TCA9546A Datasheet, PDF (17/31 Pages) Texas Instruments – Low Voltage 4-Channel I2C and SMBus Switch with Reset Function

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TCA9546A
SCPS205A – APRIL 2014 – REVISED FEBRUARY 2015
Typical Application (continued)
10.2.1 Design Requirements
The A0, A1, and A2 pins are hardware selectable to control the slave address of the TCA9546A. These pins may
be tied directly to GND or V_CC in the application.
If multiple 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 TCA9546A 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 16 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 TCA9546A 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 16, V_pass(max) is 2.7 V when the TCA9546A
supply voltage is 4 V or lower, so the TCA9546A 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 15).
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 TCA9546A, C_io(OFF), the capacitance of
wires/connections/traces, and the capacitance of each individual slave on a given channel. If multiple channels
will be activated simultaneously, each of the slaves on all channels will contribute to total bus capacitance.
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