LMX2615-SP: Thermal Resistance values

Logan Hester

Part Number: LMX2615-SP

On the datasheet for LMX2615-SP it lists the following thermal resistance values: ΘJC(top) = 7.3 °C/W, ΘJB = 7.6 °C/W, and ΘJC (bottom) = 1.0 °C/W. It appears the component is designed for the optimal heat path to be through the bottom, where a heat-sink will be soldered to the board. If that is the case, then why is the ΘJB (junction-to-board thermal resistance) 7.6 °C/W? With a component that is soldered to the board, I don't understand why this value is higher than the junction-to-top thermal resistance. How exactly is the thermal resistance from bottom of case-to-board being calculated for it to have this value, is the soldered heat sink not being considered?

over 6 years ago

0 Dean Banerjee over 6 years ago

TI__Mastermind 28001 points

Logan,

I'm no thermal expert, but found this application note especially helpful (http://www.ti.com/lit/an/spra953c/spra953c.pdf). I'm basing my response on this application note.

The Theta-JC is specifically when the heat sink is attached.

Theta-JB is an aggregate of several different resistances, but I would agree with you that it would seem that it would be less than Theta-JB, unless it was assumed that the heat sink was not soldered to the board.

Regards,

Dean

0 Logan Hester over 6 years ago in reply to Dean Banerjee

Prodigy 30 points

Thanks for the response Dean. I was mainly wondering if there was a possibility the value written down for ΘJB was incorrect, as I don't understand how it can be higher than ΘJC(top) considering ΘJC(bottom) is so low and it is heatsunk. Otherwise, there must be some factor I don't understand that is causing it to be so high.

0 Dean Banerjee over 6 years ago in reply to Logan Hester

TI__Mastermind 28001 points

Logan,

No, this isn't a typo. Or if it is, then just about all our datasheets are wrong.

I checked several TI datasheets and it was always the case that ThetaJC < ThetaJB

Regards,
Dean

0 Logan Hester over 6 years ago in reply to Dean Banerjee

Prodigy 30 points

Sure, I agree that ΘJC < ΘJB since ΘJB covers that resistance and the resistance from the case to the board. But that would be ΘJC(bottom) on Texas Instrument datasheets, my confusion is from the ΘJC(top) value being lower.

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LMX2615-SP: Thermal Resistance values