With the improvement of system robustness and sound quality, the latest class D audio solution has been significantly improved compared with previous generations of products. Class D amplifier has inherent advantages, including higher power efficiency and low calorific value, which can realize smaller and lighter consumer products.

Using these advantages, audio chips based on class D technology are having a great impact on today’s consumer products, especially in the field of flat-panel digital TV and portable applications.

Combination of digital TV and class D technology

Although the traditional analog amplifier is a good choice for CRT TV, it is not enough for flat-panel TV. This is because designers must make great concessions in limiting audio output power and quality in order to achieve lightweight products. Class A and class B amplifiers not only have low efficiency, but also produce unnecessary heat, causing difficult design challenges, but also need radiators, leading to further design problems. Most flat panel TVs are facing great challenges from the power consumption and heat dissipation of the video system itself. Therefore, many flat panel TVs can only provide about 5W power per channel. Class D digital amplification technology has been able to provide a solution to the main challenge of flat panel TV, that is, it can obtain high-quality audio effect in light and thin size while minimizing heat dissipation.

At present, class D technology can integrate success rate level and controller on one chip, which can achieve more than 85% efficiency without radiator. By integrating the new class D chip into the system, the flat panel TV can provide enhanced audio power (about 15W per channel), and the sound quality level has been comprehensively improved. Therefore, OEM manufacturers can achieve enhanced audio power and quality to better match the visual experience.

Table 1 shows the comparison between traditional linear analog amplifier and digital PWM amplifier.

Based on the application advantages of class D technology and products in portable products

Table 1 Comparison between monolithic linear circuit and PWM amplifier

At present, class D technology is close to the performance level of analog amplifier, and even exceeds the performance expectation in some aspects. Its current power level is equivalent to analog amplifier, and total harmonic distortion (THD) is at the same level as analog amplifier. In fact, its dynamic range has exceeded the average performance of the integrated analog amplifier, which is usually less than 80dB. Advanced digital amplifiers can achieve a dynamic range of more than 100dB – the difference between mute and maximum power output.

The noise that occurs during conduction is another challenge. Although the use of relays and larger DC blocking capacitors is quite expensive, digital integration helps to achieve more powerful control of amplifier startup without increasing material costs. Therefore, cirrus logic simplifies the audio channel and can obtain better sound quality without feedback. The existing switching power supply is sufficient to provide stable voltage. However, high-end applications that use traditional transformers and worry about filtering still use local power feedback.

In order to meet the unique system design requirements of flat-panel digital TV, cirrus logic recently launched a highly integrated audio amplifier cs4525 based on class D technology. It integrates stereo ADC, sample rate converter, digital audio processor, and a complete 30W class D amplifier including controller and power stage (e.g. 15W per channel). The cs4525 is unique in that it supports input analog and digital audio signals, and its efficient power level does not need to use a radiator in the system design.

In portable productsClass D technical advantages

Generally, class D technology can provide higher output power and lower voltage for portable products with smaller area. This reduced power consumption can achieve longer battery life. Class D technology also reduces the number of components in the system design, which helps to reduce the cost. In addition, the chip size of class D technology is usually small, which can better match the circuit board design of portable products with limited space.

Another problem in class D system design is filtering. The output of class D amplifier needs filtering. Some advocates take the inductance coil as part of the filter driving the speaker, while others have created a complete design of fully filtered output independent of the characteristics of the speaker. For audio systems that use speakers as part of their products, the use of inductance in speakers helps to save component costs.

For portable audio applications, because the earphone wire is relatively long, its output must be fully filtered without out of band noise. However, wires may become EMI antennas. Due to the need for inductors, it is difficult to achieve this with a class D amplifier, because the volume of inductors is relatively large, which may affect the audio performance in applications. Therefore, for some products, such as cs42l52 Stereo Class D amplifier, in order to achieve lower power consumption, the headphone output adopts a traditional low EMI analog amplifier and a higher power class D amplifier for speaker output, which are usually integrated into the product itself to minimize the amount of filtering required.

Cs42l52 adopts ground centered output amplifier. Using an amplifier output equal to the DC potential returned (grounded) by the headset, DC will not flow through the headset. In this way, it is not necessary to use expensive and large DC isolation capacitors, and the sound quality will not be affected. Because this requires the existence of positive and negative power supply voltage, a charging pump can be integrated on the headphone amplifier to generate negative power supply voltage to meet this requirement. This charge pump is more effective and requires only a few external components.

This design method not only eliminates the use of decoupling capacitors in headphones, but also helps the 1.8V system have sufficient power to produce acceptable listening volume. The charge pump can generate – 1.8V power supply voltage and effectively drive a 3.6V amplifier.

Responsible editor: GT

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