Supply and Ground Bounce

The output voltage of an IC is with respect to the ground of the chip. Hence, in a system, the tiny wire connection between the chip and the lead frame of the package contributes a small amount of inductance in the circuit. When Vout goes low, a spike of current flows through this inductance and creates a voltage spike. A device connected to the output of the circuit indicates a logical zero. This is called as ground bounce. The VDD and ground pins of the package are not at VDD and ground. VDD and ground are established and regulated at the power supply. Therefore, both a resistive component and an inductive component between the package pin and the power supply ground are exists. Therefore, the designer use heavy copper planes that make them as full as possible with as few cuts and holes as possible. But, in fact, this inductance in many high speed circuits is just too large to tolerate. So the bypass capacitors are used in VLSI integrated circuits. The purpose of bypass capacitors is to implement that looks like a regulated VDD and ground right at the package for a short time until the inductance of the planes is overcome. The transient currents when the logic device changes state will not have to flow to and from the power supply, they simply flow to and from the capacitor.

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