capacitance

The capacitance mind map is as follows ：

Capacitance has four functions ： decoupling 、 coupling （ Every other day ）、 wave filtering 、 Energy storage . Today we are mainly talking about decoupling .

Capacitor package

I believe everyone has used these capacitors , The most on the board are multilayer ceramic capacitors .

Tantalum capacitance ： It is mainly used in power circuit , Bloggers have been bombed by it many times ......

Decoupling capacitor

This is a STM32F103 Minimum system schematic ,STM32F103VET6 We need five routes 3.3V Power supply , His 3.3V Generally from LDO（ Low differential linear voltage regulator ）, such as LM1117

LDO Than DC-DC The way （TPS5430） More stable voltage , But it's still not enough for chips , We need to add... Next to the chip power supply pin 0.1uF Decoupling capacitance of , Let the high-frequency AC part of the voltage go from the capacitor to the ground , Thus, the chip can obtain a stable DC voltage . therefore , The decoupling capacitor should be placed as close to the chip pin as possible .

Why 0.1uF ？

When analyzing the integrity of power supply, our commonly used capacitance model is shown in the figure below ：

ESR Is the series equivalent resistance of the capacitor ,ESL Is the series equivalent inductance of the capacitor ,C Is the real ideal capacitor .ESR and ESL It is determined by the manufacturing process and material of the capacitor , Can't eliminate .ESR Ripple that affects the power supply ,ESL Affect the filter frequency characteristics of the capacitor .

therefore , There is a formula ：

When the frequency is very low, the capacitance works , When the frequency is high enough, the effect of inductance can not be ignored , When it is higher, the inductance plays a leading role . The capacitor loses its filtering function . therefore , At high frequency, the capacitance is not a simple capacitance . The filtering curve of the actual capacitance is shown in the figure below .

See above , The best filtering effect we want is in “ valley ” At the end of , It's the tip of the curve concave , At this sharp point , The filtering effect is good , It can filter out the interference in this frequency band , however , When the frequency is very high , This is the time 0.1uF The capacitor has no filtering effect 0.01uF Okay , And so on , Higher frequency , The magnitude of the selected filter capacitor should be smaller .

Therefore, sometimes the parallel connection of multi resistance and capacitance can be adopted , Get the best filtering effect .