1 Transient Drive Strength

Transient drive strength refers to the ability of the CMOS output buffer to deliver high current during short-duration transient events, such as switching transitions. This indicates the ability of the output to supply or sink current by quickly charging or discharging the output capacitance, while switching from LOW to HIGH or HIGH to LOW.

Transient drive strength is critical for ensuring fast rise and fall times of output signals charging and discharging larger capacitive loads, which directly impact the system's speed and signal integrity. A faster rise and fall time indicates a higher transient drive strength. In high-speed applications, such as clock distribution and data transmission, a strong transient drive capability is essential to minimize signal propagation delays for higher loads and reduce the risk of signal distortion.

Design considerations for enhancing transient drive strength includes optimizing the output stage's size – width and length of the transistors used in the pull-up and pull-down buffers and minimizing parasitic capacitance in the output network to increase transient response.

However, careful system considerations are recommended as some systems might not be able to handle strong transient drive due to sensitivities to transmission line effects. Some symptoms can include overshoots / undershoots / reflections (commonly known as ringing). Simply using series dampening resistors on the outputs help adjust accordingly, per system requirements. Appropriate dampening resistor sizing should be considered to accommodate for the reduced rise/fall times, the voltage drop across the resistor limiting the signal swing, additional propagation delay and additional power dissipation introduced.

Rise or Fall times are good indicators of a device’s transient drive strength.