Low noise output buffer

Abstract

A low noise output buffer to simultaneously reduce switching noise and output signal ringing for output ringing and maintain DC current. A temporary and a steady-state output buffers are supplied by a buffer voltage source and an internal circuit voltage source, respectively. Each driver has a pull-up and a pull-down transistors. While switching the output buffer from a high voltage level to a low voltage level or from a low voltage level to a high voltage level, a predriver and a single steady-state circuit are designed to respectively generate a large simultaneous switching noise at the buffer voltage source and a small simultaneous switching noise at the internal circuit voltage source. A Schmitt trigger circuit is also used to turn off the temporary driver, so as to reduce the output signal ringing while the steady-state driver maintains a supply of DC current. In another design of a low noise output buffer to reduce ground bounces and output signal ringing as well as to maintain a DC current, a temporary driver is used. An adaptive characteristic of the low noise output buffer under different loading conditions is achieved by a feedback circuit. The temporary driver is turned on only during the middle period of output transition time to provide an additional charging or discharging current. Since the temporary driver is always off apart from the transition period, the effect of reducing ground bounces and output signal ringing can thus be outstanding.