TCA6408A-Q1 Datasheet, PDF (27/40 Pages) Texas Instruments – Low-Voltage 8-Bit I2C and SMBus I/O Expander With Interrupt Output

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TCA6408A-Q1
SCPS234 – SEPTEMBER 2016
Typical Application (continued)
θ_JA is the standard junction to ambient thermal resistance measurement of the package, as seen in Thermal
Information table. P_d is the total power dissipation of the device, and the approximation is shown in Equation 2.
( ) _{å å} ^P_d ^{» I}_{CC _ STATIC} ^{´ V}_CC ^{+ P}_{d_PORT _L} ^{+ P}_{d_PORT _H}
(2)
Equation 2 is the approximation of power dissipation in the device. The equation is the static power plus the
summation of power dissipated by each port (with a different equation based on if the port is outputting high, or
outputting low. If the port is set as an input, then power dissipation is the input leakage of the pin multiplied by
the voltage on the pin). Note that this ignores power dissipation in the INT and SDA pins, assuming these
transients to be small. They can easily be included in the power dissipation calculation by using Equation 3 to
calculate the power dissipation in INT or SDA while they are pulling low, and this gives maximum power
dissipation.
( ) P_{d_PORT _L} = I_OL ´ V_OL
(3)
Equation 3 shows the power dissipation for a single port which is set to output low. The power dissipated by the
port is the V_OL of the port multiplied by the current it is sinking.
( ) ( ) ^P_{d_PORT _H} ^{= I}_OH ^{´ V}_CC ^{- V}_OH
(4)
Equation 4 shows the power dissipation for a single port which is set to output high. The power dissipated by the
port is the current sourced by the port multiplied by the voltage drop across the device (difference between V_CC
and the output voltage).
9.2.1.2 Minimizing I_CC When I/O is Used to 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 31. The LED acts as a diode, so when the LED is off, the I/O V_IN is about 1.2 V less than V_CC. The ΔI_CC
parameter in the Electrical Characteristics table shows how I_CC increases as V_IN becomes lower than V_CC.
Designs that must minimize current consumption, such as battery power applications, must consider maintaining
the I/O pins greater than or equal to V_CC when the LED is off.
Figure 32 shows a high-value resistor in parallel with the LED. Figure 33 shows V_CC less than the LED supply
voltage by at least 1.2 V. Both of these methods maintain the I/O V_IN at or above V_CC and prevent additional
supply current consumption when the LED is off.
V_CC
V_CC
Px
LED
100 kΩ
Figure 32. High-Value Resistor in Parallel With LED
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