DCV010505: Maximum junction temperature rating

Hello Dave, I see that we only spec' max ambient at 85degC, while, we have a thermal shutdown implemented at 150degC. With how the datasheet is stated, we dont have a max junction temperature rating. Are you looking for guidance on max junction temperature on a product guarantee (product specification)? If the latter, I may need to recommend an alternative device.

Hi, how can there be a thermal resistance to the junction from the board, case and ambient specified if there is no junction temperature rating?  I am looking for a max junction temperature rating for computing margins from a thermal analysis.  I'm unable to make the call on the alternative part number, but I will pass it along to the design engineers.  I'm only performing a thermal analysis on their design.

Hi, as we do know what the max ambient can be, we can estimate what the max junction can be as you said from estimated power dissipation and package, thermal characterization parameters.(Temperature rise from top case to junction).

Though, we don't have a specification stated in the datasheet for max Tj as we both recognized.

If that is a problem, I can suggest an alternative device or look into another solution for you.

Can you help me estimate that "max" Tj rating?  I'm interested in knowing how "close" we are.  I see 85°C max ambient and a theta_ja of 61°C/W.  I'm guessing the datasheet has a max rated power somewhere in there, where we could back out Tj, but I'm not an electrical engineer, so it doesn't stand out to me.  I expect that even if we overly conservatively assumed Tj = 85°C that'd we'd have margin, but I'd like to have a better value on that.  Thanks.

In the body of the datasheet, contained are efficiency curves for which the Pdiss can be determined: Po/eta - Po = Pdiss

Then we can estimate junction temp by:

Tj ~ Pdiss(PsijT+ RthetajA) + Tamb

The RthetajA in the datasheet seems pretty conservative, but that may be a result of the JEDEC board often used for characterization. If the engineer has used this product before, it may be good to see if the application board RthetajA was characterized.

Which figure # should I look at?

Figure 7-5 on ward...efficiency vs load.

www.ti.com/.../dcv010505.pdf