Datasheet

LTM8052/LTM8052A
21
8052fc
For more information www.linear.com/LTM8052
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 LTM8052/LTM8052A. However, these
capacitors can cause problems if the LTM8052/LTM8052A
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 volt
-
age at the V
IN
pin of the LTM8052/LTM8052A can ring
to more than twice the nominal input voltage, possibly
exceeding the LTM8052/LTM8052A’s rating and damag
-
ing the part. If the input supply is poorly controlled or the
user will be plugging the LTM8052/LTM8052A 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
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
LTM8052/LTM8052A output current may need to
be derated if it is required to operate in a high ambient
temperature. The amount of current derating is depen
-
dent 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 LTM8052/
LTM8052A 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 applica
-
tion, many
designers use finite element analysis (FEA) to
predict
thermal performance. To that end, Page 2 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 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.