MC56F81663 datasheet(68/73 Pages) NXP

68 / 73 page

PD = 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 TJ value is closer to the application depends on the power

dissipated by other components on the board.

• The TJ value obtained on a single layer board is appropriate for a tightly packed

printed circuit board.

• The TJ value obtained on a board with the internal planes is usually appropriate if the

board has low-power dissipation and if the components are well separated.

When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-

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

RΘJA = RΘJC + RΘCA

where

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

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

RΘCA = Package case-to-ambient thermal resistance (°C/W).

RΘJC is device related and cannot be adjusted. You control the thermal environment to

change the case to ambient thermal resistance, RΘCA. For instance, you 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 the following equation:

TJ = TT + (ΨJT × PD)

where

TT = Thermocouple temperature on top of package (°C/W)

ΨJT = hermal characterization parameter (°C/W)

PD = Power dissipation in package (W).

Design Considerations

MC56F81xxx, Rev. 1.1, 12/2020

NXP Semiconductors

MC56F81663 数据表(PDF) 68 Page - NXP Semiconductors

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