Detailed explanation of smartphone audio amplifier circuit design

In recent years, smartphones have integrated more and more functions, but in terms of basic audio amplification applications, there is still room for further improvement in terms of performance optimization and user audio experience. The reason is that smartphones have special audio requirements, such as: smartphones have multiple audio input sources such as baseband/application processors, FM radio, Bluetooth (headphones), etc.; the codec (CODEC) can be integrated in the analog baseband or exist independently; in most cases, at least the speaker amplifier remains separate (not integrated) to provide sufficient output power ; the headphone amplifier is external and cooperates with high-fidelity (Hi-Fi) music playback. This article will focus on the speaker amplifier and headphone amplifier performance requirements of smartphones, introduce ON Semiconductor 's corresponding audio amplification solutions, and a new audio subsystem solution that integrates stereo headphone amplifiers, Class D speaker amplifiers and I2C control - audio management integrated circuits (AMIC ) .

Speaker amplifier performance requirements and solutions

For smartphones, the desired speaker amplifier should provide low electromagnetic interference ( EMI ) to avoid interference with other radio frequency ( RF ) circuits in the smartphone. In terms of actual user applications, users sometimes want to make hands-free voice calls in public places, and sometimes want to watch videos with audio playback. This requires the speaker amplifier to provide a highly recognizable output volume while providing low distortion. In addition, low noise is also an important feature provided by the desired speaker amplifier. Specifically, this requires the speaker amplifier to have a high power supply rejection ratio (PSRR) to suppress the time division multiple access (TDMA) noise generated by battery voltage fluctuations during GSM signal transmission; it also requires no pop and click noise during turn-on and turn-off.

Figure 1: Schematic diagram of audio amplification application in smartphones

Figure 2: Different techniques for reducing EMI

To meet these expected performance requirements of smartphone speaker amplifiers, Class D amplifiers are an excellent choice. For example, Class D amplifiers provide extremely low EMI and avoid interference with other RF circuits. In fact, Class D amplifiers convert the input analog audio signal into a pulse width modulated ( PWM ) pulse signal, and then use this pulse signal to control the switching device to turn on/off the audio power amplifier. For smartphone applications, to reduce EMI in the audio output stage, it is important to reduce the higher frequency spectrum. Traditional PWM technology does not have a specific means to deal with it. However, to achieve this, two technologies can be used, one is PWM spread spectrum modulation (switching frequency variation) and the other is PWM with ramp control (delaying rise/fall time). In comparison, ramp control technology is more effective in reducing the higher frequency spectrum than spread spectrum modulation technology, and is more conducive to reducing EMI.

ON Semiconductor's NCP2824 is a 2.8 W mono Class-D amplifier that uses ramp control technology to provide low EMI. In addition, the NCP2824 provides a real-time configurable automatic gain control (AGC) function via a single-wire interface . Its automatic gain control function includes two modes, non-clipping and power limiter mode. For speaker amplifiers, clipping occurs under low battery voltage conditions in smartphones, resulting in reduced output swing and saturation. The NCP2824's automatic gain control "non-clipping" function maintains low distortion and can select a maximum total harmonic distortion (THD) threshold. On the other hand, excessive output power occurs under high output power conditions, resulting in reduced output swing and saturation. The power limiter function limits the amplifier's output power (with a selectable maximum output voltage threshold) to protect the speaker from damage caused by excessive volume.

Figure 3: The NCP2824 supports automatic gain control in both non-clip and power limiter modes

In addition to low EMI and low distortion, the NCP2824 also performs very well in other key performance indicators of audio amplifiers. For example, this device has an excellent signal-to-noise ratio (SNR) performance of up to 95 dB, providing excellent audio performance. In addition, the NCP2824 also has an excellent power supply rejection ratio (PSSR), with a PSSR of -72 dB at 217 Hz. The NCP2824 also provides energy efficiency of up to 92%, which helps extend the battery life of portable devices . The device operates from 2.5 V to 5.5 V and supports fully differential inputs (thus eliminating input coupling capacitors), requiring only one external capacitor. The device also provides short-circuit protection circuitry for applications such as smartphones and mobile Internet devices ( MIDs ), navigation devices, portable game consoles, and portable media players.

Headphone amplifier performance requirements and solutions

Smartphone users expect to enjoy music playback with high-fidelity (Hi-Fi) quality through headphones, which requires the headphone amplifier to have low distortion. Since the headphones are close to the human ear and directly affect the user's auditory experience, the headphone amplifier must have no audible noise. This feature is more important for headphone amplifiers than for speaker amplifiers. In addition, the headphone amplifier is also required to have high energy efficiency to help extend the music playback time.

