Design Articles

High Efficiency Audio Designs for Portable Devices

In order to improve efficiency, a new technology for Class D amps is now available, giving a typical Class D audio amplifier an efficiency of 80% or more.

By Henry Kwok, Senior Applications Engineer, National Semiconductor Corporation

With an ever increasing feature set found in today’s portable devices, a mobile phone can now function as a multimedia playback system, digital still camera and personal digital assistant (PDA). A portable media player (PMP) is now a navigation system, music player, global positioning system (GPS) and digital film library. Several systems are available with MP3 / MP4 playback, GPS, TV and gaming with web browser. Most manufacturers are now placing a greater emphasis on sound quality because sound is a key element to differentiate their products. Some manufacturers will even put more than one speaker in a system to improve the sound quality and output level.

This article uses a high-efficiency class D audio power amplifier to demonstrate how to lower a portable device’s heat dissipation by reducing power consumption. It will also show how to reduce the amplifier’s electromagnetic interference and improve audio quality by reducing noise.

Power savings is always a challenging mission, especially for battery-operated portable devices. Multiple speakers are becoming more popular due to the improved sound quality and louder output. In single speaker applications, products only require one power amplifier to drive the speaker, but for stereo or multiple speaker applications, systems demand two audio power amplifiers to drive the speakers. This requirement more than doubles the power consumption and the heat generated by the dual power amplifiers and is another design issue engineers need to address. Many previous portable system designs used Class AB amplifiers, which have higher power consumption and a typical efficiency in the 40-55% range. Increasing efficiency can enable a reduction in the heat generated by the audio power amplifiers. In order to improve efficiency, a new technology for Class D amps is now available, giving a typical Class D audio amplifier an efficiency of 80% or more.  

figure 1
Figure 1: Typical Class-D amplifier operation

Basic Class D amplifier theory asserts that from a small analog signal into the power amplifier input you get a digital output. The power amplifier’s internal modulator takes the conversion from analog to digital signals by using pulse width modulation (PWM) or pulse code modulation (PCM), depending on the type of modulation used. But it’s still a small digital signal. After the analog to digital conversion, bridge amplifiers are used to increase the digital signal amplitude. For converting high amplitude digital back to analog, output passive LC filters are needed. The block diagram (Figure 1) shows the details where the red block is the power amplifier device (chip) portion.

Figure 1 is a typical PWM Class D amplifier structure, which includes a PWM converter (modulator). The “H-Bridge” block is the digital signal amplifier, which functions similar to a level shifter. The final output stage (between the amplifier and speaker) is a passive low pass filter (LPF).

figure 2
Figure 2: Typical passive LC Low Pass Filter (LPF)

The low pass filter circuit in Figure 2 shows a typical implementation with two in-series inductors connected between the power amplifier and speaker. A high current flows through these two inductors when the power amplifier is operating. Due to the high current flow, the inductor’s size is rather large. But for portable products, with very limited printed circuit board (PCB) space, using two big inductors is not a good solution. Besides the inductors, three external capacitors are consuming PCB space, too.

In order to eliminate the output filter, National Semiconductor developed a series of filter-less Class D amplifiers: mono versions (LM4671, LM4673, LM4675 and LM48310) and stereo versions (LM4674, LM48410 and LM48411).

The concept is to use a moving coil speaker as a transducer. The typical transducer load on an audio amplifier is quite reactive (inductive). For this reason, the speaker load can act as its own filter. So without the usual output filter, the power amplifier outputs an aggressive waveform that can radiate or conduct to other components in the system and cause interference. It is essential to keep the audio power amplifier and output traces short and well shielded.

figure 3a
Figure 3a: Fixed Frequency FFT
figure 3b
Figure 3b: Spread Spectrum FFT

In the past, only fixed frequency technology was used for Class D products, but today electromagnetic interference (EMI) effects are more sensitive on portable designs. Fixed frequency, Class D amplifier outputs switch at a constant 300 kHz, where the output spectrum consists of the fundamental and its associated harmonics (see Figure 3a).

