LTM8026 LINER [Linear Technology], LTM8026 Datasheet - Page 20

• Manufacturer Part Number: LTM8026
• Description: 36VIN, 5A, 2-Quadrant CVCC Step-Down
• Manufacturer: LINER [Linear Technology]

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. However, these capacitors
can cause problems if the LTM8052 is plugged into a live
input supply (see Application Note 88 for a complete dis-
cussion). 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
of the LTM8052 can ring to more than twice the nominal
input voltage, possibly exceeding the LTM8052’s rating
and damaging the part. If the input supply is poorly con-
trolled or the user will be plugging the LTM8052 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
popular method of controlling input voltage overshoot is
to add an electrolytic bulk capacitor to the V
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 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 LTM8052 mounted to a 58cm
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.
20
IN
, but the most
IN
net. This
IN
pin
2
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:
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
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
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
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
value may be useful for comparing packages but the test
conditions don’t generally match the user’s application.
JA
JCbottom
JCtop
θ
θ
bottom of the product case
θ
product case
θ
circuit board.
JA
JCbottom
JCtop
JB
is the natural convection junction-to-ambient air
is determined with nearly all of the component power
– Thermal resistance from junction to ambient
– Thermal resistance from junction to the printed
– Thermal resistance from junction to top of the
is the junction-to-board thermal resistance with
– Thermal resistance from junction to the
JCbottom
, this
8052fa