TCA9538 Datasheet, PDF (24/37 Pages) Texas Instruments – Low Voltage 8-Bit I2C and SMBus Low-Power I/O Expander

TCA9538
SCPS199C – AUGUST 2014 – REVISED OCTOBER 2015
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Typical Application (continued)
9.2.1 Design Requirements
9.2.1.1 Minimizing I_CC When I/Os Control LEDs
When the I/Os are used to control LEDs, normally they are connected to V_CC through a resistor as shown in
Figure 24. For a P-port configured as an input, I_CC increases as V_I becomes lower than V_CC. The LED is a diode,
with threshold voltage V_T, and when a P-port is configured as an input the LED will be off but V_I is a V_T drop
below V_CC.
For battery-powered applications, it is essential that the voltage of P-ports controlling LEDs is greater than or
equal to V_CC 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 V_CC less than the LED supply voltage by at least V_T.
Both of these methods maintain the I/O V_I at or above V_CC and prevents additional supply current consumption
when the P-port is configured as an input and the LED is off.
V_CC
V_CC
LEDx
LED 100 k
Figure 25. High-Value Resistor in Parallel With LED
3.3 V
5V
V_CC
LED
LEDx
Figure 26. Device Supplied by a Lower Voltage
9.2.2 Detailed Design Procedure
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:
R_p(min)
=
V_CC
- 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:
R_p(max)
=
t_r
0.8473 ´ C_b
(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, C_i for SCL or
C_io for SDA, the capacitance of wires/connections/traces, and the capacitance of additional slaves on the bus.
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