In order to meet consumers' higher demands for headphone audio quality, portable consumer devices such as smartphones require high-quality stereo headphone amplifiers. When designing the output stage of a stereo headphone amplifier, designers need to choose a more suitable solution from different options such as capacitive coupling and true ground. The capacitive coupling solution is energy-efficient because the power supply only supplies the positive output signal; however, this solution requires the use of large coupling capacitors (which will breed size and cost issues), and the sound quality is poor at low frequencies. In comparison, the true ground solution does not require the use of coupling capacitors, has good low-frequency response performance, and the true grounding of the headphone is used with a conventional converter, but the energy efficiency of the true ground structure is not high. In general, the true ground solution provides lower distortion and smaller solution size, and the focus is on improving energy efficiency to help extend audio playback time.

For headphone amplifiers, in order to provide a comfortable listening level, quiescent power (i.e., quiescent current) is the main component of their overall power consumption. Therefore, minimizing quiescent current is critical to improving the energy efficiency of headphone amplifiers. NCP2815 is an ultra-low quiescent current (Iq) stereo headphone amplifier launched by ON Semiconductor, which provides an ultra-low quiescent current of 1.8 mA to help extend audio playback time.

The device also provides a high impedance (High Z) output mode to support audio input/output for audio jacks. The NCP2815 supports common-mode sensing to eliminate ground loop noise. The device supports a wide supply voltage of 1.6 V to 3.6 V, consumes only 20 mW when powered by 1.8 V and has a 16 Ω load, and has a total harmonic distortion plus noise (THD + N) of less than 0.01%. The NCP2815 provides a high power supply rejection ratio of -100 dB, offers fixed internal gain (-1.5 V/V) or externally adjustable gain, and also provides pop and click noise elimination circuitry. The 1.2 mm x 1.6 mm CSP package makes the NCP2815 the smallest device of its kind on the market.

Figure 4: NCP2815 “long play time” stereo headphone amplifier block diagram

Audio Subsystem Solution——Highly Integrated Audio Management IC

ON Semiconductor, as a leading supplier of high-performance silicon solutions for energy-efficient electronic products, not only launched the above-mentioned independent high-performance speaker amplifiers and stereo headphone amplifiers, but also launched an audio subsystem solution that integrates stereo headphone amplifiers, speaker amplifiers and I2C control - the Audio Management Integrated Circuit (AMIC), which provides flexible routing and multiplexing (muxing) between the two audio input sources of the speaker and headphone outputs, such as NCP2704 and NCP2705.

Figure 5: Audio management IC functional diagram

Among them, NCP2704 is a PWM Class D audio management integrated circuit with ramp control, which helps to effectively reduce EMI. This device provides a fully programmable automatic gain control function to ensure excellent audio output quality and protect the speaker. The headphone amplifier integrated in NCP2704 has ultra-low quiescent current consumption characteristics, which helps to extend the audio playback time. This device also provides rich input/output multiplexing control to improve the flexibility of the device. The THD+N value of the headphone amplifier integrated in NCP2704 is only 0.02%, and that of the speaker amplifier is 0.042%; the corresponding headphone amplifier power supply rejection ratio is -100 dB, and that of the speaker amplifier is -89 dB. NCP2704 provides a wide and precise gain selection (mute and -60 dB to +12 dB).

NCP2705 is also a Class D audio management IC with similar main functions to NCP2704, but NCP2705 adds common mode sensing. This feature can improve crosstalk performance, especially in the case of FM tuners with parasitic resistance. NCP2705 has a lower THD+N value of 0.01% for headphone amplifiers and 0.017% for speaker amplifiers.

Figure 6: Block diagram of the NCP2705 Class D audio management IC with common-mode sensing

Summarize:

Audio output applications for portable products such as smartphones require low EMI, low distortion, high power supply rejection ratio and high energy efficiency audio amplification solutions. Designers use the NCP2824 Class D speaker amplifier with ramp control provided by ON Semiconductor, which can effectively reduce high-frequency EMI interference to the RF circuit, while using the "no clipping" automatic gain control (AGC) function to ensure that the speaker provides low distortion when playing audio, and use the "power limiter" AGC function to protect the speaker from damage. At the same time, designers use the ultra-low quiescent current stereo headphone amplifier NCP2815 provided by ON Semiconductor to extend the audio playback time of smartphones. Both NCP2704 and NCP2705 are audio management integrated circuits with low EMI Class D amplifiers, automatic gain control and "long playback time" headphone amplifiers, providing flexible multiplexing and wiring via I2C.