SPC58NH84C3 datasheet(129/147 Pages) STMICROELECTRONICS

129 / 147 page

DS12304 Rev 5

129/147

SPC58EHx, SPC58NHx

Package information

131

5.5.5

General notes for specifications at maximum junction temperature

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

Equation 1

TJ = TA + (RθJA * PD)

where:

TA = ambient temperature for the package (°C)

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

PD = power dissipation in the package (W)

The thermal resistance values used are based on the JEDEC JESD51 series of standards

to provide consistent values for estimations and comparisons. The differences between the

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 leaves

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 + (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)

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