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LP3875-ADJ

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SNVS247E – SEPTEMBER 2003 – REVISED AUGUST 2016

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Copyright © 2003–2016, Texas Instruments Incorporated

On the TO-263 (KTT) package, the primary conduction path for heat is through the thermal tab into the PCB. In

this package, the die is connected directly to the thermal pad and the heat generated in the die (junction) has a

direct path through the large thermal tab into the PCB copper area.

In the SOT-223 (NDC) package, the primary conduction path for heat is through the GND Tab (pin 5) into the

PCB. While the die (junction) is connected directly to the GND tab metal, this thermal path is longer and has a

higher thermal resistance value than the TO-263.

To ensure the best thermal performance, place as large of a copper area directly under the thermal tab as is

possible, and connect the thermal tab, through multiple thermal vias, to an internal ground plane with an

appropriate amount of copper PCB area.

Power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance

(RθJA) of the combined PCB and device package and the temperature of the ambient air (TA), according to

Equation 4 or Equation 5:

TJ(MAX) = TA(MAX) + (RθJA × PD(MAX))

(4)

PD(MAX) = (TJ(MAX) – TA(MAX)) / RθJA

(5)

Unfortunately, this RθJA is highly dependent on the heat-spreading capability of the particular PCB design, and

therefore varies according to the total copper area, copper weight, and location of the planes. The RθJA recorded

in Thermal Information is determined by the specific EIA/JEDEC JESD51-7 standard for PCB and copper-

spreading area, and is to be used only as a relative measure of package thermal performance. For a well-

designed thermal layout layout for the TO-263 (KTT) , RθJA is actually the sum of the package junction-to-case

(bottom) thermal resistance (RθJCbot) plus the thermal resistance contribution by the PCB copper area acting as a

heat sink.

8.2.2.10 Estimating Junction Temperature

The EIA/JEDEC standard recommends the use of psi (Ψ) thermal characteristics to estimate the junction

temperatures of surface mount devices on a typical PCB board application. These characteristics are not true

thermal resistance values, but rather package specific thermal characteristics that offer practical and relative

means of estimating junction temperatures. These psi metrics are determined to be significantly independent of

copper-spreading area. The key thermal characteristics (ΨJT and ΨJB) are given in Thermal Information and are

used in accordance with Equation 6 or Equation 7.

TJ(MAX) = TTOP + (ΨJT × PD(MAX))

where

•

PD(MAX) is explained in Equation 5

•

TTOP is the temperature measured at the center-top of the device package.

(6)

TJ(MAX) = TBOARD + (ΨJB × PD(MAX))

where

•

PD(MAX) is explained in Equation 5.

•

TBOARD is the PCB surface temperature measured 1-mm from the device package and centered on the

package edge.

(7)

For more information about the thermal characteristics ΨJT and ΨJB, see Semiconductor and IC Package Thermal

Metrics; for more information about measuring TTOP and TBOARD, see Using New Thermal Metrics; and for more

information about the EIA/JEDEC JESD51 PCB used for validating RθJA, see the Thermal Characteristics of

Linear and Logic Packages Using JEDEC PCB Designs. These application notes are available at www.ti.com.