To begin the design process, determine the following:
Input voltage range
Use the supply voltage of the device that is driving the TXB0101 device to determine the input voltage range. For a valid logic HIGH the value must exceed the VIH of the input port. For a valid logic LOW the value must be less than the VIL of the input port.
Output voltage range
Use the supply voltage of the device that the TXB0101 device is driving to determine the output voltage range.
External pullup or pulldown resistors are not recommended. If mandatory, TI recommends the value should be larger than 50 kΩ.
An external pulldown or pullup resistor decreases the output VOH and VOL. Use Equation 1 and Equation 2 to draft estimate the VOH and VOL as a result of an external pulldown and pullup resistor.
Equation 1. VOH = VCCx × RPD / (RPD + 4.5 kΩ)
Equation 2. VOL = VCCx × 4.5 kΩ / (RPU + 4.5 kΩ)
where
VCCx is the output port supply voltage on either VCCA or VCCB
RPD is the value of the external pulldown resistor
RPU is the value of the external pullup resistor
4.5 kΩ is the counting the variation of the serial resistor 4 kΩ in the I/O line.