A CMOS inverter can be considered equivalent to a simplified
output driver shown in Fig. 1. Pin parasitics across the
chip ground and the system ground are represented by a
lumped inductor, a capacitor, and a resistor. The bypass
capacitor is connected between the chip power and ground.
The NMOS transistor turns on when the driver input changes
from low to high and the discharge current flows into the
chip ground node. If there is no bypass capacitor, the entire
discharge current flows into the inductor, generating more
ground bounce. But due to the presence of the bypass capacitor,
some part of the current flowing into the inductor is
driven into the bypass capacitor resulting in less current
throagh the ground inductor and hence less ground bounce.
As lieen from Fig. 2, the maximum ground bounce occurs
when the input voltage becomes VDD for both the cases, with
and without bypass capacitor. From Fig. 2 it can be observed
that when there is a bypass capacitor across the chip power
and ground, the gate to source voltage becomes closer to the
input voltage without ground bounce. Also, the output
vok3g-e falls more rapidly than without the bypass capacitor
case. Hence the use of an on-chip bypass capacitor also helps
to irnprove the speed of the circuit.