TDA8902J digital power amplifier circuit and anti-interference design

1. PCB layout and wiring principles
PCB provides electrical connections between components of the power amplifier circuit. To achieve the best performance of the power amplifier circuit, the layout of components and the layout of printed wires are the key.

1.1 Layout principles
(1) The power tube of the digital power amplifier works in a switching state, with high frequency, large current, and is close to the power supply part. Since the power amplifier (as shown in Figure 1-1) is powered by a switching power supply (not shown in the figure), the interference and ripple coefficient are large. Therefore, the position of the components on the PCB should consider anti-electromagnetic interference, and the leads between the components should be as short as possible. In terms of layout, the analog signal, digital signal and noise source should be reasonably separated to minimize the coupling between them. That is, the analog input part connected to the LM4651⑩ pin should be separated from other digital parts, and the power input and decoupling filter components should also be separated from the digital processing part. In addition, the directionality of the power transformer should also be considered to minimize the radiation to the circuit.

(2) The components should be arranged in the order of output filter, H-bridge circuit, comparator, oscillator generator, and voltage amplifier. If the various levels are arranged crosswise, they are likely to affect each other, resulting in self-excitation or absorption.
(3) Components with strong electromagnetic field radiation (such as L3, L2) and components that are sensitive to electromagnetic induction (R1, C1, R5, C3) should be shielded or kept away from electromagnetic field radiation sources to reduce interference.
(4) The connection between high-frequency components (such as R5, C3) should be shortened as much as possible to reduce their distributed parameters and mutual electromagnetic interference. Components susceptible to interference should not be too close to each other, and input and output components should be kept as far away as possible.
(5) There is a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuits due to discharge. For example, the ⑤ and ⑦ pins of TDA8902J and the ① and
③ pins of LH4652 should not be too close. Components with high voltage (such as power switches) should be placed in places that are difficult to reach during debugging.

1.2 Wiring principles
(1) The wires used for input and output terminals (such as R8 and L1) should be avoided to be adjacent and parallel. It is best to add ground wires between the wires to avoid feedback coupling.
(2) The wiring of each level should be as short as possible, and the components should be as close as possible. Pay more attention to the wiring of large signals and high impedance. For example, the wiring of R11 and C18 should be as short as possible, and the audio input (C1, R1) and output (L1, L2) lines should not be too long, otherwise they are easy to sense AC signals.
(3) The minimum width of the wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. When the copper foil thickness is 0.05mm and the width is 1-15mm, the temperature will not be higher than 3℃ when a current of 2A passes through. The power amplifier can choose a wire width of 0.5-5mm. The minimum spacing of the wires is mainly determined by the insulation resistance and breakdown voltage between the wires in the worst case, and the spacing can be as small as 5-8mm.
(4) The bends of printed conductors are generally in the shape of arcs, while right angles or included angles will affect the electrical performance in high-frequency circuits. Figure 1-2 (a), (b), and (c) show three types of corner lines. The transmission performance and reflection performance of the 45° external bevel corner line in Figure (C) are better than those of the other two types of corner lines. The performance of the arc corner line is better than these three types of routing, but the engraving of the arc has relatively high requirements for the board making process, which will increase the production cost. The power amplifier uses a 45° external bevel corner routing to meet the design requirements.

2. Anti-interference design measures
The anti-interference design of PCB has different requirements for different circuits. The following discusses the anti-interference design measures of PCB for this power amplifier from three aspects.
2.1 Power line design
In the DC power supply loop, the change of load will cause power supply noise. According to the size of the PCB current, the width of the power line should be as thick as possible to reduce the loop resistance.
For the routing of the power supply of the LM4651L part of this power amplifier circuit, the wire width of 1.5 mm can meet the requirements, while the LM4652 part requires 3 to 5 mm. At the same time, the direction of the power line and the ground line is consistent with the direction of data transmission,
which helps to enhance the anti-noise ability.

2.2 Ground wire design
The ground wire is more important than the power wire. The most important means to overcome electromagnetic interference is the design of the ground wire. The wiring of the ground wire is particularly particular, and the single-point grounding method is usually adopted. The analog ground, digital ground and high-power device ground are separated, and finally all converged to the power ground. The ground wire structure of this power amplifier includes system ground, chassis ground, digital ground and analog ground. The design principles of the ground wire are:
(1) Digital ground and analog ground are separated. This power amplifier has both logic circuits and linear circuits. They should be separated as much as possible and connected to the power supply ground wire respectively, and the grounding area of ​​the linear circuit should be increased as much as possible. The analog audio ground should be connected to the ground at a single point as much as possible.
(2) The ground wire should be as thick as possible. If the ground wire is very thin. The ground potential changes with the change of current, causing the signal level of the power amplifier circuit to be unstable and the anti-noise performance to deteriorate. Usually, the ground wire can pass three times the current. The ground wire of this power amplifier should be more than 3 to 6 mm.
(3) Correctly choose single-point grounding and multi-point grounding. The analog part of the power amplifier has a low operating frequency, so the inductance between its wiring and devices has little effect, while the loop current formed by the grounding circuit has a greater impact on interference, so single-point grounding should be used. When the operating frequency of the digital part is greater than 10MHz, the ground impedance becomes very large. At this time, the ground impedance should be reduced as much as possible and multiple points should be grounded nearby. When the operating frequency is between 1 and 10MHz, if single-point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise multi-point grounding should be used. Although the switching frequency of the digital part of the power amplifier is 125kHz, due to the influence of harmonics, multi-point grounding is better.
(4) Make the ground wire a closed loop. For the PCB of the digital power amplifier, designing the ground wire into a closed loop can significantly improve the anti-noise ability. The reason is that there are many power-consuming components in the circuit. Due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground wire, causing the anti-noise ability to decrease. If the ground is formed into a loop, the potential difference will be reduced, improving the anti-noise ability of the power amplifier circuit.

2.3 Signal line design
When connecting to weak signals outside the PCB, shielded cables are usually used. For high-frequency and digital signals, both ends of the shielded cable are grounded. The shielded cable used for the analog audio signal of this amplifier is preferably grounded at one end. The signal traces in the PCB should be as short as possible and avoid interference sources.

3. Electromagnetic compatibility (EMC) design