SCPS237 June 2018 TCA9517-Q1
VCCB and VCCA can be applied in any sequence at power up. The TCA9517-Q1 includes a power-up circuit that keeps the output drivers turned off until VCCB is above 2.5 V and the VCCA is above 0.8 V. After power up and with the EN high, a low level on the A-side (below 0.3 × VCCA) turns the corresponding B-side driver (either SDA or SCL) on and drives the B-side down to approximately 0.5 V. When the A-side rises above 0.3 × VCCA, the B-side pull-down driver is turned off and the external pull-up resistor pulls the pin high. When the B-side falls first and goes below 0.3 × VCCB, the A-side driver is turned on and the A-side pulls down to 0 V. The B-side pull-down is not enabled unless the B-side voltage goes below 0.4 V. If the B-side low voltage does not go below 0.5 V, the A-side driver turns off when the B-side voltage is above 0.7 × VCCB. If the B-side low voltage goes below 0.4 V, the B-side pull-down driver is enabled, and the B-side is able to rise to only 0.5 V until the A-side rises above 0.3 × VCCA.
TI recommends using a decoupling capacitor and placing it close to the VCCA and VCCB pins of a value of about 100 nF.