Triple-slope clock driver for reduced EMI

Abstract

A clock driver for an integrated circuit reduces electro-magnetic interference (EMI) induced in nearby metal traces yet also reduces jitter due to noise at the switching threshold. A weak driver using small n-channel and p-channel transistors initially drives the clock line. Then a pulse generator produces a short pulse to a gate of a large driver transistor. The large driver transistor is pulsed on for a very short period of time. The large driver transistor is turned off by the end of the pulse before the clock line completes its transition. The weak driver then finishes the clock-line transition. Since only the weak driver is on during the start and end of the transition, a slow voltage-slew rate occurs at the beginning and end of the transition. The large driver transistor is on only in the middle of the transition, producing a fast voltage-slew rate in the middle. A triple-slope waveform results. Since a fast voltage-slew occurs in the middle of the transition near the receiver's switching threshold, clock jitter due to supply noise is reduced. EMI is reduced because the average slew rate is reduced.

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