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I've seen this in many schematics, I also do it myself, but I don't really know why it's needed. I've also noticed that on RGB led strips, there's tiny 100nf caps, too.

Why is it needed? Also, if it's generally needed, why aren't those capacitors integrated in the IC itself already? I'm sure there's tech for doing it, so why not?

MightyPork
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2 Answers2

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  1. They are needed to supply very sudden high current changes to the IC. Especially CMOS ICs may need high current changes during switching from Low to High or vice versa. If those capacitors are not very close to the IC the line between them would represent a resistance and inductance ("impedance") which impedes quick current changes. That's why they can not be collected and located some where else as one big capacitor.

  2. It is not easy to integrate large capacitances. It is also more flexible to have the circuit designer decide how big the capacitor needs to be. Its value may depend of the application.

Curd
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The 100n caps are for bypassing undesired rf energy associated with stray inductances, capacitances, and rectified by diodes and transitors.

The answer above,

They are needed to supply very sudden high current to the IC. If they are not very close to the IC the line would represent a resistance and inductance which impededes quick current changes." is incorrect because 100n caps won't provide a squat amount of high current.

the caps are there to filter rf. in the case of the led's, they bypass ambiant rf across the diode so that it won't be rectified - generally only needed under certain circumstances.

the caps across the power supply at the IC are crucial - they help to ensure that any rf on the power supply (ususally there is some) will bypass the IC.

delme
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  • It is very unlikely that there is undesired RF energy that you have to worry about being rectified by the LEDs. Each LED has a forward voltage of > 1V. In order to get rectified the RF voltage had to be larger than that or even a multiple of that if the LEDs are connected in series. – Curd Dec 05 '14 at 11:14
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    *" they help to ensure that any rf on the power supply (ususally there is some) will bypass the IC"* - I thought they helped to stop the IC *generating* high frequency noise on the power rails due to fast internal switching. Am I mistaken? – Roger Rowland Dec 05 '14 at 11:49
  • About the leds, I meant the likes of ws2812b, there's actually a tiny driver chip in the led, that does the pwm and communication. What is RF, btw? – MightyPork Dec 05 '14 at 15:38