MSC7118 datasheet(39/60 Pages) FREESCALE

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Hardware Design Considerations

MSC7118 Low-Cost 16-bit DSP with DDR Controller Data Sheet, Rev. 7

Freescale Semiconductor

Hardware Design Considerations

This section described various areas to consider when incorporating the MSC7118 device into a system design.

3.1

Thermal Design Considerations

An estimation of the chip-junction temperature, TJ, in °C can be obtained from the following:

TJ = TA + (R

θJA × PD)Eqn. 1

where

TA = ambient temperature near the package (°C)

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

PD = PINT + PI/O = power dissipation in the package (W)

PINT = IDD × VDD = internal power dissipation (W)

PI/O = power dissipated from device on output pins (W)

The power dissipation values for the MSC7118 are listed in Table 4. The ambient temperature for the device is the air

temperature in the immediate vicinity that would cool the device. The junction-to-ambient thermal resistances are JEDEC

standard values that provide a quick and easy estimation of thermal performance. 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. The value that more closely

approximates a specific application depends on the power dissipated by other components on the printed circuit board (PCB).

The value obtained using a single layer board is appropriate for tightly packed PCB configurations. The value obtained using a

board with internal planes is more appropriate for boards with low power dissipation (less than 0.02 W/cm

2 with natural

convection) and well separated components. Based on an estimation of junction temperature using this technique, determine

whether a more detailed thermal analysis is required. Standard thermal management techniques can be used to maintain the

device thermal junction temperature below its maximum. If TJ appears to be too high, either lower the ambient temperature or

the power dissipation of the chip.

You can verify the junction temperature by measuring the case temperature using a small diameter thermocouple (40 gauge is

recommended) or an infrared temperature sensor on a spot on the device case. Use the following equation to determine TJ:

TJ = TT + (ΨJT × PD)Eqn. 2

where

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

JT = thermal characterization parameter (°C/W)

PD = power dissipation in the package (W)

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