# How can I calculate the maximum operating frequency of a level shifter (voltage translation logic IC)?

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.

• The maximum operating frequency of the bus buffer type can be calculated from the propagation delay times (tpLH, tpHL) as shown below.
Maximum operating frequency ≈ 1/(tpLH ＋ tpHL)
• The maximum operating frequency of the bus switch type can be calculated based on the CR time constant of the load and on-resistance.
Maximum operating frequency ≈ 1/(τ), where τ is a time constant:
Since τ is determined by charging and discharging time, τ is calculated from the time required for the output voltage to reaches 63 %. (See the Fig. 1.)
1/τ = 1/(C×R)

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.