CM3196
Performance Information (cont’d)
Typical Thermal Characteristics
The overall junction to ambient thermal resistance
(
θ
JA
) for device power dissipation (P
D
) consists prima-
rily of two paths in series. The first path is junction to
case (
θ
JC
) which is defined by the package style, and
instantaneous current of 2A should not be exceeded
29% of the time. For CM3196-12SB, the maximum
RMS current increases from 1.3A to 2.2A. Thus, the
maximum continuous current can be 2A all the time.
the second path is case to ambient (
θ
CA
) 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
JUNC
= T
A
+ P
D
(
θ
JC
) + P
D
(
θ
CA
)
= T
A
+ P
D
(
θ
JA
)
When a CM3196-12SN (SOIC) is mounted on a dou-
ble-sided printed circuit board with two square inches
of copper allocated for "heat spreading," the resulting
θ
JA
is 151°C/W. Based on the over temperature limit of
150°C with an ambient of 85°C, the available power of
this package will be:
P
D
= (150°C -85°C) / 151°C/W = 0.43W
For the CM3196-12SB (PSOP),
θ
JA
is 40°C/W and the
available power for this package will be:
P
D
= (150°C -85°C) / 40°C/W = 1.625W
DDR Memory Application
Since the output voltage is 1.25V, and the device can
either source current from V
DD
or sink current to
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
0.344A for CM3196-12SN and 1.3A for CM3196-12SB.
The maximum instantaneous current is specified at 2A,
so this condition should not be exceeded 17% and
65% of the time for CM3196-12SN and CM3196-12SB,
respectively. It is highly unlikely in the use of DDR
memory that this would occur, because it means the
DDR memory outputs are either all high or all low for
17% (SOIC) and 65% (PSOP) 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
D
) can be 0.73W
for CM3196-12SN and 2.75W for CM3196-12SB. So
the maximum average RMS current increases from
0.42A to 0.58A for CM3196-12SN and maximum
© 2004 California Micro Devices Corp. All rights reserved.
04/22/04
Figure 10. Duty Cycle vs. Ambient
Temperature (I
LOAD
= 2A)
Figure 11. Duty Cycle vs. Output
Current (Temp=70°C)
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9
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