RC vs RL circuit (transient behavior)

Question

I have noticed that in PCB design, usually placing Capacitors next to the IC's on the power lane, are more favorable than placing Inductors.

I was given different resoaning that why RC is more favorable than RL.

I know that usually Inductors are bigger than Capacitors and that is one of the reasons that RC are being used more on PCB than inductors.

However, I was also told that RC circuit provide protection against power supply sagging, when an IC draws current abruptly. And, I can see why.

I was also told that on the contrary, LC will not be so good, because Inductros tend to prevent transient current changes, and this is no good, because IC tends to draw transient current.

However, I read different articles explaining that both RC and RL circuits behave very similarly. Eg: http://zone.ni.com/devzone/cda/ph/p/id/217

By reading the above article (figure 7.6), I think that those two circuits behave similalry, and they can be used interchangably. Is this correct?

Because, for example, I can see that the inductor, in the RL circuit, will prevent current change inside the inductor, if let's say an IC is connected to it on the right (figure 7.6). But, this will result in a transient voltage change on the right side of the inductor, which means that transient current will be provided to the IC via the Resistor (R) path instead of the inductor (L) paht, correct?

Which will imply that both RC and RL circuit will behave the same way when connected to an IC that draws transient current, correct?

I would appreciate some help,

Thanks, R.B.

Answer 1

The simple answer for this: -

I have noticed that in PCB design, usually placing Capacitors next to the IC's on the power lane, are more favorable than placing Inductors.

ICs require a "hard" voltage supply i.e. one which has extremely low impedance when subjected to impulses of current taken by the IC. This is perfectly suited to capacitors because they can charge quickly, hold energy for a long time and then supply that energy in very short bursts if needed or as required.

This means that the inductance/resistance of tracks feeding the power rails of the IC won't cause problems. You don't need me to say this - read virtually any data sheet of any IC and they strongly suggest power supply decoupling using a capacitor.

If the power supply has a lot of noise on it (say) for instance ripple due to rectified AC being the power source then feeding the capacitor/IC via a resistor is the main option.

If there is a lot of high frequency power supply noise, feeding the capacitor/IC via an inductor is sometimes the preferred solution but be aware of resonance issues and use a small resistor in series with the inductor too.