TCA9534_16 Datasheet, PDF (24/38 Pages) Texas Instruments – Low Voltage 8-Bit I2C and SMBUS Low-Power I/O Expander with Interrupt Output and Configuration Registers

TCA9534
SCPS197B – SEPTEMBER 2014 – REVISED NOVEMBER 2016
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Typical Application (continued)
9.2.1 Design Requirements
9.2.1.1 Calculating Junction Temperature and Power Dissipation
When designing with the TCA9534, it is important that the Recommended Operating Conditions not be violated.
Many of the parameters of this device are rated based on junction temperature. So junction temperature must be
calculated in order to verify that safe operation of the device is met. The basic equation for junction temperature
is shown in Equation 1.
T_j = T_A + (q_JA ´ P_d )
(1)
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/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 33. 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 is 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 34 shows a high-
value resistor in parallel with the LED. Figure 35 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 34. High-Value Resistor in Parallel with LED
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