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Decoupling capacitor for Power Supply

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feiutm9898

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Current I read an Application Note - Hardware Design Considerations

http://www.atmel.com/dyn/resources/prod_documents/doc2521.pdf

Please refer to the Figure 2.2 at attachment.

According to the note, the figure 2.1 creating large current loop but figure 2.2 have better noise immunity. I understand that the inductor is AC device. But, I cannot understand why it will help to protect the spike noise.

I am a software guy. Can any hardware engineer explain it?
 

Hi feiutm9898,

In microcontroller there are thousands of transistors which are switching simulteniosly on high frequency due to which these type of spikes produces on I of VCC.It produces due to microconroller & may affect other circuit/peripherals of system.When you connect L(blocks AC & bypass DC) & C(blocks DC & bypass AC) combination as shown in figure 2-2 ,effect of spikes remain local to microcontroller while ripple on power plane gets reduce significantly.

Regards,
tramp
 

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    feiutm9898

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well, the capacitor will soothen out voltage ripples, inductors will soothen out current ripples, and filter out AC current on the line,
 

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    feiutm9898

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note: in second figure capacitor locate very close to microcontroller power pins.
in this way, noise and ripple absorbed by capacitor.
inductor behave like open circuit in this frequency and prevent pass high frequency component to power lines.
 

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    feiutm9898

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HERE, INDUCTOR IS USED FOR BLOCKING a.c. WHILE ALLOWING d.c.
So, only high grequency spikes are bolcked.
The capacitor near pins of the IC , grounds the remaining spikes which saves the threshold device to be blown connected inside the IC.
 

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