8.0 Package Specifications
The thermal characteristics and mechanical dimensions
for the Geode GXm processor are provided on the follow-
ing pages.
As stated previously, a maximum junction temperature is
not specified since a maximum case temperature is.
Therefore, the following equation can be used to calculate
the maximum thermal resistance required of the thermal
solution for a given maximum ambient temperature:
8.1 THERMAL CHARACTERISTICS
Table 8-1 shows the junction-to-case thermal resistance
of the SPGA and BGA package and can be used to calcu-
late the junction (die) temperature under any given cir-
cumstance.
T
– T
C
A
θ
+ θ
= ---------------------
CS
SA
P
where:
θCS = Max case-to-heatsink thermal resistance
(°C/W) allowed for thermal solution.
Table 8-1. Junction-to-Case Thermal Resistance
for SPGA and BGA Packages
θSA = Max heatsink-to-ambient thermal resistance
(°C/W) allowed for thermal solution.
Package
θ
J
C
TA = Max ambient temperature (°C)
SPGA
BGA
1.7 °C/W
1.1 °C/W
TC = Max case temperature at top center of package
(°C)
P = Max power dissipation (W)
Note that there is no specification for maximum junction
temperature given since the operation of both SPGA and
BGA devices are guaranteed to a case temperature range
of 0°C to 85°C. As long as the case temperature of the
device is maintained within this range, the junction tem-
perature of the die will also be maintained within its allow-
able operating range. However, the die (junction)
temperature under a given operating condition can be cal-
culated by using the following equation:
If thermal grease is used between the case and heatsink,
θCS will reduce to about 0.01 °C/W. Therefore, the above
equation can be simplified to:
T
– T
C
A
θ
= ---------------------
CA
P
where:
θCA = θCS = Max case-to-ambient thermal resistance
TJ = TC + (P * θJC
where:
TJ = Junction temperature (°C)
C = Case temperature at top center of package (°C)
)
(°C/W) allowed for thermal solution.
The calculated θCA value (examples shown in Table 8-2)
represents the maximum allowed thermal resistance of
the selected cooling solution which is required to maintain
the 85°C case temperature for the application in which the
device is used.
T
P = Power dissipation (W)
These examples are given for reference only. The actual
value used for Maximum Power (P) and ambient tempera-
ture (TA) is determined by the system designer based on
system configuration, extremes of the operating environ-
ment, and whether active thermal management (via Sus-
pend Modulation) of the processor is employed.
θJ = Junction-to-case thermal resistance (°C/W)
C
Revision 3.1
195
www.national.com
NSC [ National Semiconductor ]
