JAJSUQ7 データシート

JAJSUQ7B June 2024 – September 2024 TMUXS7614D

PRODUCTION DATA

パッケージ・オプション

デバイスごとのパッケージ図は、PDF版データシートをご参照ください。

メカニカル・データ（パッケージ｜ピン）

ZEM|30

サーマルパッド・メカニカル・データ

発注情報

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8.3 Thermal Considerations

For analog switches in many applications, several 100s of mA 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 you want to calculate for your specific use case (temperature, supply voltage, channels in parallel) you can use the following equations and scheme.

There are mainly 2 limitations to this maximum current:

Inherent metal limitations of the device
Thermal self-heating limitations

To calculate maximum current for your specific setup you need the following information:

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 datasheet

Below is an example using TMUXS7614D specifications:

Device maximum T_J=150°C

R_ϴJA=53.5 °C/W

For this example we assume 20°C of self-heating at a maximum T_A=105°C and operating with 4 channels at once at ±15V. We can assume worst case R_ON = 2.2Ω. This number is taken from the maximum specified value at T_A = 125°C where T_J=125°C since the specification assumes no self-heating. Using the following equation we can calculate the maximum thermal limitation.

Equation 2.

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

The current calculated from this example is 0.206A, but due to the inherent metal limitation we must 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 in this case. This means that we can only pass a maximum of 0.143A through each of the 4 channels.

As another example where the device is thermally limited: While operating with 8 channels at once at ±15V with a maxmium T_A = 50°C, we can assume worst case R_ON = 2.2Ω by taking the maximum specified value at T_A = 125°C by assuming 55°C of self-heating and taking the closest maximum R_ON specification to 105°C. Using the previous equation we can calculate the maximum thermal limitation.

The current calculated from this example is 0.291A which means that we can pass a maximum of 0.291A through each of the 8 channels at once. This means across all 8 channels the device can handle 2.33A total. If we were to only run with one channel then the equation outputs 0.824A, but due to the inherent metal limitation we must take the lower of the value calculated and the value provided in the maximum current table based on T_J in the datasheet which is 0.309A in this case.

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

Equation 3.

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

Equation 4.

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 5.

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

Equation 6.

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