Patch capacitors are recommended for switching power supply and LDO output. Patch capacitor has good high frequency performance and relatively low dielectric loss, especially its resonant equivalent series resistance ESR is very small, such as 33pF, and the equivalent resistance under resonance is only 0.15 ohm. This is unmatched by electrolytic capacitors, including tantalum capacitors. There is basically no leakage current and no polarity. In the past, the patch capacitor was not very large, so it was usually necessary to connect a patch capacitor in parallel with a tantalum capacitor. At present, the SMD capacitor is close to the tantalum capacitor with small capacity, so many of them are directly replaced by SMD capacitor.
In addition, the chip capacitor itself has very small inductance and good high-frequency performance. Many power supplies are directly processed with the chip capacitor of 10uF, and there is no need to connect 100nF. At this time, because the resonance point is of little significance, the impedance of 0805 packaged 10uF patch capacitor is 0.03 ohms at 10MHz, 0.32 ohms at 100MHz and 3.2 ohms at 1000MHz. From these data, as long as the power below 100 MHz is not too large, there is no need to connect the small capacitors at the resonance frequency point in parallel.
It should be noted that many friends know that the wiring capacitor is close to the equipment and the wire is as short as possible when wiring, but they forget to consider the circuit of the floor pad of the patch capacitor when wiring. Generally speaking, the bottom surface of PCB is the reference of one layer. It must be remembered that all front pads must have holes in the main ground. Otherwise, if the ground circuit is too long, this patch capacitor will be meaningless, and good things will be wasted because of the problem of design and wiring. Many experienced PCB engineers will make this mistake.
