In recent years, smartphones have integrated more and more functions. In terms of basic audio amplification applications, consumers are also more demanding on audio quality. This article will focus on the performance requirements of audio amplifiers for smartphones and corresponding audio amplification solutions.

Digital input audio amplifier

At present, the sound sources of all smart phone platforms are still mainly analog sound sources. The technology is quite mature and can provide high-quality sound sources. The main problem is that the analog sound sources are susceptible to various interferences on the transmission path, resulting in noise or noise. For example, noise coupling on the PCB, or TDD-Noise caused by space radiation. As more and more functions are integrated on smartphones, the sources of audio interference will be more and more complex, and analog audio sources will be incapable. Digital audio sources will be a future development trend, such as I2S, PDM or PCM and other digital audio sources, I2S The format should become a mainstream format supported by all platforms.

In addition to the strong anti-interference ability of digital audio sources, another huge advantage is that it is easy to implement various sound effects processing in the digital domain, such as tone control, multi-band EQ, 3D surround sound, etc. The cost and difficulty of implementing these functions in the analog domain is almost unimaginable.

At present, Aiwei Electronics has begun to exert efforts in this field, and it will launch power amplifier products with digital audio input in due course. It is expected that it will usher in explosive growth within one to two years, gradually replacing analog audio power amplifiers. Entered the field and seized the opportunity.

Development trend of speakers

Metal composite film speakers are commonly used on smart phones, mainly in 1115 and 1420 specifications. Their main characteristics are good bass, high sensitivity, small size, thin thickness, only 3mm, especially suitable for ultra-thin machines, and their disadvantages are also obvious. The rated power is too small, only about 0.5W, and it is easy to burn; it must be driven by a power amplifier with anti-breaking sound function, and the output power of the power amplifier must be strictly controlled. .

At present, the volume of smart phones is generally low, and the loudness of the speaker needs to be improved. The most direct way is to increase the power of the speaker. Now some manufacturers have first introduced 0.8W or even 1W metal composite film speakers, another way to increase the loudness The better diaphragm material is used to enhance the sensitivity of the speaker, so that the loudness of the speaker can be greatly improved under the same power condition.

Tailor-made audio power amplifier for smart phones: K6

Aiwei Electronics has always been committed to providing high-performance audio solutions for mobile phones. Since the launch of the first K-type amplifier in 2009, the super-volume as the main feature has been widely recognized by customers, and it has quickly become popular in the north and south. The power amplifier technology is more mature and the product performance is more perfect. It has become the standard product in the functional machine era, with a cumulative shipment of 300 million units.

At present, the sound effects of smart phones are far from meeting the expectations of users, and there are generally problems such as low volume, lack of bass or noise.

In response to the challenges faced by the audio design of smart phones, Aiwei launched a high-efficiency, high-volume, and anti-breaking audio power amplifier tailored for smart phones: K6, using the first 1.5X open loop charge The overall efficiency of the pump structure is up to 80%, which can greatly extend the use time of the smart machine; K6 provides three power levels of 0.8W, 1W and 1.2W for users to choose, matching metal membrane speakers of different powers and sizes to achieve beautiful music Enjoy, the anti-breaking function strictly controls the output power of the power amplifier, and maximizes the volume when the speaker is effectively protected; adopts Aiwei's patented RNS (RFI TDD-Noise Suppression) technology and sound purification technology to effectively suppress TDD-Noise problem caused by radio frequency interference; K6 uses a tiny 2mm & TImes; 2mm FC-16 package, FC-16 is a high-performance package form widely used in the field of RF PA, which is characterized by small package resistance and overcurrent capability Strong, good heat dissipation, also suitable for the packaging of audio power amplifier products.

The functional block diagram of K6 is as follows

1.5X open loop charge pump

Compared with the traditional charge pump structure, the 1.5X open-loop charge pump has the advantages of strong driving ability, large output current, and high efficiency. The adjustment tube of the traditional charge pump works in the linear region, while the 1.5X open-loop charge pump has no devices. In this area, all work in the switching state, the power consumed by the charge pump itself is very small, so the 1.5X open-loop charge pump has extremely high efficiency, the average efficiency is above 85%, and the peak efficiency can reach 95%. The operating frequency of up to 1.2MHz allows the use of small-sized capacitors on the periphery; at the same time, the charge pump has fault protection functions such as overvoltage protection, heavy load current limiting, and ground protection to protect the chip from damage under abnormal conditions.

