SPC564L54L5 datasheet(101/160 Pages) STMICROELECTRONICS

101 / 160 page

SPC56XL60/54

Electrical characteristics

Doc ID 15457 Rev 8

101/160

3.4.1

General notes for specifications at maximum junction temperature

An estimation of the chip junction temperature, TJ, can be obtained from Equation 1:

Equation 1 TJ =TA +(RθJA ×PD)

where:

= ambient temperature for the package (oC)

RθJA

= junction to ambient thermal resistance (oC/W)

= power dissipation in the package (W)

The junction to ambient thermal resistance is an industry standard value that provides a

quick and easy estimation of thermal performance. Unfortunately, there are two values in

common usage: the value determined on a single layer board and the value obtained on a

board with two planes. For packages such as the PBGA, these values can be different by a

factor of two. Which value is closer to the application depends on the power dissipated by

other components on the board. The value obtained on a single layer board is appropriate

for the tightly packed printed circuit board. The value obtained on the board with the internal

planes is usually appropriate if the board has low power dissipation and the components are

well separated.

When a heat sink is used, the thermal resistance is expressed in Equation 2 as the sum of a

junction to case thermal resistance and a case to ambient thermal resistance:

Equation 2

RθJA =RθJC + RθCA

where:

RθJA

= junction to ambient thermal resistance (°C/W)

RθJC

= junction to case thermal resistance (°C/W)

RθCA

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

RθJC is device related and cannot be influenced by the user. The user controls the thermal

environment to change the case to ambient thermal resistance, RθCA. For instance, the user

can change the size of the heat sink, the air flow around the device, the interface material,

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

printed circuit board surrounding the device.

To determine the junction temperature of the device in the application when heat sinks are

not used, the Thermal Characterization Parameter (

ΨJT) can be used to determine the

junction temperature with a measurement of the temperature at the top center of the

package case using Equation 3:

Equation 3 TJ =TT +(ΨJT ×PD)

where:

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

ΨJT

= thermal characterization parameter (°C/W)

= power dissipation in the package (W)

The thermal characterization parameter is measured per JESD51-2 specification using a 40

gauge type T thermocouple epoxied to the top center of the package case. The

thermocouple should be positioned so that the thermocouple junction rests on the package.

A small amount of epoxy is placed over the thermocouple junction and over about 1 mm of

wire extending from the junction. The thermocouple wire is placed flat against the package

case to avoid measurement errors caused by cooling effects of the thermocouple wire.

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