SCDS486 Data sheet

SCDS486 April 2025 TMUX6612-Q1

PRODUCTION DATA

18.5 Thermal Considerations

For analog switches in many applications, several 100mA of current needs to be supported through the switch (from source to drain, or NO/NC to COM). Many devices already have a maximum current specified based on ambient temperature, but if a device specifies with junction temperature or to calculate for a specific use case (temperature, supply voltage, channels in parallel), use the following equations and scheme.

There are 2 main limitations to this maximum current:

Inherent metal limitations of the device
Thermal self-heating limitations

The following information is needed to calcualate maximum current for a specific setup:

T_A = maximum ambient temperature
R_ϴJA = package thermal coefficients
R_ON = on resistance
n = number of channels in parallel
Limitations on maximum current based on junction temperature from the data sheet

Below is an example using TMUX6612-Q1 specifications:

Device maximum T_J=150°C

R_ϴJA=99.3 °C/W

While operating with 4 channels in parallel at ±15V, assume R_ON = 1.8Ω by taking the maximum specified value at T_A = 105°C. Use T_J = 125°C. Using the following equation, calculate the maximum thermal limitation.

Equation 1.

I = \sqrt{\frac{T_{J} - T_{A}}{R_{θ J A} \times R_{O N} \times n}}

The current calculated from this example is 0.167A, so only a maximum of 0.167A can pass through each of the 4 channels in parallel. If only running with one channel then the equation outputs 0.334A, but due to the inherent metal limitation take the lower of the value calculated and the value provided in the maximum current table based on T_J in the data sheet, which is 0.143A.

Similarly, calculate the T_J and total power dissipated in these examples with the following equations. Note there is some small power dissipated from the supply current consumption of the device, which is ignored here.

Equation 2.

T_{J} = R_{θ J A} \times I^{2} \times R_{O N} \times n + T_{A}

Equation 3.

P_{t o t a l} = \frac{T_{J} - T_{A}}{R_{θ J A}}

Pulse current can be calculated the same way, but using the duty cycle, d. Typically, pulse current is specified at a 10% duty cycle; however, do not exceed the maximum current provided in the pulse current table even with a shorter duty cycle.

Equation 4.

I = \frac{1}{d} \sqrt{\frac{T_{J} - T_{A}}{R_{θ J A} \times R_{O N} \times n}}

Equation 5.

T_{J} = R_{θ J A} \times {(d \times I)}^{2} \times R_{O N} \times n + T_{A}