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Class D amps liven up audio in flat-panels

Posted: 16 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:audio power amplifier? Class D amplifier? flat-panel display?

By Eric L. Droge
Texas Instruments Inc.

The latest developments in audio power amplifier (APA) technology are increasing the sound quality in flat-panel displays to match the superb image quality. As DLP, LCD TV, LCD monitor and plasma display TV screens increase in size, audio performance must continue to keep pace. However, heat dissipation and power consumption problems prohibit traditional Class-AB or linear APAs from being the best solutions in the latest flat-panels.

Due to the inefficiencies of linear amplifiers, the flat-panel industry is choosing Class D APAs for its audio solutions. Class D APAs operate much cooler and consume a lot less power compared to Class-AB amplifiers. Designers who use Class D APAs can increase the audio power performance in their applications without increasing heat or power consumption, keeping transformers and voltage regulators small and eliminating heat sinks. In fact, Class D APAs can lower heat and power consumption even while increasing power performance.

Linear APA problems
Linear APAs are inherently inefficient due to the voltage drop from the power supply across the linear amplifier's output stage transistors [1]. In most cases, they dissipate more power in heat than they supply to the speaker to produce sound. When the linear amplifier's output voltage signal is not equal to the supply voltage, there is internal power loss in the amplifier, which lowers the amplifier's efficiency. When the amplifier's output voltage signal equals the supply voltage, its efficiency is much higher; however, the amplifier is distorting because the voltage signal is being "clipped" by the supply rail (Figure 1). Distortion caused by clipping the supply rail produces an unpleasant sound and can permanently damage the speaker.

Figure 1: 10V, 1kHz sinewave clips into power supply rails.

Therefore, there will always be an internal voltage drop from the supply voltage to the output voltage signal. The voltage drop is calculated by subtracting the RMS value of the output voltage from the supply voltage (VDD). The voltage drop multiplied by the average supply current, IDD(avg), determines the internal power dissipation of the linear amplifier. The larger the voltage drop, the lower the amplifier's efficiency. A simple formula for calculating the efficiency of a differential output linear amplifier is:

From Equation 1, the efficiency of a linear amplifier driving 3W into an 8? speaker from a 12V supply is only 45 percent, which means the total power consumption for a stereo solution is 13.3W. The total power consumption of a typical 17-inch LCD monitor, which needs a 3W amplifier, is 90W. In this example, the linear amplifier consumes nearly 15 percent of the total power supplied to the LCD monitor. Furthermore, the amplifier is dissipating 7.3W as heat, which requires a large heat-sink.

Class D to the rescue
Unlike linear APAs, the Class D APA's efficiency is not dependent on PL or VDD. Theoretically, a Class D APA is 100 percent efficient because the Class D output transistors function as switches that turn on-and-off very quickly. When the transistors are switched on, the output voltage is equal to the supply voltage. If the transistor is ideal, no voltage drop occurs that would cause power dissipation when multiplied by the average supply current. Moreover, when the transistors are switched "off", they are an open circuit, through which no current flows and therefore no power is dissipated. Figures 2 through 5 illustrate output switching states of the TPA3200D1. Advantages of this amplifier's four-stage Class D switching modulation compared to other two-stage Class D modulations are lower noise floor and reduced filter requirements. The shaded transistors are switched on and the non-shaded transistors are switched "off", and are switching on-and-off at 250 kHz. The 250 kHz switching frequency achieves the best balance of performance while minimizing switching losses.

Figures 2 and 3

Figures 4 and 5

TPA3200D1, 20W Mono Class D APA output waveform
Figure 6

The TPA3200D1 positive output waveform is shown in Figure 6. Also shown is the ringing caused by ferrite beads, which are used to minimize EMI. Additional information on how to minimize EMI is given in the How to Design with Class D APAs section.

To calculate the efficiency of a Class D amplifier, we assume non-ideal transistors with a finite resistance when switched "on". The on-resistance of the output transistors, rds(on), is the primary factor that determines a Class D amplifiers efficiency if the switching frequency is optimized around 250kHz. A simple formula for calculating the efficiency of a Class D amplifier is [2]:

Applying the same operating conditions used previously to determine the 45 percent efficiency of the linear amplifier and assuming an rds(on) of 0.6?, the Class D efficiency is calculated at 93 percent. However, 87 percent is used to more accurately reflect an increase in rds(on) as the device warms to a steady operating state. The total power consumed by the Class D amplifier is only 6.8W for a stereo solution, compared to 13.3W, for the linear amplifier. The 49 percent reduction in power consumption allows designers to use smaller transformers and regulators. In addition, the mere 0.8W of power dissipation, compared to the 7.3W for the linear amplifier eliminates the need for a heat sink.

As previously stated, the TPA3200D1 output switches at 250kHz. The 250kHz switching frequency over samples the audio signal a minimum of 10 times to ensure high quality audio that is equal to or better than the linear amplifiers used in these applications today. The high frequency PWM output waveform shown in Figure 6 carries the audio signal. The speaker functions as a low-pass filter that only reproduces the audio frequencies. In most applications, minimal filtering to reduce EMI is required prior to the speaker.

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