TPS7A74: How to calculate junction temperature

Hi Hailiang,

Yes, after running some calculations what the customer is seeing as their resultant temperature of 124 deg C, that value is correct.

(0.62*55.3)+90

This resource should help with when a heat sink is involved in thermal measurements: Semiconductor and IC Package Thermal Metrics (Rev. D) (ti.com)

EIA/JESD51-1 states that RθJC is, “the thermal resistance from the operating portion of a semiconductor device to outside surface of the package (case) closest to the chip mounting area when that same surface is properly heat sunk so as to minimize temperature variation across that surface.”

This method forces almost all the power of the test device through a defined surface of the package. Depending on how a heat sink will be applied to the device, this may be the top or bottom of the package. Most generally, it is the top surface of the package.

RθJA is a measure of the thermal performance of an IC package mounted on a specific test coupon. The intent of RθJA is to give a metric by which the relative thermal performance of a package can be compared.

So, I would assume to use RθJC with a heat sink. 

It is reasonable to have RθJC be bigger than RθJA. Many of our devices show a similar characteristic.

For example: 

Rth-JA has different heat path than Rth-JC. 

• Rth-JA: die to leadframe to PCB to air
• Rth-JC: die through mold to cold plate

When mold is sufficiently thick or when mold thermal conductivity is low, Rth-JC value can be larger than Rth-JA.

Best,

Hannah

Hi Hailiang,

"Small Psi parameter values are common for LDOs, as most of the internal integrated circuit is the pass device, which is dissipating the heat."

Additionally, "Psi parameters provide an estimate of the junction temperature almost independent of the board layout."

While RθJC is the die through the mold compound to the cold plate. 

RθJC and the psi parameter are acting as two completely different values due to the independence the dependence on board layout.

You should add a heat sink, as you will have better thermal dissipation while using a heat sink. 

Especially if using the heat sink, I would recommend using 1.6 as the value for ψJT. The EVM should more closely relate the heat dissipation of the customer's board with a heat sink. 

Using the 55.3C for RθJC should work for this application. We do not have an exact value for RθJC with using the EVM, most likely because typically you would use RθJA. 

Best,

Hannah

Hi Hailang, 

I misspoke. Please see Using New Thermal Metrics (ti.com) to learn why TI does not recommend using  RθJC. We typically recommend using  RθJA parameter, so you should use the 34.9 C/W parameter to estimate what the junction temperature. If using the Psi JT parameter to estimate junction temperature, then yes use the 1.6 value.

Best,

Hannah