r

p(max)

b

t

R

0.8473 C

=

´

DPUX

OL(max)

p(min)

OL

V

V

R

I

-

=

18

TCA9543A

SCPS206B – MARCH 2014 – REVISED NOVEMBER 2019

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Copyright © 2014–2019, Texas Instruments Incorporated

Typical Application (continued)

The A0 and A1 terminals are hardware selectable to control the slave address of the TCA9543A. These

terminals may be tied directly to GND or VCC in the application.

If both slave channels will be activated simultaneously in the application, then the total IOL from SCL/SDA to GND

on the master side will be the sum of the currents through all pull-up resistors, Rp.

The pass-gate transistors of the TCA9543A are constructed such that the VCC voltage can be used to limit the

maximum voltage that is passed from one I2C bus to another.

Figure 17 shows the voltage characteristics of the pass-gate transistors (note that the graph was generated using

data specified in the Electrical Characteristics section of this data sheet). In order for the TCA9543A 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, Vpass must be equal to or below 2.7 V

to effectively clamp the downstream bus voltages. As shown in Figure 17, Vpass(max) is 2.7 V when the TCA9543A

supply voltage is 4 V or lower, so the TCA9543A supply voltage could be set to 3.3 V. Pull-up resistors then can

be used to bring the bus voltages to their appropriate levels (see Figure 16).

9.2.2 Detailed Design Procedure

Once all the slaves are assigned to the appropriate slave channels and bus voltages are identified, the pull-up

resistors, Rp, for each of the buses need to be selected appropriately. The minimum pull-up resistance is a

function of VDPUX, VOL,(max), and IOL:

(1)

The maximum pull-up resistance is a function of the maximum rise time, tr (300 ns for fast-mode operation, fSCL =

400 kHz) and bus capacitance, Cb:

(2)

The maximum bus capacitance for an I2C bus must not exceed 400 pF for fast-mode operation. The bus

capacitance can be approximated by adding the capacitance of the TCA9543A, Cio(OFF), the capacitance of

wires/connections/traces, and the capacitance of each individual slave on a given channel. If both channels will

be activated simultaneously, each of the slaves on both channels will contribute to total bus capacitance.