Decoupling Capacitor - Switching Subcircuits

Switching Subcircuits

In a switching subcircuit switching noise must be suppressed. When a load is applied to a voltage source, it draws a certain amount of current. Typical power supply lines show inherent inductance, which results in a slower response to change in current. This in turn affects the transient voltage levels, since if the load current is zero the voltage across the load is zero as well. This sudden voltage drop would be seen by other loads as well if the inductance between two loads is much lower compared to the inductance between the loads and the output capacitors of the power supply. This is only temporary; the inductor ultimately saturates (that is the magnetic field around the conductor reaches its max), the voltage drop across the inductor reaches zero, and the supply voltage comes back to normal. But even a temporary reduction in voltage can disturb adjacent subcircuits. Decoupling caps provide instantenous current jolt which helps maintain constant voltage across a subcircuit (or provide a low impedance path for the transient currents; different descriptions are used by different industries).

To decouple other subcircuits from the effect of the sudden current demand, a decoupling capacitor can be placed between the supply voltage line and its reference (ground) next to the switched load. While the load is switched out, the capacitor charges up to full power supply voltage and otherwise does nothing. When the load is applied, the capacitor initially supplies demanded current. Ideally, by the time the capacitor runs out of charge, the power supply line inductance is saturated, and the load can draw full current at normal voltage from the power supply (and the capacitor can recharge too). Note that the voltage dip is reduced but not eliminated; i.e., the decoupling is not perfect and sometimes parallel combinations of caps are used to improve response. The best way to reduce switching noise is to design a PCB as a giant capacitor by sandwiching the power and ground planes across a dielectric material.

The size of the capacitor must be reasonable, and there is a tradeoff between capacitor size and signal quality at a given frequency. If a cap is too large it would distort the signal by charging too slowly and filtering out the signal's most needed high-frequency components.