Designing with Class D amplifier ICs

CLASS D audio amplification is becoming the solution of choice in consumer audio, by enabling long battery life and compact dimensions. However, designers must take care to avoid complexity elsewhere in the system, including the power supply, output filtering and input signal conditioning.

The switching amplifier, or Class D amplifier, has risen quickly to prominence in consumer audio applications, from MP3 devices including mobile phone handsets to games consoles, LCD-TVs and home cinema. The ace in the pack for Class D is its vastly superior efficiency, which can be as high as 85-90 percent in practice. A linear Class AB implementation will normally achieve around 25 percent at typical listening levels.

In handheld applications the low power dissipation of Class D allows designers to combine high audio performance with a long battery recharge interval. Battery life is a key figure of merit for all personal communication and audio devices. For mains powered equipment, such as audio-visual (AV) products and games consoles, the high power efficiency of Class D brings the advantage of reduced heat dissipation.

Hence designers can specify smaller heatsinks, to achieve lower profile styles as well as lower bill of materials and assembly costs. In fact, careful design of the power supply can allow heatsink-less operation up to several Watts per channel of output power.

The basic topology of a Class D amplifier comprises a pulse width modulator, a power bridge output circuit and a low pass filter. Class D amplifier ICs currently available take away much of the design effort, such as managing the EMI produced by the amplifier's switching operation, and selecting the optimal switching frequency. Increasing the switching frequency reduces output filtering requirements, but results in greater losses due to MOSFET gate capacitance. Hence switching frequency selection requires a balance between external components and power efficiency.

The design of the power bridge depends upon the desired output power of the amplifier. For instance, Class D ICs are available with headphone drivers or with drivers for loudspeakers, with the design of the output stage being one of the key differences between these various configurations.

Amplifiers designed for use with loudspeakers are capable of producing from less than 1W up to several Watts of output power, without requiring a heatsink. These ICs enable a single chip solution in many consumer applications, from portable media players to games consoles and some LCD-TVs. In the majority of these applications, particularly handheld products, a single chip solution is essential.

However, for very high output power, a Class D amplifier IC can be combined with an external output stage built using audio power MOSFETs. The IC must provide a suitable pre-driver, and the chosen discrete MOSFETs must be optimised for digital audio operation.

The output from the Class D MOSFET H-bridge is a square wave representation of the audio signal. The switching frequency components must be attenuated, to prevent interference and to ensure the end product will pass electromagnetic compatibility (EMC) certification. Low pass filtering, with a cut-off frequency just above the audible band, is required. Hence attenuation of these components is greater with higher switching frequencies. This allows smaller external filtering components.

On the other hand, MOSFET losses tend to increase with switching frequency, thereby driving down efficiency and leading to increased power dissipation and associated thermal management issues. In particular, switching losses are incurred in the MOSFET gate capacitance, increasing linearly with the operating frequency. Hence the design of a Class D amplifier IC output stage is predicated on fabricating low loss MOSFETs and setting the switching frequency low enoughcorrectly to meet the specified target for electromagnetic interference (EMI).

Filterless connection to a loudspeaker, such as a cellular phone speaker, is a distinct advantage in size-and-cost sensitive applications. When the Class D output is physically close to the speaker, the parasitic resistance and inductance of the speaker coil may be used as a suitable R-L low pass filter, enabling one inductor and one capacitor to be eliminated from each output connection. An example of a Class D amplifier chip that may be used in a filterless configuration is the Wolfson WM8960. If the distance from the amplifier output to the speaker is longer, a small amount of additional inductance, in the form of a ferrite bead, will be required to improve EMC performance. Figure 1 compares headphone driver outputs with and without a ferrite bead.

Designers using Class D amplifier ICs also must pay closer attention to the effect of power supply behaviour on audio output quality than is the case for linear amplifiers. Because the Class D output is a switching stage, effectively connecting the supply rail directly to the audio output, audio-band fluctuations in the supply will modulate the output signal directly. Designers, therefore, must ensure high load regulation in the audio band, or take steps to eliminate the effects of mains or audio band ripple.

A number of manufacturers provide floating regulators that can be added to existing supplies to improve load regulation if necessary. Using a separate regulator for each amplifier output has the additional benefit of reducing crosstalk between audio channels. However, an additional regulator, or pair of regulators, adds to the overall cost of the implementation.

Also, power dissipation in the voltage regulator offsets the efficiency gains that are the key justification for a Class D implementation.

Alternatively, increasing the power supply rejection ratio (PSRR) of the amplifier reduces the effect of load regulation on the audio output signal. Adding feedback from the PWM output to the analog audio input raises the PSRR by compensating for supply voltage variations. This can achieve a PSRR of up to around 80 dB, which is very close to the PSRR of a differential Class AB amplifier for portable applications.

If the Class D input signal is pure digital audio, however, this technique cannot be applied. The PSRR of an all-digital Class D amplifier is 0 dB, and the designer must ensure close regulation of the supply voltage.

The transient performance of the power supply should also be considered. To reproduce the PWM waveform accurately, the power supply must be capable of reacting quickly to sudden changes in current draw. A linear amplifier is much less demanding in this respect, since the bandwidth of the output stage is limited to the audio range.

In a power supply for a Class D amplifier, voltage fluctuations outside the audio band, resulting from poor transient response, will modulate the PWM signal, introducing harmonic distortion that can be heard in the audio output.

Of course, high value capacitors can be used to deal with these fluctuations. However, capacitors that are physically large are not desirable in handheld products. On the other hand, high value capacitors in small outline packages are expensive.

A helpful technique is to arrange for the MOSFETs in the different output stages to switch at different times, thus reducing the peak supply current. For example, the Wolfson WM8608, a 5 to 7.1 channel digital power amplifier controller, has a built-in 'PWM output phase' function that introduces a delay of 160ns between the PWM signals for each output channel.

This has the effect of spreading the switching transients around the PWM cycle as shown in figure 2, although the added delay of 160ns is far too short to make an audible difference to the output. In a multi-channel system with six channels, this technique significantly diminishes the maximum instantaneous load current and reduces crosstalk.

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One potential concern with switched supplies is electromagnetic interference (EMI) due to the rapid switching of large currents. This problem is exacerbated when a switching supply and switching amplifier are operated in the same system at different switching frequencies.
Intermodulation produces tones that may be audible in the output. Synchronizing power supply switching with that of the Class D PWM modulator can eliminate this effect. As an example, the Wolfson WM8608, which is compatible with integrated or discrete switching output stages, is the only PWM controller available that provides this synchronisation capability.

Alternatively, a Class D amplifier may be powered from a regulated linear supply. This may be attractive where extremely low cost targets dominate design.

However, switching power supplies are usually preferred, for the very same efficiency and small size advantages that have drawn Class D amplifiers into the consumer audio space.

Eric Haber is senior product architect at Wolfson Microelectronics.

This story appeared in the February 2009 print edition of EE Times Europe

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