Buffer type products and bus switch type products have different calculation methods for the operating frequency. Buffer type products can be calculated based on the propagation delay time obtained from the CR values of the load and on-resistance. On the other hand, bus switch type products can be calculated based on the load and on-resistance CR time constant.
Each calculation method is briefly described below.
For example, the TC7SPB9306 with a switch I/O capacitance (CI/O) of 14 pF. Assume a load capacitance (CL) of 30 pF and a pullup resistor (Rpu) of 1 kΩ. Then, τ is calculated as follows:
τ = (14 pF + 30 pF) × 1 kΩ
Hence, the maximum operating frequency is calculated as roughly 20 MHz
The result of this calculation should be considered merely as an estimate under the conditions shown in the datasheet.
Level shifters should be sufficiently evaluated to ensure that they work properly under the actual operating conditions because their propagation delay times change with the board capacitance and other conditions.
The following documents also contain related information.