Cancellation of injected charge in a bus switch

Abstract

A bus switch has control of the timing of turning on and off the main p-channel and n-channel transistors that connect two network nodes. A pair of cross-coupled NAND gates form a set-reset S-R latch that controls the gates of the main p-channel and n-channel transistors. The S-R latch controls the timing so that the main p-channel and n-channel transistors switch at about the same time, canceling much of each other's injected charge. Since the main p-channel is larger due to the lower hole mobility, an excess of injected charge from the p-channel transistor remains. This excess charge is cancelled by opposite charge injected by compensating transistors. The compensating transistors are also p-channel devices, but are driven with a logical inverse of the gate of the main p-channel transistor. This produces a charge with opposite polarity to the excess charge from the main p-channel transistor. The sources and drains of the compensating transistors are connected together so that they transistors act as capacitors. A connecting p-channel transistor is added in parallel with the main p-channel transistor. The connecting p-channel transistors is turned on early, before the main p-channel transistor, to increase the capacitance by connecting the two network nodes. The increased capacitance decreases the voltage spike caused by a fixed amount of injected charge.