The PCA9540B is a 1-of-2 bidirectional translating multiplexer controlled by the IC bus. The SCL/SDA upstream pair expands into two SCx/SDx downstream pairs, or channels. The contents of the programmable control register decide which SCx/SDx channel is active at any given time. The power-on reset operation restores the registers to their default state and starts the IC-bus state machine with no channels chosen.
The pass gates of the multiplexer are designed so that the VDD pin can be utilized to limit the maximum high voltage passed by the PCA9540B. This enables the use of multiple bus voltages on each SCx/SDx pair, allowing 1.8 V, 2.5 V, or 3.3 V parts to communicate with 5 V parts without requiring additional safety. External pull-up resistors can bring the bus voltage up to the necessary level for this channel. All I/O pins are 5 V tolerant.
Datasheet