SPC574S60 datasheet(24/77 Pages) STMICROELECTRONICS

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Electrical characteristics

SPC574Sx

24/77

DS10601 Rev 6

values determined for the single-layer (1s) board compared to a four-layer board that has

two signal layers, a power and a ground plane (2s2p), demonstrate that the effective

thermal resistance is not a constant. The thermal resistance depends on the:

•

Construction of the application board (number of planes)

•

Effective size of the board which cools the component

•

Quality of the thermal and electrical connections to the planes

•

Power dissipated by adjacent components

Connect all the ground and power balls to the respective planes with one via per ball. Using

fewer vias to connect the package to the planes reduces the thermal performance. Thinner

planes also reduce the thermal performance. When the clearance between the vias leave

the planes virtually disconnected, the thermal performance is also greatly reduced.

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 + (RqJB × PD)

where:

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

RqJB = 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: RqJA = RqJC + RqCA

where:

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

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

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

RqJC is device related and is not affected by other factors. The thermal environment can be

controlled to change the case-to-ambient thermal resistance, RqCA. For example, change

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