CM3109
Performance Information (cont’d)
Typical Thermal Characteristics
The overall junction to ambient thermal resistance
(θJA) for device power dissipation (P ) consists prima-
D
rily of two paths in series. The first path is the junction
to the case (θJC) which is defined by the package style,
and the second path is case to ambient (θCA) thermal
resistance which is dependent on board layout. The
final operating junction temperature for any set of con-
ditions can be estimated by the following thermal equa-
tion:
T
= T
= T
+ P (θJC) + P (θCA
)
JUNC
AMB
D
D
+ P (θJA)
AMB
D
When a CM3109-00SB/SH (PSOP) is mounted on a
double sided printed circuit board with two square
inches of copper allocated for "heat spreading", the
resulting θJA is 40°C/W. Based on the over tempera-
ture limit of 150°C with an ambient of 85°C, the avail-
able power of this package will be:
Figure 12. Duty Cycle vs. Ambient
Temperature (I = 2A)
LOAD
P = (150°C -85°C) / 40°C/W = 1.625W
D
DDR Memory Application
Since the output voltage is 1.25V, and the device can
either source current from V or sink current to
DDQ
Ground, the power dissipated in the device at any time
is 1.25V times the current load. This means the maxi-
mum average RMS current (in either direction) is 1.3A
for CM3109-00SB/SH. The maximum instantaneous
current is specified at 2A, so this condition should not
be exceeded 65% of the time. It is highly unlikely in
most usage of DDR memory that this might occur,
because it means the DDR memory outputs are either
all high or all low for 65% of the time.
If the ambient temperature is 40°C instead of 85°C,
which is typically the maximum in most DDR memory
applications, the power dissipated (P can be 2.75W.
D)
So the maximum RMS current increases from 1.3A to
2.2A. Thus, the maximum continuous current can be
2A all the time.
Figure 13. Duty Cycle vs. Output
Current (Temp=70°C)
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01/15/04
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