SN74LVC1G04: Input transition rise or fall rate (dt/dv)

The input transition rise or fall rate is not an electrical characteristic. It is listed in the Recommended Oprating Conditions table because it is a limit on how slow the input edges are allowed to be. The actual edges can (and should) be faster than the limit.

Understanding and Interpreting Standard-Logic Data Sheets says:

4.5.13 Δt/Δv Input Transition Rise or Fall Rate

The rate of change of the input voltage waveform during a logic transition (low-to-high or high-to-low).

To avoid output-waveform abnormalities, input voltage transitions should be within the range set forth in the recommended operating conditions.

Customers often place external capacitors on a trace to ensure the driver does not switch rapidly from one logic state to another. This is sometimes done to prevent unwanted overshoot and undershoot voltage conditions that could cause ringing and degrade signal integrity, or in switch debounce circuits. However, this could cause problems at the input; therefore, TI provides input transition rise or fall rates. The problem may not arise due to external capacitive loading, however, but may be the result of choosing a device with a weak driver. In either case, the end result is a voltage waveform that is too slow for the device.

Slow transition rates wreak havoc on CMOS inputs because a slowly changing input voltage will induce a large amount of current from the power supply to ground. This phenomenon is known as through current. Through currents are normal ac transient currents, but when they are sustained indefinitely-as are those caused by slow input transition rates-the device will not perform as expected, and its output voltage may oscillate or, even worse, damage the device. This surge of current, if large enough, will disturb the ground reference because of the inductive nature of the package [V = L × (di/dt)] and produce a positive-going glitch on the ground reference. The glitch may, in turn, reduce the relative magnitude, causing the output node of the input inverter to switch states. Ultimately, this erroneous data propagates to the output of the device, thereby causing oscillations. The more inputs that are being switched in the same manner, the worse this condition becomes, as more current is being forced into ground during a short time. TI data sheets specify the slowest input transition rate to avoid this problem. For additional information, refer to the TI application report, Implications of Slow or Floating CMOS Inputs, literature number SCBA004.

Helpful Hint:

If you must supply a slowly changing voltage to the input of a logic device, select a device that has Schmitt-trigger inputs. These inputs have been specifically designed to tolerate slow edges. An example of such a device in the LVC family is the SN74LVC14A.

HI Brian,

The input transition rate limit is for the input and doesn't impact your outputs. The output frequency will be the same as your input frequency.

CMOS devices can't have slow input edges since if the input is at half Vcc for too long, then the output doesn't know what state to be in. So the input has to have a fast transition. This limit on how slow of an edge rate is spec'd in the datasheet as input transition rate.

Thanks!

-Karan