The main sources of integrated circuit EMI in PCB are: EMl signal voltage and signal caused by the square wave signal frequency generated at the output during the transition of digital integrated circuit from logic high to logic low or from logic low to logic high. The capacitance and inductance of the current electric field and magnetic field chip itself.
The square wave generated at the output of the integrated circuit chip contains sinusoidal harmonic components with a wide frequency range. These sinusoidal harmonic components constitute the EMI frequency components that engineers care about. The highest EMI frequency is also called the EMI emission bandwidth, and it is a function of the signal rise time (not the signal frequency).
The formula for calculating the EMI emission bandwidth is: f = 0.35/Tr where the factory is the frequency, and the unit is GHz; 7r is the signal rise time or fall time, and the unit is ns.
From, L: What: It can be seen from the formula that if the switching frequency of the circuit is 50MHz, and the rise time of the integrated circuit chip used is 1ns, then the highest EMI emission frequency of the circuit will reach 350MHz, which is much larger than the circuit The switching frequency. And if the sink-rise time is 5 ribs Fs, then the highest EMI emission frequency of the circuit will be as high as 700MHz.
Every voltage value in the circuit corresponds to a certain current, and also every current has a corresponding voltage. When the output of the IC changes from logic high to logic low or logic low to logic high, these signal voltages and signal currents will generate electric and magnetic fields, and the highest frequency of these electric and magnetic fields is the emission bandwidth. The strength of the electric and magnetic field and the percentage of external radiation are not only a function of the signal rise time, but also depend on the control of the capacitance and inductance on the signal path between the signal source and the load point. Therefore, the signal source is located on the PCB board. The sink is inside, and the load is inside other ICs. These ICs may or may not be on the PCB. In order to effectively control EMI, it is not only necessary to pay attention to the sink; the capacitance and inductance of the baffle itself, but also to the capacitance and inductance that exist on the PCB.
When the pot close between the signal voltage and the signal circuit is not close, the capacitance of the circuit will decrease, so the suppression effect on the electric field will be weakened, thereby increasing the EMI; the current in the circuit also has the same situation, if The current and the return path are not properly connected; better, it will inevitably increase the inductance on the loop, thereby enhancing the magnetic field, and ultimately leading to an increase in EMI. This fully shows that poor control of the electric field usually leads to poor suppression of the magnetic field. The measures used to control the electromagnetic field in the circuit board are generally similar to the measures used to suppress the electromagnetic field in the IC package. As in the case of PCB design, IC package design will greatly affect EMI.
A considerable part of the electromagnetic radiation in the circuit is caused by voltage transients in the power bus. When the output stage of the sink: jumps and drives the connected PCB line to logic "high", the sink chip will absorb current from the power supply and provide the energy required by the output stage. For the ultra-high frequency current generated by the continuous conversion of the IC, the de-rolling network on the PCB of the power bus stops at the output stage of the sink. If the signal rise time of the output stage is 1.0 ns, then the IC must draw enough current from the power supply to drive the transmission line on the PCB within a short time P of 1.0 ns. The voltage transient on the power bus depends on the application on the power j line path. Inductance, the current absorbed, and the transmission time of the current.
Since the IC pins and internal circuits are part of the power bus, and the time for absorbing current and outputting signals also depends on the process technology of the sink to a certain extent, selecting the appropriate sink can control the power to a large extent. The three elements mentioned in the above formula.