LTM8028
17
8028fb
For more information www.linear.com/LTM8028
applicaTions inForMaTion
Thermal Considerations
The LTM8028 relies on two thermal safety features. At about
145°C, the device is designed to pull the PGOOD output
low providing an early warning of an impending thermal
shutdown condition. At 165°C typically, the LTM8028 is
designed to engage its thermal shutdown and the output is
turned off until the IC temperature falls below the thermal
hysteresis limit. Note that these temperature thresholds
are above the 125°C absolute maximum rating to avoid
interfering with normal operation. Thus, prolonged or
repetitive operation under a condition in which the thermal
shutdown activates may damage or impair the reliability
of the device.
The LTM8028 output current may need to be derated if it
is required to operate in a high ambient temperature. The
amount of current derating is dependent upon the input
voltage, output power and ambient temperature. The
temperature rise curves given in the Typical Performance
Characteristics section can be used as a guide. These curves
were generated by the LTM8028 mounted to a 58cm
2
4-layer FR4 printed circuit board. Boards of other sizes
and layer count can exhibit different thermal behavior, so
it is incumbent upon the user to verify proper operation
over the intended system’s line, load and environmental
operating conditions.
For increased accuracy and fidelity to the actual application,
many designers use finite element analysis (FEA) to predict
thermal performance. To that end, the Pin Configuration
of the data sheet typically gives four thermal coefficients:
θ
JA
– Thermal resistance from junction to ambient
θ
JCbottom
– Thermal resistance from junction to the bottom
of the product case
θ
JCtop
– Thermal resistance from junction to top of the
product case
θ
JBoard
– Thermal resistance from junction to the printed
circuit board.
While the meaning of each of these coefficients may seem
to be intuitive, JEDEC has defined each to avoid confusion
and inconsistency. These definitions are given in JESD
51-12, and are quoted or paraphrased below:
θ
JA
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.
θ
JCbottom
is the junction-to-board thermal resistance with
all of the component power dissipation flowing through the
bottom of the package. In the typical µModule regulator,
the bulk of the heat flows out the bottom of the package,
but there is always heat flow out into the ambient envi
-
ronment. As a result, this thermal resistance value may
be
useful
for comparing packages but the test conditions
don’t generally match the user’s application.
θ
JCtop
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 regulator are
on the bottom of the package, it is rare for an application
to operate such that most of the heat flows from the junc
-
tion 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.
θ
JB
is the junction-to-board thermal resistance where
almost all of the heat flows through the bottom of the
µModule regulator 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
portion of the board. The board temperature is measured
a specified distance from the package, using a 2-sided,
2-layer board. This board is described in JESD 51-9.
