Linear Technology LTM8048 Demo Board, 725VDC Isolated µModule, 4.5V ≤ VIN ≤ 30V, VOUT = 5V @ 110mA to 370mA, LDO Post-Regulated Flyback Li DC1560A Ficha De Dados

Códigos do produto

DC1560A

LTM8048

8048ff

For more information

www.linear.com/LTM8048

APPLICATIONS INFORMATION

Thermal Considerations
The LTM8048 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 LTM8048 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 application,

many designers use FEA to predict thermal performance.

To that end, the Pin Configuration section of the data sheet

typically gives four thermal coefficients:
θ

: Thermal resistance from junction to ambient

JCbottom

: Thermal resistance from junction to the bot-

tom of the product case

JCtop

: Thermal resistance from junction to top of the

product case

JCboard

: 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 confu-

sion and inconsistency. These definitions are given in

JESD 51-12, and are quoted or paraphrased as follows:
θ

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.

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 converter,

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 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

JCbottom

, this

value may be useful for comparing packages but the test

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

JCboard

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

JCbottom

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 two-sided,

two-layer board. This board is described in JESD 51-9.
Given these definitions, it should now be apparent that none

of these thermal coefficients reflects an actual physical

operating condition of a µModule converter. Thus, none

of them can be individually used to accurately predict the

thermal performance of the product. Likewise, it would

be inappropriate to attempt to use any one coefficient to

correlate to the junction temperature vs load graphs given

in the product’s data sheet. The only appropriate way to

use the coefficients is when running a detailed thermal

analysis, such as FEA, which considers all of the thermal

resistances simultaneously.