# How to do heat sink calculation, when junction to case thermal resistance is not provided

I am not very knowledgeable, so please bear with me.

For heat sink calculation, you need the junction-to-case thermal resistance of the chip, then the case-to-ambient thermal resistance of the heat sink. Why are there datasheets that don't contain the junction-to-case thermal resistance? One example is this LTC1732-8.4:

How can you do heat sink calculation without the junction-to-case thermal resistance? Since only the junction-to-ambient resistance is provided, does that mean this chip package doesn't support a heat sink?

The formula I know for heat sink calculation is:

$$T_j=T_a+P_w(\theta_{JC}+\theta_{CA})$$

where $$\T_j\$$ is the junction temperature, $$\T_a\$$ is the ambient temperature, $$\\theta_{JC}\$$ is the junction-to-case resistance of the chip, and $$\\theta_{CA}\$$ is the heat sink case-to-ambient resistance.

• Is this a specific or general question? LTC1832 draws a couple mA, you'd be hard pressed to get it self-heated any at all. Dec 10, 2022 at 22:13
• Yeah, "Input Supply Current: 3 mA" maximum.
– pipe
Dec 10, 2022 at 22:33
• packages without exterior exposed metal surfaces aren't designed to take heatsinks, usually out of a lack of need or an internal bottleneck that runs out of margin long before case temp is problematic. Sometimes tacking on a heatsink can result in somewhat better operation (chip amps, RAM, LEDs, motor controllers, PIs, etc) but these are more subjective improvements than design requirements with engineering specs. Dec 10, 2022 at 23:42
• @dandavis so, is it safe to assume then that if not $\theta_{JC}$ is provided, then the chip is not meant to be used with a heat sink? Dec 11, 2022 at 0:27
• As pipe says, input current is 3mA maximum, so why do you think it needs a heatsink? Dec 11, 2022 at 3:12