Based on the design suggestions provided by MPS, a few points are summarized;
1. The input capacitor should be as close as possible to the IN and GND pins, the connection line should be as short and thick as possible, and the loop area should be small;
2. SW and BST lines are far away from sensitive areas, and the inductors are placed close to SW, and the copper is thick between them;
3. The FB trace trajectory is as short as possible, and the feedback resistance is close to the chip pins;
4. And so on.
Generally, these principles will be followed in the process of PCB layout, adhering to the principle of asking a few more whys, why the original factory put forward these suggestions, and what will be the impact if these suggestions are not implemented?
To understand these problems, we must have "rings" and "nodes" in mind. The circuit of switch on and off is different, there are different circuits, and the area can also be different.
In the process of switching on and off, the switch produces a sudden change in current and voltage in the input loop and the switch node, which is what we often call DI/DT, DV/DT signals, which leads to the occurrence of switching frequency and its frequency doubling, as well as more high-frequency noise components, which will produce EMI interference.
What is the impact of a large loop in the PCB design process?
From the di/dt analysis, the alternating current will produce a magnetic field, then the magnetic field generated in this loop will change with the change of current, the larger the dI/dt, then the magnetic field in the loop will also change drastically, magnetism generates electricity, electricity generates magnetism, and electromagnetic radiation is formed, this radiation is also called differential mode radiation, which is mainly based on the spatial coupling of magnetic fields.
Radiant intensity E, current I, loop area A, current frequency f, distance from the loop R from the test point.
From the formula, it can be seen that the magnitude of the radiant intensity is proportional to the magnitude of the current and the size of the loop area. In a nutshell, the magnitude of the magnetic field formed by a high-frequency current loop depends on the area of the loop and the magnitude of the current.
From the dv/dt analysis, the switch tube is constantly turned on and off, and the voltage waveform at this position is a rectangular wave, which will produce a relatively wide frequency band of high-frequency harmonic components. At the switching node, there will be distributed capacitance in the length and width of the trace, and the harmonic component will enter the ground through the distributed coupling capacitance to form common-mode radiation.
The principle of di/dt, dv/dt is different, the reduction of differential mode radiation intensity needs to be dealt with from the angle of loop area, current size and frequency, the size of the current is based on the load situation, and the loop area can be controlled and reduced as much as possible in the process of wiring.
The size of the circuit area is a MPQ4430 layout.
1. The input capacitor is placed as close to the switch as possible to reduce the loop;
2. The input capacitor is placed symmetrically to form two small loops to reduce the radiation of the loops;
3. The area of the switch junction is reduced, the inductance size is reduced, and the distributed capacitance is reduced;
4. The bottom layer of the loop is paved with a complete ground plane, and the induced current formed cancels out the magnetic field generated by the high-frequency loop;
In summaryIn the process of PCB layout, it is necessary to have a clear understanding of the loop, understand how the current flows in the dynamic changes, and solve the critical loops in the process. Combined with the path of the current, you can understand the layout suggestions provided by the official data sheet, and it can be flexible in actual projects.