Bus voltage translation is when the bus voltage is translated up or down. This presents a unique challenge with I3C for FET-based translators, like the TCA39306, because they rely on a pull-up resistor to translate the voltage up from the low-voltage side. The pull-up resistor selected must be strong enough to meet the timing requirements (based on bus capacitance and translation voltages), but not so strong to violate the V_IL requirements of the I3C devices.

The pull-up resistors are needed on both sides. The reason for this is that with the normal translation setup, the switch is "on" when either side's bus voltage drops to roughly V_{PU_1}. This means that the pull-up resistors are required to pull the bus voltage on the high-voltage side from V_{PU_1} to V_{PU_2}. When the device on the high-voltage side is controlling the bus, the switch will turn off at V_{PU_1}. The pull-up resistors on the low-voltage side are used to bleed off any additional current that might "leak" through the switch.

Figure 9-7 I3C Bus Translation