Switching subcircuits

In a switching subcircuit switching noise must be suppressed. When a load is suddenly applied to a voltage source, the circuit tries to suddenly increase its current , but the inductance in the power supply line acts to oppose that increase. It opposes it by lowering the voltage the power line supplies. This is not just the voltage that the load in question sees, but the voltage that every other subcircuit that shares that power supply line sees. This is only temporary -- the inductance ultimately loses the battle and the voltage comes back to normal. But even a temporary reduction in voltage can disturb other subcircuits.

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. By the time the capacitor runs out of charge, the power supply line inductance cannot maintain the previous current, so the load can draw full current at normal voltage from the power supply (and the capacitor can recharge too). The voltage dip is reduced but not eliminated; i.e. the decoupling is not perfect.

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