LTM8046
13
8046fb
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applicaTions inForMaTion
GND planes at these locations, due to their proximity
to internal power handling components. The optimum
number of thermal vias depends upon the printed
circuit board design. For example, a board might use
very small via holes. It should employ more thermal
vias than a board that uses larger holes.
The printed circuit board construction has an impact on
the isolation performance of the end product. For example,
increased trace and layer spacing, as well as the choice
of core and prepreg materials (such as using polyimide
versus FR4) can significantly affect the isolation withstand
of the end product.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of the LTM8046. However, these capaci
-
tors can cause problems if the LTM8046 is plugged into a
live supply (see
Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor
combined with stray inductance in series with the power
source forms an underdamped tank circuit, and the volt
-
age at
the V
IN
pin of the LTM8046 can ring to more than
twice the nominal input voltage, possibly exceeding the
LTM8046’s
rating and damaging the part. If the input
supply is poorly controlled or
the user will be plugging
the LTM8046 into an energized supply, the input network
should be designed to prevent this overshoot. This can
be accomplished by installing a small resistor in series
to V
IN
, but the most popular method of controlling input
voltage overshoot is adding an electrolytic bulk capacitor
to V
IN
. This capacitor’s relatively high equivalent series
resistance damps the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency
ripple filtering and can slightly improve the efficiency of the
circuit, though it can be a large component in the circuit.
Thermal Considerations
The LTM8046 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 LTM8046 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 FEA to predict thermal performance.
To that end, the Pin Configuration section 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 bot-
tom of the product case
θ
JCtop
: Thermal resistance from junction to top of the
product case
θ
JB
: 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 as follows:
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 converter,
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.
