TCA9555_16 Datasheet, PDF (27/46 Pages) Texas Instruments – Low-Voltage 16-Bit I2C and SMBus I/O Expander with Interrupt Output and Configuration Registers

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TCA9555
SCPS200D – JULY 2009 – REVISED JULY 2016
V_CC
V_CC
Pn
LED
100 kΩ
Figure 35. High-Value Resistor in Parallel With LED
3.3 V
5V
V_CC
Pn
LED
Figure 36. Device Supplied by Lower Voltage
10.2.2.3 Pull-Up Resistor Calculation
The pull-up resistors, R_P, for the SCL and SDA lines need to be selected appropriately and take into
consideration the total capacitance of all slaves on the I²C bus. The minimum pull-up resistance is a function of
V_CC, V_OL,(max), and I_OL as shown in Equation 5.
R_p(min)
=
V_CC
- V_OL(max)
I_OL
(5)
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 as shown in Equation 6.
R_p(max)
=
t_r
0.8473 ´ C_b
(6)
The maximum bus capacitance for an I²C bus must not exceed 400 pF for standard-mode or fast-mode
operation. The bus capacitance can be approximated by adding the capacitance of the TCA9555, C_i for SCL or
C_io for SDA, the capacitance of wires, connections and traces, and the capacitance of additional slaves on the
bus. For further details, see the I²C Pull-up Resistor Calculation application report, SLVA689.
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