LTM8045
16
8045fc
For more information www.linear.com/LTM8045
applicaTions inForMaTion
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 LTM8045. However, these capaci
-
tors can cause problems if the LTM8045 is plugged into a
live
input supply (see 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 voltage at the V
IN
pin
of the LTM8045 can ring to more than twice the nominal
input voltage, possibly exceeding the LTM8045’s rating
and damaging the part. If the input supply is poorly con
-
trolled or the user will be plugging the LTM8045 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 with V
IN
, but the most
popular method of controlling input voltage overshoot is
to add 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 filter
-
ing and can slightly improve the efficiency of the circuit,
though it is physically large.
Thermal Considerations
The
LTM8045 output current may need to be derated if
it is required to operate in a high ambient temperature or
deliver a large amount of continuous power. 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 Character
-
istics section can be used as a guide. These curves were
generated by a LTM8045 mounted to a 25.8cm
2
4-layer
FR4 printed circuit board with a copper thickness of 2oz
for the top and bottom layer and 1oz for the inner layers.
Boards of other sizes and layer count can exhibit differ
-
ent thermal behavior, so it is incumbent upon the user to
verify
proper operation over the intended system’s line,
load and environmental operating conditions.
The thermal resistance numbers listed in the Pin Configura-
tion section
of the data sheet are based on modeling the
µModule package mounted on a test board specified per
JESD
51-9 (“Test Boards for Area Array Surface Mount
Package Thermal Measurements”). The thermal coefficients
provided in this page are based on JESD 51-12 (“Guide-
lines
for Reporting and Using Electronic Package
Thermal
Information”).
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
bottom 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 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 thermal resistance between the junction
and bottom of the package 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 environment.
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.
