4 Why High DC Strength Does Not Always Mean High Transient Strength

To make sure a high static current strength, larger MOSFETs are typically deployed with wider routing and connections to avoid reliability concerns like electro-migration. These large size FETs poses two challenges to designers:

• The large FETs with high DC strength often lead to higher amplitude of ringing at the output during transients.
• The wider routing incorporates additional parasitic capacitance which slows down transient response.

To control ringing, current limiter circuits are added in CMOS buffers to limit the transient current and thus reduce ringing. Hence, the buffer with high static current strength can have a low transient strength.

Figure 4-1 and Figure 4-2 compares SN74AVCxT245 (faster transient drive - rise or fall times, lower static drive - specified for up to 12mA) and SN74LXCxT245 (slower transient drive - rise or fall times, higher static drive - specified for up to 32mA).

Figure 4-1 High Transient Drive Strength Buffer (AVC)

Figure 4-2 High DC Drive Strength Buffer With Transient Current Limiter Circuit (LXC)

Figure 4-1, compares AVC and LXC transient drive strengths. AVC switches faster than LXC, indicating a higher transient drive strength.

Figure 4-3 Rise or Fall Times of SN74AVCxT245 and SN74LXCxT245

Figure 4-4, compares AVC and LXC transient currents required for their switching transitions. Although specified for a lower static current, the faster AVC device requires higher transient currents to switch between the fast HIGH or LOW transitions.

Figure 4-4 Transient Switching Current for SN74AVCxT245 and SN74LXCxT245