TCA9555_16 Datasheet, PDF (26/46 Pages) Texas Instruments – Low-Voltage 16-Bit I2C and SMBus I/O Expander with Interrupt Output and Configuration Registers

TCA9555
SCPS200D – JULY 2009 – REVISED JULY 2016
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Typical Application (continued)
10.2.1 Design Requirements
The designer must take into consideration the system, to be sure not to violate any of the parameters. Table 8
shows some key parameters which must not be violated.
DESIGN PARAMETER
I²C and Subsystem Voltage (V_CC)
Output current rating, P-port sinking (I_OL)
I²C bus clock (SCL) speed
Table 8. Design Parameters
EXAMPLE VALUE
5V
25 mA
400 kHz
10.2.2 Detailed Design Procedure
10.2.2.1 Calculating Junction Temperature and Power Dissipation
When designing with this device, 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 the 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).
10.2.2.2 Minimizing I_CC When I/O Is Used to Control LED
When an I/O is used to control an LED, normally it is connected to V_CC through a resistor as shown in Figure 34.
Because the LED acts as a diode, 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. For
battery-powered applications, it is essential that the voltage of I/O pins is greater than or equal to V_CC when the
LED is off to minimize current consumption.
Figure 35 shows a high-value resistor in parallel with the LED. Figure 36 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.
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