- The maximum operating frequency of the bus buffer type can be calculated from the propagation delay times that are determined by the CR values of the load and on-resistance.
Maximum operating frequency ≈ 1/(tpLH ＋ tpHL)
- The maximum operating frequency of the bus switch type can be calculated based on the rise waveform.
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 figure.)
1/τ = 1/(C×R)
Take, 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.
For the recommended level shifters compatible with communication standards, please refer to “Are there level shifters compatible with communication standards?” in the Frequently Asked Questions (FAQ) page.
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.