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Thermal Considerations

MSC8122 Technical Data, Rev. 13

Freescale Semiconductor

4-5

4.5 Thermal Considerations

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

TJ = TA + (R

θJA × PD)

Equation 1

where

TA = ambient temperature near the package (°C)

RθJA = 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 MSC8122 are listed in Table 2-3. 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/cm2 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 that is painted black. The MSC8122 device case surface is too shiny (low emissivity) to

yield an accurate infrared temperature measurement. Use the following equation to determine TJ:

TJ = TT + (θJA × PD)

Equation 2

where

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

JA = thermal characterization parameter (°C/W)

PD = power dissipation in the package (W)

Note:

See MSC8102, MSC8122, and MSC8126 Thermal Management Design Guidelines (AN2601/D).

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