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# TPS7A90: Thermal resistance and power dissipation

Part Number: TPS7A90

Hi Team

I don't find the power dissipation graph for TPS7A90, do we have any available information regarding this?

The datasheet does mention RθJA56.9（℃/W)  though, if to draw the graph, then PDmax =125(℃)/ 56. 9 (℃/W)=2.197 W, with the slope=1/ RθJA =1/56.9 W/℃.

For the case when the power dissipation PD＝0.7W, ambient temperature Ta=45℃, the junction temperature would be Tj=Ta + RθJA✕PD=45 + 56.9 X 0.7 = 84.8（℃）.

If calculate with the power dissipation graph obtain from above, then the power dissipation would be 1.933W, and therefore the power dissipation margin would be 1-(0.7／1.933) = 64％.

Could you confirm if the calculation above is correct?

Regards

David

• Hi David,

There's a problem with the way you calculated the P_Dmax. The way you calculated it is assuming a 0C ambient temperature. The correct way to calculate it would be P_Dmax = (125C - 45C) / 56.9C/W = 1.4W.

If calculate with the power dissipation graph obtain from above, then the power dissipation would be 1.933W, and therefore the power dissipation margin would be 1-(0.7／1.933) = 64％.

I'm confused by this statement. You mentioned the case when P_D = 0.7W. Why did you calculate 1.933W then? What value were you intending to calculate?

Regards,

Nick

• Hi Nick

Sorry for the confusing statement.

This was actually a question I received from my customer and I didn't look through it before I translated and posted it here.

For the part PDmax =125(℃)/ 56. 9 (℃/W)=2.197 W I believe the customer thinks that the maximum temperature is 150C so it's like PDmax =(150-25)(℃)/ 56. 9 (℃/W)=2.197 W.

Also for the P_D = 0.7W part, I believe that the customer was trying to make a graph below, so he can easily calculate how much more power he can put on the LDO under different situations such as: 45C environment temperature, 0.7W power(if the PDmax is 1.4W then it's at 50% so he can add 0.7W more to it). That's why he called it the PD margin Regards

David

• Hi David,

Why do they assume that the P_DMax is flat from 0C - 25C? Can you point me to where they got this idea from?

For the part PDmax =125(℃)/ 56. 9 (℃/W)=2.197 W I believe the customer thinks that the maximum temperature is 150C so it's like PDmax =(150-25)(℃)/ 56. 9 (℃/W)=2.197 W.

The abs max junction temperature is 150C and the recommended max junction temperature is 125C, so it depends on what they are trying to do here. If they want to ensure that the junction temperature stays within the recommended Tj spec they should use 125C, but if they are just trying to ensure that they do not violate the abs max Tj, then they should use 150C. I'm also not sure why they use (150C - 25C) in the P_DMax calculation if their expected Ta is 45C - there's nothing special about 25C.

Regards,

Nick