[Design Application]
Class-D Amps Raise Low-Power Systems To The Next Level
Use Integrated Class-D Amplifiers To Reduce Size, Cost, And Heat Dissipation In Battery-Powered Audio Systems.
Class-D audio power amplifiers (APAs) were first introduced nearly 50 years ago. Since then, they have been used sparingly in a relatively small number of applications with limited bandwidth, such as public address systems and telephony equipment. This will soon change as a new class of integrated Class-D APAs make their way into such mainstream applications as portable computers, battery-operated music systems, wireless communication devices, and other compact low-power systems.
It's becoming clear that moving to next-generation designs requires taking advantage of this technology's greater power efficiency and its resulting reduction in heat dissipation. The bottom line is that Class-D amplifiers have the potential to reduce system size and cost while extending the life of battery-powered systems.
Only recently have advances in semiconductor fabrication processes made integrated Class-D audio amplifiers possible. Fast-switching, rugged DMOS power MOSFETs can now be integrated with analog circuitry, eliminating the need for a discrete output power stage. The resulting Class-D APA is an effective, highly efficient solution for compact, battery-powered audio applications in the music bandwidth.
In a laboratory test designed to compare the power efficiency of Class-AB and Class-D APAs, a Class-D amplifier extended the life of a battery by 2.5 times. The test took a linear Class-AB APA and placed it on an evaluation platform in a test system with bass/treble volume control modules, dc-dc converter modules, and one 9-V alkaline battery. The test system ran until the dc/dc converter tripped its undervoltage lock-outlockout at 5.2 V. Subsequently, the voltage in the battery would drift upwards and the system would turn on again. After three such incidents, the test was deemed complete. Then, a Class-D APA was substituted for the Class-AB amplifier and the same procedure was repeated.
It's important to note that the test utilizes a real-world signal. Music was used instead of the sine waves or tones that are typically used in a lab to assess the power efficiency of audio amplifiers. Unlike the widely varying and often unruly music signals, tones are uniform and well behaved. In other words, the crest factor of music is much higher than that of a tone.
Using Crest Factor Crest factor can be used to analyze the differences between the amplifiers. Essentially, crest factor represents the difference between a signal's peaks and its RMS power:
Crest factor = 10 log (PPK/PRMS)
This is sometimes referred to as headroom. Music signals can have crest factors as high as 15 dB, which means that peaks of over 30 times the RMS value can occur. There is a dramatic difference in test results when music signals rather than sine waves are used to determine the power-efficiency of Class-AB and Class-D amplifiers (Fig. 1).
The simulations outlined in Figure 1 were done at full power, which occurs when the input signal is large enough to drive the output to the rails without clipping. The results are even more dramatic when the same simulation is done at normal listening levels, which are usually much less than full power. At less-than-maximum power, the efficiency of linear amplifiers drops considerably faster than does the efficiency of Class-D devices. These simulation results support the empirical battery-life tests that found that Class-D APAs are two to three times more power efficient than linear devices.
Basically, an audio power amplifier is a special type of operational amplifier optimized to drive low-impedance loads—typically speakers or headphones—at frequencies in the 20-Hz to 20-kHz range. Consider the architecture of a typical Class-AB linear APA used in a bridge-tied-load (BTL) configuration (Fig. 2). The input capacitor forms an RC high-pass filter with the input resistance of the amplifier, and attenuates signals below 20 Hz.
Linear amplifiers derive their name from the fact that they produce an instantaneous output that's equal to a given input multiplied by a constant, known as the gain of the amplifier. This requires that the output transistors be biased to operate in the linear region. The output transistors are analogous to variable resistors in which the input voltage adjusts the resistance to create the required output voltage. The output voltage of an amplifier must be derived from the supply voltage, with the difference being dropped across one of the device's output transistors to attain the output voltage level.
Linear Amps Always "On" Even when there is no input signal present, the output transistors are on and drawing precious quiescent current. This results in inefficient power dissipation and the generation of a great deal of heat. Heat sinks are required to transfer the excess heat to the ambient air.
The only way to improve the power efficiency of such an APA is to operate the output transistors as switches rather than as variable resistors. This means that when the output transistors are turned on, current is passed through the circuit but very little voltage is developed across it. When the switches are off, the circuit has the full supply voltage across it and virtually no current, minimizing I2R power losses. This type of switching arrangement is precisely how Class-D APAs operate.
So a Class-D amplifier is essentially a switch-mode power delivery circuit, much like the switch-mode voltage regulators that are found in most personal computers. Rather than using a dc reference to set the output voltage, as switch-mode regulators do, Class-D amplifiers use the audio input signal as the reference.
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