TCA9543A_15 Datasheet, PDF (18/32 Pages) Texas Instruments – TCA9543A Low Voltage 2-Channel I2C Bus Switch With Interrupt Logic And Reset

TCA9543A
SCPS206A – MARCH 2014 – REVISED FEBRUARY 2015
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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 V_CC in the application.
If both slave channels will be activated simultaneously in the application, then the total I_OL from SCL/SDA to GND
on the master side will be the sum of the currents through all pull-up resistors, R_p.
The pass-gate transistors of the TCA9543A are constructed such that the V_CC 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 V_pass 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, V_pass must be equal to or below 2.7 V
to effectively clamp the downstream bus voltages. As shown in Figure 17, V_pass(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).
10.2.2 Detailed Design Procedure
Once all the slaves are assigned to the appropriate slave channels and bus voltages are identified, the pull-up
resistors, R_p, for each of the buses need to be selected appropriately. The minimum pull-up resistance is a
function of V_DPUX, V_OL,(max), and I_OL:
R_p(min)
=
V_DPUX
- V_OL(max)
I_OL
(1)
The maximum pull-up resistance is a function of the maximum rise time, t_r (300 ns for fast-mode operation, f_SCL =
400 kHz) and bus capacitance, C_b:
t
R_p(max)
=
r
0.8473 ´ C_b
(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, C_io(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.
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