to play music and videos for more than 2 hours; second, the audio experience on mobile phones will be close to that of home audio and video systems. Mobile audio playback can get clear, three-dimensional and powerful audio with heavy bass.

Currently, the mainstream mobile phone speakers are driven by Class AB audio amplifiers. When MP3 becomes a popular application and the playback time increases from a few minutes to a few hours, the low efficiency and poor heat dissipation of Class AB amplifiers can no longer meet the requirements. Therefore, more and more new designs use Class D audio amplifiers.

Audio amplifier power consumption in a mid-priced mobile phone is generally less than 100mW. The maximum output power can reach 500mW. As shown in Figure 2, we can compare a typical Class AB and a Class D audio amplifier NCP2820. At 50mW, the efficiency of the NCP2820 is 80%, while the Class AB is only 20%. For the higher power operating range between 100mW and 500mW, the Class D still provides a stable 85% to 90% efficiency, but the Class AB is still very low, between 30% and 60%. This clearly shows that the excellent efficiency of the Class D amplifier helps to extend the audio playback time.

Figure 2: Efficiency of Class AB and Class D audio amplifiers (NCP2820).

Due to low efficiency and high heat dissipation, Class AB amplifiers cannot provide output power above 1W without saturation or distortion. Class D amplifiers work in a switching state, so they are highly efficient in amplifying audio signals and can therefore provide greater output power to support high-capacity audio playback.

For the NCP2820, it can deliver up to 1.4W of power into an 8-ohm speaker while still keeping THD+N below 1%. Since the speaker output response rolls off low-frequency signals, the extra power helps boost bass sounds - important in music and gaming audio playback.

For stereo 4-ohm speakers, a common accessory in MP3 players, the NCP2820 can deliver up to 2.56W per channel with an input voltage of 5V. In this case, it is recommended to power the amplifier with a constant voltage of 5V to 5.5V to ensure constant high output power. The DC/DC boost converter NCP1422 provides a constant voltage of 5V to power 2 Class-D amplifiers in stereo applications.

EMI Considerations in Class D Amplifier Design

Class D amplifiers operating in a constant frequency PWM switching state can generate EMI that interferes with RF operations on the same board. Two key technologies help prevent EMI from interfering with RF systems. First, the Class D amplifier should be placed very close to the speakers. In stereo applications, it is recommended to use two mono amplifiers, allowing designers to place the two amplifiers next to the two speakers at both ends of the phone. In addition, designers should also connect an EMI filter, such as a ferrite bead, to the output of the amplifier. The EMI filter acts as a bandpass filter and can eliminate the switching signal before it propagates along the wire to the RF circuit.