Figure 1: Working principle of 1.5X open-loop charge pump

The operation of the 1.5X open-loop charge pump is divided into two stages. In the charging stage, the flying capacitors CF1 and CF2 are charged. The charge is stored in the flying capacitor. In the discharging stage, the two flying capacitors are connected in parallel and superimposed on the Power Supply voltage. To achieve the boost function, the output voltage is 1.5 times the power supply voltage.

Anti-break function

In audio applications, factors such as excessive input signals or a drop in battery voltage can cause undesirable sound-breaking distortion in the output signal of the audio amplifier, and overloaded signals can cause permanent damage to the speakers. Aiwei's unique anti-breaking sound (NCN) function can automatically adjust the system gain by detecting the broken sound distortion of the amplifier, so that the output audio signal remains smooth and smooth, which not only effectively avoids the damage of the high-power overload output to the speaker, but also brings Comfortable listening experience.

Figure 1: Working principle diagram of anti-breaking sound

The start time and release time are two important parameters of the anti-breaking sound function, which directly affect the anti-breaking sound effect. The start-up time refers to the time required for the anti-breaking function to detect the output of the broken sound distortion to the gain attenuation of -10dB. The release time refers to the time required for the anti-breaking function from detecting that the output is no longer broken and distorted until the gain is restored to 10dB. Too small start-up time and release time may cause problems with wheezing; too large may cause problems such as broken sounds or "suffocating" sounds. In view of the audio characteristics of mobile devices such as mobile phones, K6 has a startup time of 40ms and a release time of 2s, which effectively eliminates broken sounds, protects speakers, maintains the inherent rhythm of music, and achieves wonderful music enjoyment.

Super strong TDD suppression technology

TDD-Noise, commonly known as "current sound", TDD-Noise can interfere with the audio circuit through two ways, one is the conduction interference of the power line and the ground wire; the other is the antenna space radiation interference. K6 integrates two major technologies to suppress current sound: RNS technology and clean tone technology

K6 adopts the original completely symmetrical power amplifier architecture. The internal components and traces of the chip are completely symmetrically arranged. At the same time, any circuit seen on the sensitive pins of the power amplifier uses RNS (RF TDD Noise Suppression) technology, which not only makes the RF signal difficult The interference into the chip, even if there is a very small amount of interference signal, it will be completely cancelled by the completely symmetrical power amplifier architecture.

Figure 2: RNS technology

The input TDD caused by ground conduction interference is the most difficult to filter, because it will be superimposed with the input signal, so it is recommended to use differential input on platforms that can use differential input, and the traces should be parallel and symmetrical. At the same time, K6 adopts the "clean tone" technology to effectively filter out this TDD Noise. It is recommended to use it only in the hands-free call mode.

Figure 2: Clean tone technology

Typical application of K6

The application of K6 is extremely simple. Only a small amount of resistance and capacitance is needed on the periphery. The peripheral devices are required to be placed as close as possible to the chip pins. K6 has multiple states for users to choose. One-line pulse control is used and only one GPIO port is required.

K6 adopts slim 2mm & TImes; 2mm FC-16 package, which has the advantages of small parasitic resistance, strong overcurrent capability, good heat dissipation, small board area, etc. It is especially suitable for portable devices such as smart phones with limited space.

K6 typical application diagram is as follows

to sum up

The integration of mobile phone functions is more and more, and the volume is developing towards thin and light. The space left on the board for audio is getting smaller and smaller, which is difficult to obtain good sound quality. Ai Wei Electronics has launched a complete set of audio together with major speaker manufacturers. The solution, K6 with high-power, high-sensitivity metal composite film speakers will bring a shocking volume and high-fidelity sound quality to the smartphone, and at the same time up to 80% of the overall efficiency, greatly extend the use time of the smartphone, K6 is a smartphone The only choice for audio design.

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