LTM4645 datasheet(19/34 Pages) LINER

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LTM4645

4645f

For more information www.linear.com/LTM4645

applicaTions inForMaTion

Combining like terms, then simplifying the natural log

terms yields:

∆VD = T(KELVIN) • KD • lN(10)

and redefining constant

K'D=KD•IN(10) =

198µV

yields

∆VD = K'D • T(KELVIN)

Solving for temperature:

T(KELVIN)

∆VD

K'D

(

°CELSIUS)= T(KELVIN)–273.15

where

300°K = 27°C

means that is we take the difference in voltage across the

diode measured at two currents with a ratio of 10, the

resulting voltage is 198μV per Kelvin of the junction with

a zero intercept at 0 Kelvin.

The diode connected PNP transistor between the TEMP+

and TEMP– pin can be used to monitor the internal tem-

perature of the LTM4645. See Figure 23 for an example.

Thermal Considerations

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters defined by JESD51-12 and are intended for use with

finite element analysis (FEA) software modeling tools that

leverage the outcome of thermal modeling, simulation,

and correlation to hardware evaluation performed on

an µModule package mounted to a hardware test board.

The motivation for providing these thermal coefficients in

found in JESD 51-12 (“Guidelines for Reporting and Using

Electronic Package Thermal Information”).

Manydesignersmayopttouselaboratoryequipmentanda

testvehiclesuchasthedemoboardtopredicttheµModule

regulator’s thermal performance in their application at

various electrical and environmental operating conditions

to compliment any FEA activities. Without FEA software,

the thermal resistances reported in the Pin Configuration

section are, in and of themselves, not relevant to providing

guidance of thermal performance; instead, the derating

curves provided in this data sheet can be used in a man-

ner that yields insight and guidance pertaining to one’s

application usage, and can be adapted to correlate thermal

performance to one’s own application.

The Pin Configuration section gives four thermal coeffi-

cients explicitly defined in JESD 51-12; these coefficients

are quoted or paraphrased below:

θJA, the thermal resistance from junction to ambient, is

the natural convection junction-to-ambient air thermal

resistance measured in a one cubic foot sealed enclo-

sure. 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 95mm

× 76mm PCB with six layers.

θJCbottom, the thermal resistance from junction to the

bottom of the product case, is determined with all of

the component power dissipation flowing through the

bottomofthepackage.InthetypicalµModuleregulator,

the bulk of the heat flows out the bottom of the pack-

age, 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.

θJCtop, the thermal resistance from junction to top of

the product case, is determined with nearly all of the

componentpowerdissipationflowingthroughthetopof

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 junction 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, the thermal resistance from junction to the printed

circuitboard,isthejunction-to-boardthermalresistance

where almost all of the heat flows through the bottom

of the µModule package 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 a

portionoftheboard.Theboardtemperatureismeasured

a specified distance from the package.

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