Fast, low-noise CMOS output buffer

Claim 1

An improved CMOS output buffer comprising: transient pull-up circuit means responsive to a high drive enable pulse signal for generating a transition from a low logic level to a high logic level at an output node; said pull-up circuit means including a transient pull-up output driver transistor having one of its main electrodes connected to a high transient supply voltage, its other main electrode connected to the output node, and its control electrode coupled to receive the high drive enable pulse signal; transient pull-down circuit means responsive to a low drive enable pulse signal for generating a transition from the high logic level to the low logic level at the output node; said pull-down circuit means including a transient pull-down output driver transistor having one of its main electrodes connected to a low transient supply voltage, its other main electrode connected to the output node, and its control electrode coupled to receive the low drive enable pulse signal; keeper circuit means being responsive to the high and low drive enable pulse signals for maintaining the output node at the high logic level after the output node has made the low-to-high transition and for maintaining the output node at the low logic level after the output node has made the high-to-low transition; said keeper circuit means including a P-channel output driver MOS transistor and an N-channel output driver MOS transistor; said keeper circuit means further including delay means responsive to the high and low drive enable pulse signals and having first and second outputs for delaying the turning on of the N-channel output drive transistor until after the output node has completed the high-to-low transition; said P-channel output driver transistor having its source connected to a supply potential, its drain connected to the output node, and its gate coupled to the first output of said delay means; said N-channel output driver transistor having its drain connected to the output node, its source connected to a holding low supply voltage, and its gate coupled to the second output of said delay means; said keeper circuit means further including speed-up means responsive to the high drive enable pulse signal for quickly turning off the N-channel output drive transistor when the output node is making the low-to-high transition; said P-channel and N-channel output driver transistors being sized so as to be relatively smaller than said transient pull-up and pull-down output driver transistors; the turning on of the N-channel output drive transistor being delayed relative to the turning on of said transient pull-down output drive transistor during the high-to-low transition so that noise generated on the low transient supply voltage will be isolated from the holding low supply voltage; and said N-channel output drive transistor being quickly turned off when said transient pull-up output drive transistor is turned on so that noise generated on the high transient supply voltage during the low-to-high transition will be isolated from the holding low supply voltage.