“When I recently discussed with car audio design engineers about car radio solutions instead of traditional class AB amplifiers and switch to class D amplifiers, they also had this concern. Now let’s talk about the two main issues I hear most often: the impact on the size of the printed circuit board (PCB) and potential electromagnetic interference (EMI) issues.
Have you ever wanted to do a significant and meaningful thing but worried that it would be difficult to do? Then, when you finally gather the courage to do it, and look back, you will be surprised to find that it is actually easy.
When I recently discussed with car audio design engineers about car radio solutions instead of traditional class AB amplifiers and switch to class D amplifiers, they also had this concern. Now let’s talk about the two main issues I hear most often: the impact on the size of the printed circuit board (PCB) and potential electromagnetic interference (EMI) issues.
Question 1: Class D amplifiers will take up more PCB space
Conventional Class D audio amplifiers use a switching frequency of about 400 kHz, and require 8.2-µH or 10-µH inductors to obtain ideal audio effects.
And TI’s TPA6304-Q1D class amplifier uses 2.1-MHz switching frequency. The reduction in ripple current means that smaller and lighter 3.3-µH inductors can be used, as shown in Figure 1.
Figure 1: Comparison of Inductor size and switching frequency
TPA6304-Q1 uses TI’s latest mixed-signal manufacturing technology. When used with 3.3-μH inductors, the total size of the entire 4-channel amplifier solution (including all necessary passive components) will be reduced to 272 mm2,as shown in picture 2.
Figure 2: TPA6304-Q14 channel class D amplifier
In this regard, the entire TPA6304-Q1 solution shown in Figure 3 is smaller than a traditional Class AB amplifier.
Figure 3: Size comparison of TPA6304-Q1D class amplifier solution and class AB amplifier
Question 2: Class D amplifiers can cause electromagnetic compatibility (EMC) problems
In essence, a Class D audio amplifier turns its output on and off in each clock cycle, while a Class AB amplifier does not turn on and off. But this does not mean that Class D amplifiers will cause unsolvable electromagnetic compatibility (EMC) problems.
I would like to review in particular several methods of TPA6304-Q1 amplifier design to alleviate EMC problems:
The TPA6304-Q1 amplifier design is highly optimized to handle overall EMC behavior. In addition, the aforementioned 3.3-µH inductor is part of an inductive capacitor (LC) filter, which helps minimize the electromagnetic compatibility of high-speed switching transients on the output stage of the Class D amplifier.
As shown in Figure 4, a traditional Class D amplifier that switches in the 400-kHz range produces harmonics directly in the AM band. These harmonics can produce interference signals that reduce the sensitivity of AM receivers, thereby hindering AM broadcast stations’ reception. Therefore, some type of EMI avoidance technology must be implemented on the design of these 400-kHz Class D amplifiers to mitigate the impact of these AM band harmonics.
Figure 4: Typical 400-kHz Class D amplifier harmonics
By working at a higher 2.1-MHz switching frequency, the TPA6304-Q1 no longer needs to implement EMI avoidance techniques for the AM band, because the TPA6304-Q1 provides significant margin above the AM band. This design also does not have any low-frequency spikes that will interfere with the AM band, as shown in Figure 5.
Figure 5: The high switching frequency of TPA6304-Q1 is higher than the AM band
In order to prevent EMI problems in some PCB layout designs, TPA6304-Q1 has implemented the proprietary spread spectrum technology developed by Kilby Labs. Figure 6 illustrates how this feature can help spread the narrowband energy source over a larger band, thereby reducing peak energy.
Figure 6: Spread spectrum technology background
TPA6304-Q12.1-MHz high switching frequency automotive Class D audio amplifier meets the industry’s needs for next-generation car radios and external amplifiers. In addition to reducing the thermal load in the system, the design of this amplifier also solves the PCB size and EMC issues that have attracted much attention when converting from a class AB amplifier to a class D amplifier.