This invention relates generally to integrated circuits and more particularly, it relates to output buffer circuits for reducing significantly ground bounce noise.
As is well known, digital logic circuits are widely used in the electronics field. One such use is for the interfacing between the logic of one integrated circuit device and another integrated circuit device. An output buffer circuit is an important component for this interface function. The output buffer provides, when enabled, an output signal which is a function of data signal received from other logic circuitry of the integrated circuit.
Output buffer circuits typically use a pull-up fieldeffect transistor (FET) and a pull-down field-effect transistor connected to an output terminal. Depending upon the state of the input data signal and an enable signal, either the pull-up or pull-down field-effect transistor is quickly turned on and the other one of them is turned off.
In general, the output buffer circuits of the prior art have not been capable of providing output drive current of the magnitude possible with bipolar integrated circuits. There have been attempts made to achieve higher output drive currents from output buffers by increasing the size of the pull-down field-effect transistor to approximately twice the size of the pull-up field-effect transistor. However, this has resulted in sacrifices in speed and output signal distortion due to the large instantaneous current, when the pull-down transistor is turned on, flowing through the impedance and inductance components of the power supply lines to cause inductive noise at the internal ground potential node of the output buffer.
It would therefore be desirable to provide improved output buffer circuits which have a high current drive capability comparable to the bipolar integrated circuits but yet reduces significantly the distortion in the output signal due to ground bounce noise. The output buffer circuits of the present invention include a first pull-down transistor having a size equal to the size of the pull-up transistor and a second pull-down transistor having a larger size to accommodate the high current drive capability. The reduction in the bounce noise is achieved by delaying the turn-on of the second larger pull-down transistor until after the output has reached the low or logic "0" level.