I'm wondering why on so many circuit designs, particularly ones involving ICs, there is a point, often right before the connection to the supply or drain where it branches to a capacitor, often 100nF or so, which connects to ground. Is this like a cushion for changes in current?

here is an example:


The odd thing is that looking at motor control designs, I have seen many that are almost identicle except for including or excluding these capacitors. And many chips have a similar branch to ground on their connection diagrams. So what does it do?

 

It kills high frequency noise (spikes) and keeps it from entering the IC.

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Mike2545

 

That makes sense, Thank you

 

It's also something of a shock absorber for IC power. The IC is typically pulling a lot of current when a logic state changes. The cap, being close to the IC, acts as a storage tank to fill in peak demands at those moments.
kenjj

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.1mf does not act like you describe. It's sole purpose in this application is to squelch frequency noise.

What kind of IC's are you using that use a lot of current when the logic state changes?

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Mike2545

 

Note the 100u one mike, that's definitely a power supply buffer.

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Yes, but that is on the other side of the voltage regulator and the original question is about the .1 next to the IC.

The 100uf is resisting changes in the current, kind of smoothing out any ripples in the DC.

But .1, no.

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Mike2545

 

google decoupling capacitor

 

sometimes more than 100nF are required, for instance if you drive LED displays.

1uF, 47uF or even 220uF, if problems persist, add capacitors at multiple places until they disappear.

fast computer circuits (let say, 10MHz), which have RAMs, GALs, etc.,
need 100nF, 220nF or 330nF everywhere, not just one or two.

a CPU will introduce current surges in the 10s and 100s of mA, and if there are no capacitors, it can and will introduce logic glitches, means, the design can not work correctly.

for low-power PICs including some LEDs, i often just use 1 uF (note i supply via USB or battery, if you have 7805 etc., situation is completely different).

 

What C4 does depends on where it is located. If it is right at the regulator, then it is there to ensure regulator stability. If it is right next to the chip, it is a simple bypass cap for the logic IC to keep noise off the lines/provide low impedance to ground for AC noise. If it is close enough to both, it will act as both.

 

Doesn't C4 also help in keeping the regulator from oscillating?

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an oscillating regulator is an unstable regulator. If a capacitor is being used to help maintain stability then it is preventing oscillatory behavior.

 

What? I know what you are saying, but why are you saying it?

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Mike2545

 

He's explaining in response to Krumlink that what he said as an alternative explaination is really the same thing.

 

thanks, yes

oh - that was in response to Triode's response to the response about my response to Krumlink.