# How do I determine the junction-to-ambient thermal resistance?

For a project I'm working on, I was considering using an AS7805 to supply low-noise 5V power to some sensors. (due to the uncertainty that resulted in asking this question, I've since changed to a 7805 from another manufacturer, but that doesn't really matter to the question.) However, the datasheet doesn't tell me what the junction-to-ambient thermal resistance is. These sensors don't take much current, but since I'm dropping 10V in this 7805 it still does reach 0.6 watts, and I would like to know if I can get away with not using a heatsink.

Can I assume the 60 °C/W figure given in the absolute maximum ratings to be the heatsink-less thermal resistance of the device? Or is that just saying that you need to ensure your heatsink is sufficiently good that you don't exceed 60 °C/W?

Most devices I see specify both a junction-to-case and a junction-to-ambient thermal resistance. Why does this one, apparently, not? Is it just not intended to be used without a heatsink?

• Some heat can flow OUT the leads. Have the 3 leads to large pieces of foil on the PCB. – analogsystemsrf Apr 11 '19 at 16:47

It appears to me that $$\\Theta_{JA}\$$ is given in the table on page 4.

The $$\\Theta_{JC}\$$ values are given in the later tables, for each output voltage.

• That was my initial thought, but reading it again that is the absolute maximum ratings table, which generally means "you should never exceed these metrics", not "this is a typical metric for the device". – Hearth Apr 11 '19 at 14:27
• Ah, yes, that is confusing. My guess is that those numbers are the best guidance you will get from this manufacturer. You might compare the thermal resistance values with those for similar devices from other manufacturers to see if the numbers are reasonable. – Elliot Alderson Apr 11 '19 at 14:32
• I think you should interpret it like: don't exceed $\Theta_{JA}$ maximum value. So, given the typical values of $\Theta_{JC}$, $\Theta_{JC}$+$\Theta_{CA}$ should not exceed $\Theta_{JA}$ – Huisman Apr 11 '19 at 14:34
• @Huisman you cannot use the published junction-to-ambient for this purpose. That is unless your PCB is designed exactly like the JEDEC test fixture use to measure the published junction-to-ambient. – Misunderstood May 12 '19 at 19:44

However, the datasheet doesn't tell me what the thermal resistance is.

The thing about the published junction-to-ambient (θJA), it is only a reference used for comparison purposes. You need to estimate the thermal resistance of Your PCB (θCA). The published θJA is based on the device being mounted on a JEDEC test fixture.

This Texas Instrument App Note, Thermal Design By Insight,Not Hindsight, is very good and explains how to calculate θJA.

From page 3 of the app note:

Use the value of θJA given in the data sheet to compare different packages, and use it along with the IC power dissipation for a sanity check in your design. The high thermal resistance of the plastic packaging ensures that most of the heat travels from the exposed copper pad to the PCB, which usually has a much lower thermal resistance. A heat sink can be added to either the top of the package or directly beneath the exposed pad on the backside of the PCB. Again, because of the high thermal resistance of plastic, a heatsink will be more effective when connected to an exposed metal pad, either directly or, through thermal vias.

Since most of the heat transfer is through the exposed pad to the PCB it becomes immediately apparent that the value of θJA is highly PCB dependent. In other words, the most critical value to determine in any design is thermal resistance of the PCB (θCA). Well what, exactly, is θCA and how is it calculated? θCA is the equivalent resistance of a thermal resistive lattice that centers on the IC and ends at the surfaces of the board. It is the final of your freshman year, Circuits 101 class, all over again. Figure 3 below shows the details.

UPDATE:

This document describes traditional and new thermal metrics and puts their application in perspective with respect to system level junction temperature estimation.

Semiconductor and IC Package Thermal Metrics

• I would expect a datasheet value for θja for a TO-220 mounted in free air, at least. I understand that it's going to be more complex with a PCB heatsink, but a vertical TO-220 is what I was asking about. – Hearth May 12 '19 at 20:20
• @Hearth If the is no PCB in the θja thermal path wouldn't θja then equal θjc or 9 C°/W? I believe by definition θja is mounted to a JEDEC fixture. If the is no exposed thermal pad then the junction-to-ambient parameter is labeled θta. See the 3rd paragraph on page 3 of the app note that begins with "For DC-DC converters without an exposed pad..." – Misunderstood May 13 '19 at 19:17
• I'm not convinced of that, because the heat transfer from the case to the air is very different and far less efficient than the transfer from the junction to the case. What, exactly, is a JEDEC fixture? I get that it's some sort of standard heatsink, but I don't know what sort of standard heatsink. – Hearth May 13 '19 at 19:29
• @Hearth The JEDEC spec EIA/JESD51-x is where the fixture is defined. I added another link to a 2016 TI app note that attempts to clarify datasheet terminology. See page 2 section 1. – Misunderstood May 13 '19 at 19:49