PCA9546A datasheet(5/29 Pages) TI

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RESET Input

Power-On Reset

Voltage Translation

Maximum

Typical

Minimum

VCC (V)

4.5

3.5

2.5

5.5

4.5

3.5

2.5

1.5

PCA9546A

4-CHANNEL I2C AND SMBus SWITCH

WITH RESET FUNCTION

SCPS148E – OCTOBER 2005 – REVISED JANUARY 2008

Table 1. Control Register Write (Channel Selection), Control Register Read (Channel Status)(1)

COMMAND

Channel 0 disabled

Channel 0 enabled

Channel 1 disabled

Channel 1 enabled

Channel 2 disabled

Channel 2 enabled

Channel 3 disabled

Channel 3 enabled

No channel selected,

power-up/reset default state

(1)

Several channels can be enabled at the same time. For example, B3 =0, B2 = 1, B1 = 1, B0 = 0 means that channels 0 and 3 are

disabled, and channels 1 and 2 are enabled. Care should be taken not to exceed the maximum bus capacity.

The RESET input is an active-low signal that may be used to recover from a bus-fault condition. When this signal

is asserted low for a minimum of tWL, the PCA9446A resets its registers and I

2C state machine and deselects all

channels. The RESET input must be connected to VCC through a pullup resistor.

When power is applied to VCC, an internal power-on reset holds the PCA9546A in a reset condition until VCC has

reached VPOR. At this point, the reset condition is released, and the PCA9546A registers and I

2C state machine

are initialized to their default states, all zeroes, causing all the channels to be deselected. Thereafter, VCC must

be lowered below 0.2 V to reset the device.

The pass-gate transistors of the PCA9546A are constructed such that the VCC voltage can be used to limit the

maximum voltage that will be passed from one I2C bus to another.

Figure 3 shows the voltage characteristics of the pass-gate transistors (note that the graph was generated using

the data specified in the electrical characteristics section of this data sheet). In order for the PCA9546A to act as

a voltage translator, the Vpass voltage must be equal to or lower than the lowest bus voltage. For example, if the

main bus is running at 5 V, and the downstream buses are 3.3 V and 2.7 V, then Vpass must be equal to or below

2.7 V to effectively clamp the downstream bus voltages. As shown in Figure 3, Vpass (max) is at 2.7 V when the

PCA9546A supply voltage is 3.5 V or lower, so the PCA9546A supply voltage could be set to 3.3 V. Pullup

resistors then can be used to bring the bus voltages to their appropriate levels (see Figure 12).

Figure 3. Vpass Voltage vs VCC

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