Manufacturers want an improved solution to resolve these EMI issues. Using spread spectrum is one method to reduce EMI. In spread spectrum mode, the switching frequency varies randomly by 30% (about a 300 kHz center frequency), reducing the wideband spectral content and improving EMI emissions radiated by the speaker and associated cables and traces. A fixed frequency Class D amplifier exhibits large amounts of spectral energy at multiples of the switching frequency. The spread spectrum architecture spreads the same energy over a larger bandwidth (see Figure 3b). The cycle-to-cycle variation of the switching period does not affect the audio reproduction, efficiency, or power supply rejection ratio (PSRR).

With this new technology available, manufacturers’ end products are more easily able to pass EMI testing. Figure 4 shows the EMI report for the LM48410 audio amplifier using 3-inch length stereo speaker cables. It passes the FCC EMI test with at least a 6 dB margin.

figure 4
Figure 4: LM48410 EMI report
figure 5
Figure 5: LM48410 Application Circuit

Figure 5 shows the LM48410 focused on a speaker application (there is no headphone output capability). The LM48410 features four selectable gain (6dB, 12dB, 18dB & 24dB) options to choose from.

EMI reduces handling

With a better PCB layout, EMI performance can be further improved. The traces from the amplifier outputs to the speaker have the greatest potential to couple the EMI to other circuits. Since most Class D audio amplifiers use a bridge-tied-load (BTL) configuration to maximize the output power, both speaker outputs have switching frequency included. Below are some methods to improve EMI performance:

  1. Layout the speaker traces side by side (this configuration is very similar to differential pair).

  2. Make the speaker output trace on an internal PCB layer, with ground layer shielding above and below (this method is similar to RF shielding).

  3. Keep the device to speaker trace length as short as possible (using a twisted pair) to minimize EMI.
External components selection

Input capacitors may be required for some applications, or when the audio source is single-ended. Input capacitors block the DC component of the audio signal, eliminating any conflict between the DC component of the audio source and the bias voltage of the LM48410. The input capacitors create a high-pass filter with the input resistance RIN. The -3dB point of the high-pass filter is found using the equation below.

f = 1 / 2πRINCIN

The values for RIN are the reference to calculate the input capacitor value.

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The input capacitors can also be used to remove low frequency content from the audio signal. Small speakers cannot reproduce, and may even be damaged, by low frequencies. High-pass filtering the audio signal helps protect the speakers.

When the LM48410 is configured to using a single-ended signal source, power supply noise on the ground is seen as an input signal. Setting the high-pass filter point above the power supply noise frequencies (217 Hz in a GSM phone) filters out the noise such that it is not amplified nor heard on the output. Capacitors with a tolerance of 10% or better are recommended for impedance matching and improved common-mode rejection ratio (CMRR) and power supply rejection ratio (PSRR).

PCB layout handling for high power design

Another main parameter portable product designers must focus on is output power. The PCB trace current ratings are one way to evaluate and indicate an opportunity to improve efficiency. As output power increases, interconnect resistance (PCB traces and wires) between the amplifiers, load and power supply create a voltage drop. The voltage loss due to the traces between the LM48410 and the load results in lower output power and decreased efficiency. Higher trace resistance between the supply and the LM48410 has the same effect as a poorly regulated supply, increasing ripple on the supply line, and reducing peak output power. The effects of residual trace resistance increases as output current increases due to higher output power or decreased load impedance or both. To maintain the highest output voltage swing and corresponding peak output power, the PCB traces that connect the output pins to the load and the supply pins to the power supply should be as wide as possible to minimize trace resistance.

The use of power and ground planes will give the best total harmonic distortion plus noise (THD +N) performance. In addition to reducing trace resistance, the use of power planes creates parasitic capacitance that helps filter the power supply line. The inductive nature of the transducer load can also result in overshoot on one or both edges, clamped by the parasitic diodes to GND and VDD in each case.

In summary, as our technologies evolve, new ideas and consumer requirements will continue to drive the product advancement. As these new features continue to expand and improve performance, designers will need to create better solutions, to be creative to solve complex problems, including new ways to get better sound in portable devices.

This never ending cycle involves using better filters and board layout techniques for EMI and of course, power savings through better heat management. Designers need to listen to the requests from the real world. And they are listening – but now with much better quality.

National Semiconductor Corporation
Santa Clara, CA
(408) 721-5000

This article first appreared in the June, 2008 issue of Portable Design. Reprinted with permission.

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