CM3106 Datasheet, PDF (6/9 Pages) California Micro Devices Corp – 2 Amp Source/ Sink Bus Termination Regulator

CM3106
PERFORMANCE INFORMATION (Cont’d)
Typical Thermal Characteristics (nominal conditions unless otherwise specified)
The overall junction to ambient thermal resistance (q_JA)
for device power dissipation (P_D) consists primarily of two
paths in series. The first path is the junction to the case (q_JC)
which is defined by the package style, and the second path
is case to ambient (q_CA) thermal resistance which is
dependent on board layout. The final operating junction
temperature for any set of conditions can be estimated by the
following thermal equation:
T_JUNC = T_AMB + P_D (q_JC) + P_D (q_CA)
= T_AMB + P_D (q_JA)
When a CM3106−12SM is mounted on a double sided
printed circuit board with two square inches of copper
allocated for “heat spreading”, the resulting q_JA is 151°C/W.
Based on the over temperature limit of 150°C with an
ambient of 70°C, the available power of this package will be:
P_D = (150°C − 85°C) / 151°C/W = 0.43 W
Since the q_JA of the CM3106−12SB (PSOP) is 40°C/W,
the available power for this package will be:
P_D = (150°C − 85°C) / 40°C/W = 1.625 W
DDR Memory Application
Since the output voltage is 1.25 V, 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 1.25 V times
the current load. This means the the maximum average RMS
current (in either direction) is 0.344 A for the
CM3106−12SM and 1.3 A for the CM3106−12SB. The
maximum instantaneous current is specified at 2 A, so this
condition should not be exceeded for more than 17% of the
time for the CM3106−12SM and 65% of the time for the
CM3106−12SB. 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 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 0.73 W, for
the CM3106−12SM and 2.75 W for the CM3106−12SB. So
the maximum average RMS current increases from 0.42 A
to 0.58 A for the CM3106−12SM and a maximum
instantaneous current of 2 A should not be exceeded for
more than 29% of the time. For CM3106−12SB, the
maximum RMS current increases from 1.3 A to 2.2 A. Thus,
the maximum continuous current can be 2 A all the time.
Figure 10. Duty Cycle vs. Ambient Temperature
(I_LOAD = 2.0 A)
Figure 11. Duty Cycle vs. Output Current
(Temp = 705C)
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