TCA6408ARSVR datasheet(29/50 Pages) TI2

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9.2.2 Detailed Design Procedure

The pull-up resistors, RP, for the SCL and SDA lines need to be selected appropriately and take into

consideration the total capacitance of all targets on the I2C bus. The minimum pull-up resistance is a function of

VCC, VOL,(max), and IOL:

OL(max)

p(min)

(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:

p(max)

0.8473 C

(2)

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 TCA9538, Ci for SCL or

CIO for SDA, the capacitance of wires, connections, and traces, and the capacitance of additional targets on the

bus.

9.2.2.1 Minimizing ICC When I/O is Used to Control LEDs

When the I/Os are used to control LEDs, normally they are connected to VCC through a resistor as shown in

Figure 9-1. The LED acts as a diode, so when the LED is off, the I/O VIN is about 1.2 V less than VCC. The ΔICC

parameter in Section 6.5 shows how ICC increases as VIN becomes lower than VCC. Designs that must minimize

current consumption, such as battery power applications, should consider maintaining the I/O pins greater than

or equal to VCC when the LED is off.

Figure 9-2 shows a high-value resistor in parallel with the LED. Figure 9-3 shows VCC less than the LED supply

voltage by at least 1.2 V. Both of these methods maintain the I/O VIN at or above VCC and prevent additional

supply current consumption when the LED is off.

LED

100 kΩ

Figure 9-2. High-Value Resistor in Parallel With LED

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TCA6408A

SCPS192E – APRIL 2009 – REVISED JANUARY 2023

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