Datasheet

LTM4649
17
4649f
For more information www.linear.com/LTM4649
APPLICATIONS INFORMATION
Thermal Considerations and Output Current Derating
The thermal resistances reported in the Pin Configuration
section of the data sheet are consistent with those param-
eters defined by JESD51-9 and are intended for use with
finite element analysis (FEA) software modeling tools that
leverage the outcome of thermal modeling, simulation, and
correlation to hardware evaluation performed on a µModule
package mounted to a hardware test board—also defined
by JESD51-9 (“Test Boards for Area Array Surface Mount
Package Thermal Measurements”). The motivation for
providing these thermal coefficients in found in JESD51-12
(“Guidelines for Reporting and Using Electronic Package
Thermal Information”).
Many designers may opt to use laboratory equipment
and a test vehicle such as the demo board to anticipate
the µModule regulator’s thermal performance in their ap-
plication at various electrical and environmental operating
conditions to compliment any FEA activities. Without FEA
software, the thermal resistances reported in the Pin Con-
figuration section are in-and-of themselves not relevant to
providing guidance of thermal performance; instead, the
derating curves provided in the data sheet can be used in
a manner that yields insight and guidance pertaining to
one’s application usage, and can be adapted to correlate
thermal performance to one’s own application.
The Pin Configuration section typically gives four thermal
coefficients explicitly defined in JESD 51-12; these coef-
ficients are quoted or paraphrased below:
1. θ
JA
: the thermal resistance from junction to ambient, is
the natural convection junction-to-ambient air thermal
resistance measured in a one cubic foot sealed enclo-
sure. This environment is sometimes referred to as
“still air” although natural convection causes the air to
move. This value is determined with the part mounted to
a JESD 51-9 defined test board, which does not reflect
an actual application or viable operating condition.
2. θ
JCbottom
: the thermal resistance from junction to ambi-
ent, is the natural convection junction-to-ambient air
thermal resistance measured in a one cubic foot sealed
enclosure. This environment is sometimes referred to as
“still air” although natural convection causes the air to
move. This value is determined with the part mounted to
a JESD 51-9 defined test board, which does not reflect
an actual application or viable operating condition.
3. θ
JCtop
: the thermal resistance from junction to top of
the product case, is determined with nearly all of the
component power dissipation flowing through the top
of the package. As the electrical connections of the
typical µModule are on the bottom of the package, it
is rare for an application to operate such that most of
the heat flows from the junction to the top of the part.
As in the case of θ
JCbottom
, this value may be useful
for comparing packages but the test conditions don’t
generally match the user’s application.
4. θ
JB
: the thermal resistance from junction to the printed
circuit board, is the junction-to-board thermal resistance
where almost all of the heat flows through the bottom of
the µModule and into the board, and is really the sum of
the θ
JCbottom
and the thermal resistance of the bottom
of the part through the solder joints and through a por-
tion of the board. The board temperature is measured a
specified distance from the package, using a two sided,
two layer board. This board is described in JESD 51-9.
4649 F07
TEMPERATURE (°C)
–173 –73 27 127
DIODE VOLTAGE (V)
0.4
0.6
0.8
1.0
∆V
D
I
D
= 100µA
Figure 7. Diode Voltage V
D
vs Temperature
T(°C) for Different Bias Currents