SPC584BX datasheet(126/142 Pages) STMICROELECTRONICS

126 / 142 page

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SPC584Bx

126/142

DS11701 Rev 4

Equation 2

TJ = TB + (RJB * PD)

where:

TB = board temperature for the package perimeter (°C)

RJB = junction-to-board thermal resistance (°C/W) per JESD51-8

PD = power dissipation in the package (W)

When the heat loss from the package case to the air does not factor into the calculation, the

junction temperature is predictable if the application board is similar to the thermal test

condition, with the component soldered to a board with internal planes.

The thermal resistance is expressed as the sum of a junction-to-case thermal resistance

plus a case-to-ambient thermal resistance:

Equation 3

RJA = RJC + RCA

where:

RJA = junction-to-ambient thermal resistance (°C/W)

RJC = junction-to-case thermal resistance (°C/W)

RCA = case to ambient thermal resistance (°C/W)

RJC is device related and is not affected by other factors. The thermal environment can be

controlled to change the case-to-ambient thermal resistance, RCA. For example, change

the air flow around the device, add a heat sink, change the mounting arrangement on the

printed circuit board, or change the thermal dissipation on the printed circuit board

surrounding the device. This description is most useful for packages with heat sinks where

90% of the heat flow is through the case to heat sink to ambient. For most packages, a

better model is required.

A more accurate two-resistor thermal model can be constructed from the junction-to-board

thermal resistance and the junction-to-case thermal resistance. The junction-to-case

thermal resistance describes when using a heat sink or where a substantial amount of heat

is dissipated from the top of the package. The junction-to-board thermal resistance

describes the thermal performance when most of the heat is conducted to the printed circuit

board. This model can be used to generate simple estimations and for computational fluid

dynamics (CFD) thermal models. More accurate compact Flotherm models can be

generated upon request.

To determine the junction temperature of the device in the application on a prototype board,

use the thermal characterization parameter (

JT) to determine the junction temperature by

measuring the temperature at the top center of the package case using the following

equation:

Equation 4

TJ = TT + (JT x PD)

where:

TT = thermocouple temperature on top of the package (°C)

JT = thermal characterization parameter (°C/W)

PD = power dissipation in the package (W)

SPC584BX Datasheet(PDF) 126 Page - STMicroelectronics

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