Electrical – How to determine the junction-to-ambient thermal resistance

heatsinkthermal

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?

Best Answer

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.

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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

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