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This is the link towards the D FF datasheet I am working with: SN74LVC1G80 My ambient temperature of working is 100C. Operating temperature for DBV package is -40C to 125C.

Junction-to-ambient thermal resistance = 243.4 C/W (for DBV package).

Intention is to find the maximum rated power of the device, following is the method used:

Range from ambient temperature to maximum operating temperature = 125C -100C = 25C.

Maximum rated power = (Above Calculated Range)/Junction-to-ambient thermal resistance = 25C/(243.4C/W) = 102.7 mW.

Kindly confirm or correct the above method used to calculate the maximum rated power calculation.

Or how to do the maximum rated power calculation if there is no rated power information in the datasheet.

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  • \$\begingroup\$ That assumes that the local ambient (around the chip) doesn't rise with heat from the chip. \$\endgroup\$
    – Andy aka
    Commented Mar 15, 2018 at 17:11
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    \$\begingroup\$ Basically @Andyaka's response means that you assumed an ideal heat sink at the package. You need to calculate with the thermal resistance from the package to the "air" as well. \$\endgroup\$
    – Horror Vacui
    Commented Mar 15, 2018 at 17:15
  • \$\begingroup\$ This also assume no margin for MTBF which may be improved with a heat sink \$\endgroup\$
    – D.A.S.
    Commented Mar 15, 2018 at 17:17
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    \$\begingroup\$ How will you cause this high internal power dissipation? \$\endgroup\$
    – analogsystemsrf
    Commented Mar 15, 2018 at 17:22
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    \$\begingroup\$ @vt673 read the data sheet again in the thermal section and note that you are probably better using junction to board thermal resistance - then read the linked document under the table to see how this works. Also please explain how you think you are going to produce a power dissipation of over 100 mW. \$\endgroup\$
    – Andy aka
    Commented Mar 15, 2018 at 18:10

1 Answer 1

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Most folk would assume that the chip is adequately soldered to a PCB that will conduct heat away without much of a local temperature rise (maybe a couple of degrees). So, use the "junction to board thermal resistance" (77.6 °C/W) and read the linked document (Semiconductor and IC Package Thermal Metrics application report) listed on the data sheet to understand how this works.

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  • \$\begingroup\$ For me adding Junction-to-board thermal resistance' with 'Junction-to-ambient thermal resistance' seems more accurate. Apologies if this is an oversight. \$\endgroup\$
    – vt673
    Commented Mar 15, 2018 at 18:33
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    \$\begingroup\$ They don't add in series, they add in parallel just in case you thought otherwise. \$\endgroup\$
    – Andy aka
    Commented Mar 15, 2018 at 18:35

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