r

p(max)

b

t

R

0.8473 C

=

´

CC

OL(max)

p(min)

OL

V

V

R

I

-

=

LED

3.3 V

5 V

LEDx

VCC

LED

LEDx

VCC

100 k

VCC

24

TCA9538

SCPS199D – AUGUST 2014 – REVISED OCTOBER 2016

www.ti.com

Product Folder Links: TCA9538

Submit Documentation Feedback

Copyright © 2014–2016, Texas Instruments Incorporated

Typical Application (continued)

9.2.1 Design Requirements

9.2.1.1 Minimizing ICC When I/Os Control LEDs

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

Figure 24. For a P-port configured as an input, ICC increases as VI becomes lower than VCC. The LED is a diode,

with threshold voltage VT, and when a P-port is configured as an input the LED is off but VI is a VT drop below

VCC.

For battery-powered applications, it is essential that the voltage of P-ports controlling LEDs is greater than or

equal to VCC when the P-ports are configured as input to minimize current consumption. Figure 25 shows a high-

value resistor in parallel with the LED. Figure 26 shows VCC less than the LED supply voltage by at least VT.

Both of these methods maintain the I/O VI at or above VCC and prevents additional supply current consumption

when the P-port is configured as an input and the LED is off.

Figure 25. High-Value Resistor in Parallel with LED

Figure 26. Device Supplied by a Lower Voltage

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 slaves on the I2C bus. The minimum pull-up resistance is a function of

VCC, VOL,(max), and IOL as shown in Equation 1:

(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 as shown in Equation 2:

(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/traces, and the capacitance of additional slaves on the bus.