The pull-up resistor value needs to limit the current through the pass transistor when it is in the ON state to about 15 mA. This ensures a pass voltage of 260 mV to 350 mV. If the current through the pass transistor is higher than 15 mA, the pass voltage also is higher in the ON state. To set the current through each pass transistor at 15 mA, to calculate the pull-up resistor value use Equation 1.

Table 8-3 summarizes resistor values, reference voltages, and currents at 15 mA, 10 mA, and 3 mA. The resistor value shown in the +10% column (or a larger value) should be used to ensure that the pass voltage of the transistor is 350 mV or less. The external driver must be able to sink the total current from the resistors on both sides of the LSF0204-Q1 device at 0.175 V, although the 15 mA applies only to current flowing through the LSF0204-Q1 device.

The LSF0204-Q1 does not provide any drive capability. Therefore higher frequency applications will require higher drive strength from the host side. No pullup resistor is needed on the host side (3.3 V) if the LSF0204-Q1 is being driven by standard CMOS totem pole output driver. Best practice is to minimize the trace length from the LSF0204-Q1 on the sink side (1.8 V) to minimize signal degradation.