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Knowledge Base Articles

Calculation of Junction Temperature for Clock Devices – KBA230751


Calculation of Junction Temperature for Clock Devices – KBA230751

Author: PradiptaB_11         Version: **

Translation - Japanese: クロック製品のジャンクション温度の計算 – KBA230751 - Community Translated (JA)

Note: For spec numbers 001-00001 to 001-99999, the KBA number is five digits; for spec numbers 002-xxxxx, add '2' to make it a 6-digit number. For example, for spec 002-12345, the KBA number is 002-212345 to distinguish it from KBA12345 for spec 001-12345.

Question: How can we calculate the maximum operating junction temperature for clock devices?

Answer: Maximum Operating Junction temperature (this is the maximum junction temperature at which the thermal characteristics of the IC are guaranteed).

We can use the below formula to calculate the maximum operating junction temperature:

Tj max = Theta Ja * Pd (max) + Ta (max)


Tj is the Junction Temperature

Theta Ja is the Junction to ambient thermal resistance

Ta = Ambient temperature = 85°C

Pd= Power dissipation = VDD * Itot

Total current, Itot = Idc + Iac

Idc value can be found in the datasheet

Iac = K*C*V*F

K = 1/2 for 50% duty cycle. You can modify this parameter accordingly.

C = Load Capacitance


F = Frequency

Then calculate total power by multiplying by VDD.

Please note this is the max operating junction temperature that the device will reach as per the application or as per the values you enter or select.

Some datasheet doesn’t show the current, IDC and IAC separately, it shows the Total current “Itot” at a specific frequency. For this case we recommend the linear interpolation by C, F and V because that will be the worst case.

For example in the CY27410 datasheet, IDDO for CMOS, specified 6.0mA(max) at 10-pF 33MHz, so if you use 66MHz, the IDDO will be 6*2=12mA for 66MHz operation, and so-on.

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