LTM8026_1208 LINER [Linear Technology], LTM8026_1208 Datasheet - Page 18

• Manufacturer Part Number: LTM8026_1208
• Description: 36VIN, 5A CVCC Step-Down ?Module Regulator
• Manufacturer: LINER [Linear Technology]

LTM8026
applicaTions inForMaTion
3. Place the C
4. Place the C
5. Connect all of the GND connections to as large a copper
6. Use vias to connect the GND copper area to the board’s
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8026. However, these capacitors
can cause problems if the LTM8026 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 LTM8026 can ring to more than twice the nominal
input voltage, possibly exceeding the LTM8026’s rating
and damaging the part. If the input supply is poorly con-
trolled or the user will be plugging the LTM8026 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
18
V
ground current flow directly adjacent or underneath
the LTM8026.
pour or plane area as possible on the top layer. Avoid
breaking the ground connection between the external
components and the LTM8026.
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 5. The LTM8026 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.
OUT
and GND connection of the LTM8026.
OUT
IN
and C
capacitor as close as possible to the
OUT
capacitors such that their
IN
, but the most
IN
net. This
IN
pin
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 LTM8026 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 LTM8026 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.
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.
JA
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
– Thermal resistance from junction to ambient
– Thermal resistance from junction to the printed
– Thermal resistance from junction to top of the
– Thermal resistance from junction to the
8026fa
2