LM317A: Measurement location/conditions for package (case) temperature and ambient temperature

Hi Conor,

I'll get back to you tomorrow about this.

Regards,

Nick

Hi Nick,

Thank you. Looking forward to your reply.

Hi Conor,

How do you intend to perform the measurement in general? Will you put the device in an oven or something similar? The reason I ask is because it is much easier to do the measurement when the device can be left at room temperature. In this case the ambient temperature is the air temperature away from the device (at least more than a few inches such that the measured air temperature is not affected by radiated or convected heat). I find that the easiest way to measure the case temperature is to use a thermal imaging device, and also using a high-emissivity spray to calibrate out reflected radiation. Then the case temperature to use is the hottest point on the top of the case. 

Regards,

Nick

Hi Nick,

Thank you for your detailed response.

Regarding Q1, I would like to know more about the test conditions for IC shipping tests (measurement environment, IC measurement points, etc.).
What about Q2?

Thanks,

Conor

Hi Conor, 

Sorry, I forgot to address your questions directly. 

Conor said:

Q1. In order to verify the temperature rise, could you please tell me the location and conditions for measuring the package (case) temperature and ambient temperature?
Also, the case is divided into Top and Bottom, so please tell us specifically where you are measuring.

The case temperature measurement is most easily done on the top of the case. On a real board, it is best to use the ΨJT parameter to estimate the junction temperature because the Ψ metrics are designed to be largely independent of board layout, while the θ metrics are known to be largely dependent on board layout. 

When measuring the case temperature, the ambient air should not be moving substantially near the device, i.e. there should not be air being blown onto the device, because the thermal metrics are simulated assuming no convective cooling near the device. As I mentioned in my previous comment, the hottest spot on the top of the case should be used in the calculation, which is the reason I prefer to use a thermal imaging device because knowing the hottest spot ahead of time is not trivial. Furthermore, other methods of temperature measurement, e.g. a thermocouple, has many challenges and limitations as well, such as accuracy of measurement vs. accidental heat sinking due to the thermal mass of the thermocouple itself. 

Conor said:

Q2. The maximum power consumption (PD) is listed as Internally Limited, but it is only a reference value, so could you please disclose it?

This is something of a misnomer, because this device has a thermal shutdown circuit that shuts down the output when the junction temperature reaches a certain point, but the power dissipation to reach thermal shutdown depends on the operating conditions - in particular, the ambient temperature and board layout/heat sinking available. The thermal shutdown for this device should be around 150C; the LM317 is quite old (as I'm sure you're aware), so we have limited documentation and I was not able to find a characterization report that would have the thermal shutdown temperature. 

Regards,

Nick