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LTM8055

8055fb

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to prevent this overshoot. This can be accomplished by

installing a small resistor in series with VIN, but the most

popular method of controlling input voltage overshoot is

to add an electrolytic bulk capacitor to the VIN 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 likely to be the largest component in the circuit.

Thermal Considerations

The LTM8055 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 LTM8055 mounted to a 58cm2 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’sline,loadandenvironmentaloperatingconditions.

ThethermalresistancenumberslistedinthePinConfigura-

tion of the data sheet are based on modeling the µModule

package mounted on a test board specified per JESD 51-9

(TestBoardsforAreaArraySurfaceMountPackageThermal

Measurements). The thermal coefficients provided on this

page are based on JESD 51-12 (Guidelines for Reporting

and Using Electronic Package Thermal Information).

Forincreasedaccuracyandfidelitytotheactualapplication,

many designers use FEA to predict thermal performance.

To that end, the Pin Configuration of the data sheet typi-

cally gives four thermal coefficients:

θJA – Thermal resistance from junction to ambient.

θJCbottom–Thermalresistancefromjunctiontothebottom

of the product case.

θJCtop – Thermal resistance from junction to top of the

product case.

θJB – 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:

θJA 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.

θJCbottom is the thermal resistance between the junction

andbottomofthepackagewithallofthecomponentpower

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.

θJCtopisdeterminedwithnearlyallofthecomponentpower

dissipation flowing through the top of the package. As the

electricalconnectionsofthetypicalµModuleconverterare

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 θJCbottom, this

value may be useful for comparing packages but the test

conditions don’t generally match the user’s application.

θJB is the junction-to-board thermal resistance where

almost all of the heat flows through the bottom of the

µModule converter and into the board, and is really the

sum of the θJCbottom and the thermal resistance of the

bottom of the part through the solder joints and through a

portion of the board. The board temperature is measured

a specified distance from the package, using a 2-sided,

2-layer board. This board is described in JESD 51-9.

Giventhesedefinitions,itshouldnowbeapparentthatnone

of these thermal coefficients reflects an actual physical

operating condition of a µModule converter. Thus, none

APPLICATIONS INFORMATION

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