LTM8021
12
8021fd
For more information www.linear.com/LTM8021
applicaTions inForMaTion
the circuit. In Figure 5b an aluminum electrolytic capacitor
has been added. This capacitor’s 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 is likely to be the largest component in the
circuit. An alternative solution is shown in Figure 5c. A 0.7W
resistor is added in series with the input to eliminate the
voltage overshoot (it also reduces the peak input current).
A 0.1µF capacitor improves high frequency filtering. This
solution is smaller and less expensive than the electrolytic
capacitor. For high input voltages its impact on efficiency
is minor, reducing efficiency less than one-half percent for
a 5V output at full load operating from 24V.
Thermal Considerations
The LTM8021 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 Charac
-
teristics section can be used as a guide. These curves
were generated by a LTM8021 mounted to a 40.3cm
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.
The thermal resistance numbers listed in Page 2 of the
data sheet are based on modeling the µModule package
mounted on a test board specified per JESD51-9 (Test
Boards for Area Array Surface Mount Package Thermal
Measurements). The thermal coefficients provided in this
page are based on JESD 51-12 (Guidelines 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, Page 2 of the data sheet typically gives four
thermal coefcients:
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.
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 converter 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.
