DD 2.X
PowerPC 750FX RISC Microprocessor
Preliminary
TJ = TA + TR + (θJC + θINT + θSA) × PD
Where:
TJ is the die-junction temperature
TA is the inlet cabinet ambient temperature
TR is the air temperature rise within the system cabinet
θJC is the junction-to-case thermal resistance
θINT is the thermal resistance of the thermal interface material
θSA is the heat sink-to-ambient thermal resistance
PD is the power dissipated by the device
Typical die-junction temperatures (TJ) should be maintained less than the value specified in Table 3-3
Package Thermal Characteristics1 on page 10. 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. An
electronic 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 (θINT) is typically
about 1°C/W. Assuming a TA of 30°C, a TR of 5°C, a CBGA package θJC = 0.03, and a power dissipation (PD) of
5.0 watts, the following expression for TJ is obtained.
Die-junction temperature: TJ = 30°C + 5°C + (0.03°C/W +1.0°C/W + θSA) × 5W
For a Thermalloy heat sink #2328B, the heat sink-to-ambient thermal resistance (θSA) versus air flow velocity
is shown in Figure 5-8 Thermalloy #2328B Pin-Fin Heat Sink-to-Ambient Thermal Resistance vs.
Airflow Velocity on page 51.
5. System Design Information
Page 50 of 63
Body_750FX_DS_DD2.X.fm.2.0
June 9, 2003
IBM [ IBM ]
