Digital Circuit Boards: Mach 1 GHz

4.9 The IC Decoupling Problem

IC dies are extremely dense component packages. Trace lengths are very short so that reflections in the die are normally not an issue. Consider the case where an IC must drive an external 50-ohm transmission line. The energy to drive the line would normally come from decoupling capacitors and the ground/power plane capacitance, both of which are external to the IC package. Assume that the path between the die and an external decoupling capacitor is a short section of 50-ohm line. When the logic switch closes, the voltage to the IC and to the external transmission line must drop. If the rise time of the switch is long enough (the clock rate is low enough), the voltage will not sag. For rise times less than 1 ns, there needs to be local decoupling on the die. Without this decoupling, the reduced voltage after a logic switch closure can cause the logic to malfunction.

If the die must drive a 16-bit parallel port with a short rise time, the current demand at switch closure can exceed 1 A. Assume that these lines are series terminated. This current must flow until the waves on the transmission lines have made one round trip. The energy demand is directly proportional to the length of the transmitting path. This level of energy cannot be supplied through a short section of 50-ohm line even if it is only 5 mm long. Supplying this energy from a decoupling capacitor located on or at the die is one solution. Connecting the die to an external decoupling capacitor ...