LTM8057
14
8057f
For more information www.linear.com/LTM8057
APPLICATIONS INFORMATION
6. Use vias to connect the GND copper area to the board’s
internal ground planes. Liberally distribute these GND
vias to provide both a good ground connection and
thermal path to the internal planes of the printed circuit
board. Pay attention to the location and density of the
thermal vias in Figure 3. The LTM8057 can benefit from
the heat sinking afforded by vias that connect to internal
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.
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 LTM8057. However, these capaci
-
tors can cause problems if the LTM8057 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 LTM8057 can ring to more than
twice the nominal input voltage, possibly exceeding the
LTM8057’s rating and damaging the part. If the input
supply is poorly controlled or the user will be plugging
the LTM8057 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 the
V
IN
net. 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 LTM8057 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 LTM8057 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
JCboard
: 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 confu
-
sion 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.
