604 Hardware Specifications

29

Preliminary/Subject to Change without Notice

1.8.2 Thermal Management Example

For preliminary heat sink sizing, the die-junction temperature can be expressed as follows:

Tj = Ta + Tr + (Rθjc +Ra + Rsa) * Q

Where:

Tj is the die-junction temperature

Ta is the inlet cabinet ambient temperature

Tr is the air temperature rise within the system cabinet

Rθjc is the die-junction-to-case (top of die) thermal resistance of the device

Ra is the thermal resistance of the thermal interface material (thermal grease or thermal compound)

Rsa is the heat sink-to-ambient thermal resistance

Q is the power dissipated by the device

Typical die-junction temperatures (Tj) should be maintained less than 105 °C. The temperature of the air

cooling the component greatly depends upon the ambient inlet air temperature and the air temperature rise

within the computer cabinet. A computer cabinet inlet-air temperature (Ta) may range from 30 to 40 °C. The

air temperature rise within a cabinet (Tr) may be in the range of 5 to 10 °C. The thermal resistance of the

interface material (Ra) is typically about 1 °C/W. Assuming a Ta of 30 °C, a Tr of 5 °C, and a power

dissipation (Q) of 18 watts, the following expression for Tj is obtained:

Junction temperature: Tj = 30 °C + 5 °C + (0.03 °C/W +1.0 °C/W + Rsa) * 18 W

For a Thermalloy heat sink #2333B, the heatsink-to-ambient thermal resistance (Rsa) versus airflow velocity

is shown in Figure 19.

Figure 19. Thermalloy #2333B Pin-Fin Heat Sink-to-Ambient Thermal Resistance Versus Airflow

Velocity

0

2

4

6

8

10

0

0.5

1

1.5

2

2.5

3

3.5

Thermalloy #2333 Pin-fin Heat Sink

Airflow Velocity

(m/s)

Airflow Velocity (m/s)

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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