SPC572L64F2 datasheet(99/112 Pages) STMICROELECTRONICS

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SPC572Lx

Package characteristics

111

As a general rule, the value obtained on a single-layer board is within the normal range for

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

is usually within the normal range if the application board has:

•

One oz. (35 micron nominal thickness) internal planes

•

Components are well separated

•

Overall power dissipation on the board is less than 0.02 W/cm2

The thermal performance of any component depends on the power dissipation of the

surrounding components. In addition, the ambient temperature varies widely within the

application. For many natural convection and especially closed box applications, the board

temperature at the perimeter (edge) of the package is approximately the same as the local

air temperature near the device. Specifying the local ambient conditions explicitly as the

board temperature provides a more precise description of the local ambient conditions that

determine the temperature of the device.

At a known board temperature, the junction temperature is estimated using the following

equation:

Equation 2 TJ = TB + (RθJB * PD)

where:

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

RθJB = 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 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 is not affected by other factors. The thermal environment can be

controlled to change the case-to-ambient thermal resistance, RθCA. 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.

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