PCA9546AD datasheet(7/25 Pages) PHILIPS

7 / 25 page

9397 750 14318

Product data sheet

Rev. 03 — 6 April 2005

7 of 25

Philips Semiconductors

PCA9546A

4-channel I2C switch with reset

7.3 RESET input

The RESET input is an active LOW signal which may be used to recover from a bus fault

condition. By asserting this signal LOW for a minimum of tWL, the PCA9546A will reset its

registers and I2C-bus state machine and will deselect all channels. The RESET input must

be connected to VDD through a pull-up resistor.

7.4 Power-on reset

When power is applied to VDD, an internal Power-On Reset (POR) holds the PCA9546A in

a reset condition until VDD has reached VPOR. At this point, the reset condition is released

and the PCA9546A registers and I2C-bus state machine are initialized to their default

states—all zeroes—causing all the channels to be deselected. Thereafter, VDD must be

lowered below 0.2 V to reset the device.

7.5 Voltage translation

The pass gate transistors of the PCA9546A are constructed such that the VDD voltage can

be used to limit the maximum voltage that will be passed from one I2C-bus to another.

Figure 7 shows the voltage characteristics of the pass gate transistors (note that the graph

was generated using the data speciﬁed in Section 11 “Static characteristics” of this data

sheet). In order for the PCA9546A to act as a voltage translator, the Vo(sw) voltage should

be equal to, or lower than the lowest bus voltage. For example, if the main bus was

running at 5 V, and the downstream buses were 3.3 V and 2.7 V, then Vo(sw) should be

equal to or below 2.7 V to effectively clamp the downstream bus voltages. Looking at

Figure 7, we see that Vo(sw)(max) will be 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. Pull-up resistors

can then be used to bring the bus voltages to their appropriate levels (see Figure 14).

(1) maximum

(2) typical

(3) minimum

Fig 7.

Pass gate voltage versus supply voltage

VDD (V)

2.0

5.5

4.5

3.0

4.0

002aaa964

3.0

2.0

4.0

5.0

Vo(sw)

(V)

1.0

3.5

5.0

2.5

(1)

(2)

(3)

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