Why Do ICs Need Bypass Capacitors?

Any electronic design engineer will vouch for the necessity of supplementing integrated circuits on their PCB with bypass capacitors, although they may not understand the reason to do so very well. As a rule of thumb, engineers provide every IC with a 0.1µF ceramic capacitor next to its power pins in each circuit board they design. Along with proper PCB layout techniques, adding a bypass capacitor improves circuit performance and maximizes the efficacy of the ICs.

The trouble lies with transition currents. Circuits handling digital signals produce rapid transitions when their signals switch states. When digital circuits output a high state, the signal voltage is very close to the supply voltage. When they output a low state, the signal voltage reaches very near the ground voltage. When transiting from a low to high or a high to low, the voltage swing from supply to ground or from ground to supply, causes a transient current to be drawn from the supply.

Usually, power to an electronic circuit on a PCB is fed at a single point and traces on the PCB carry this power to each IC. Traces on the PCB have their own parasitic inductance, which, when coupled with the source impedance of the power supply, react to transient currents by creating voltage transients.

The trouble aggravates when ICs have to drive low-resistance or high-capacitance loads. The low-resistance demands high currents when the digital state changes from low to high. Again, when the digital state changes from high to low, there is a demand for the load current to reduce suddenly. However, according to Lenz’s Law, an induced current will flow such as to oppose the change that produced it.

The net effect of transient currents and the parasitic inductance of PCB traces and wires are to create high-magnitude voltage transients, ringing or severe oscillations in the power lines. This can lead to suboptimal circuit performance or even to system failure. Engineers at Texas Instruments have demonstrated an improperly bypassed line driver IC switching at 33MHz can induce ringing amplitude of the order of 2V peak-to-peak on a 5V power rail.

Placing a 0.1µF ceramic capacitor close to the IC power pins improves the situation, because capacitors store charge. Placing the bypass capacitor close to the IC allows low resistance and series inductance. The bypass capacitor is therefore in a better situation to supply or absorb the transients on the PCB traces, which have a comparatively larger resistance and series inductance.

Although engineers refer to such components as both bypass and decoupling capacitors, there is a subtle distinction between the two terms. Decoupling refers to the amount by which one part of the circuit influences another. Bypassing provides a low-impedance path allowing noise to pass by an IC on its way to ground. A capacitor, placed close to the IC supply pins, accomplishes both decoupling and bypassing. However, a decoupling capacitor has an additional task. It blocks the DC component of a signal and prevents it from traveling through to the next part of the circuit, while allowing the AC component little or no resistance at all.