Decoupling capacitor and transient current

Suppressing transient current

When a digital IC switches, it draws a short pulse of current (di/dt very high) from the supply. The inductance of the supply trace gives V = L·di/dt, a voltage dip or spike on the rails, and a ground bounce. A decoupling capacitor (e.g. 0.1 uF) placed right across the IC's power and ground pins stores charge. During a transient it supplies the current locally (I = C·dV/dt), so the demand on the distant supply is not sudden; the voltage across the capacitor changes little (ΔV = I·Δt/C). The capacitor also gives a low-impedance path to ground for the high-frequency noise present on the supply line. Hence supply ripple and ground bounce are reduced.

 Vcc ----+------- IC Vcc
         |
        ===  C (0.1 uF, close to pin)
         |
 GND ----+------- IC GND

Effects of a very large decoupling capacitor

1. High inrush current at power-up: the uncharged capacitor behaves as a short circuit, so I = C·dV/dt becomes large, stressing the supply, fuse and connectors, and may trigger the supply's protection.
2. Slow rise of the supply voltage (longer start-up), which may cause improper reset.
3. Lower self-resonant frequency: a large capacitor has higher parasitic inductance (ESL), so it is ineffective at high frequency (inductive above its resonance), and does not remove fast noise.
4. Possible resonance (anti-resonance) with other capacitors and supply instability with regulators.
5. Larger size, higher cost, more leakage current and board area.

Therefore, a combination of a small capacitor (0.1 uF / 0.01 uF) for high frequency and a moderate bulk capacitor (10 uF) for low frequency is used.