Application of High Efficiency Class D Audio Amplifier in Portable Devices

As energy becomes increasingly scarce, the requirements for energy efficiency of electronic systems are gradually increasing, especially for battery-powered systems, which have more stringent requirements on power consumption. With the popularity of mobile phones, MP3/4 and other multimedia portable devices, audio amplifiers have become standard configurations for audio parts. Both system engineers and end users are no longer satisfied with playing personalized music loudly, and have put forward higher requirements for audio amplifiers: consume less current to extend battery life, provide perfect sound quality in the entire audio range, good RF suppression ability to reduce current noise, and stable output power to ensure that the speakers are not damaged.

In the future, the market demand for high-efficiency Class D power amplifiers will gradually increase, and the application of Class D amplifiers in portable devices has a very broad prospect. According to a report by market research firm Gartner: Class D amplifiers will have a compound annual growth rate of 15.6% between 2006 and 2011, growing from US$334 million to US$688 million. The main growth drivers come from portable devices and space-constrained consumer electronics products, and the mobile phone market is an important growth point.

This article introduces a highly efficient Class-D audio amplifier BL6311 developed by Shanghai Belling. This product is used in portable devices such as mobile phones and can provide a perfect solution with only a few peripheral components.

BL6311 Application Schematic Diagram

Portable devices require audio amplifiers to be small in size and have simple peripheral circuits. BL6311 can be configured to use single-ended input (Figure 1) or differential input (Figure 2) according to system requirements.

Figure 1 BL6311 single-ended input application schematic

Figure 2 BL6311 differential input application schematic

Features of BL6311

1. High efficiency and energy saving

Compared with linear amplifiers (Class A, Class B, Class AB), Class D amplifiers have great advantages in power efficiency. For linear amplifiers, the linear operation of bias components and output tubes will consume a lot of power, while the output tubes of Class D amplifiers work in a switching state, and the power consumption mainly comes from the on-resistance, switching loss and quiescent current of the output tubes. The efficiency can reach 90%, and no heat dissipation is required, thus extending the battery life and life of portable devices.

2. RF noise suppression

Mobile phones, personal digital assistants (PDAs) and other portable communication devices often operate in harsh and noisy environments. For audio amplifiers that do not solve RF interference, the RF carrier information will be demodulated into the audio band, which will produce undesirable current sound at the output of the speaker. The RF signal will couple to the power supply, input and output of the amplifier. The good PSRR and CMRR of the amplifier can reduce the requirements for the system PCB and structure, minimize the current sound, and ensure the normal operation of the equipment. Generally, the output of 10mV can be felt by the human ear, and as shown in Figure 3, when a 500mV GSM signal is superimposed on the power supply voltage (VDD), the output end (Vo) of BL6311 is still well suppressed.

Figure 3 BL6311 has a suppressive effect on the GSM signal coupled from the power supply end.

3. Extremely low distortion and noise

For audio amplifiers, although all test indicators are measured with sine waves, the final judge is still beautiful and smooth music. Beautiful sound depends on small harmonic distortion and noise. The ideal Class D amplifier has no distortion, no noise in the audible band and 100% efficiency. However, the actual Class D amplifier is not perfect and will have distortion and noise. This is caused by the distorted switching waveform generated by the Class D amplifier. The reasons are as follows:

> The PWM modulator causes nonlinearity of the PWM signal due to sampling frequency limitation and time jitter;

> Dead time, on time, off time, rise and fall time of power driver tube;

> Non-ideal characteristics of switching devices, such as on-resistance, limited switching speed and body diode characteristics;

> Parasitic spurious parameters lead to excessive marginal oscillations;

> Non-linearity of the output low-pass filter.

Figure 4 BL6311 THDN vs Frequency

BL6311 solves the above problems very well. Figure 4 shows the sweep frequency curve of THDN, which shows that BL6311 has low distortion in the entire audio range and can meet the sound quality requirements of portable devices.

Since BL6311 has high efficiency, low distortion and strong RF noise suppression capability, it is particularly suitable for mobile phones and other portable products. Figure 5 is a block diagram of the application of BL6311 in mobile phones.

Figure 5 Application of BL6311 in mobile phones.