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LTM4650

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APPLICATIONS INFORMATION

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters defined by JESD51-9 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 a

µModulepackagemountedtoahardwaretestboard—also

defined by JESD51-9 (“Test Boards for Area Array Surface

MountPackageThermalMeasurements”).Themotivation

for providing these thermal coefficients is found in JESD

51-12 (“Guidelines for Reporting and Using Electronic

Package Thermal Information”).

Many designers may opt to use laboratory equipment

and a test vehicle such as the demo board to anticipate

the µModule regulator’s thermal performance in their ap-

plication at various electrical and environmental operating

conditions to compliment any FEA activities. Without FEA

software, the thermal resistances reported in the Pin Con-

figuration section are in-and-of themselves not relevant to

providing guidance of thermal performance; instead, the

derating curves provided in the data sheet can be used in

a manner 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 typically gives four thermal

coefficients explicitly defined in JESD 51-12; these coef-

ficients are quoted or paraphrased below:

1. 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.Thisenvironmentissometimesreferredtoas“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.

2. θJCbottom, the thermal resistance from junction to the

bottom of the product case, 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, the bulk of the heat flows out

Combining like terms, then simplifying the natural log

terms yields:

ΔVD = T(KELVIN) • KD • lN(10)

and redefining constant

K'D=KD•IN(10) =

198µV

yields

ΔVD = K'D • T(KELVIN)

Solving for temperature:

T(KELVIN)=

ΔVD

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.

ThediodeconnectedPNPtransistorattheTEMPpincanbe

used to monitor the internal temperature of the LTM4650.

See Figure 23 for an example.

Figure 8. Diode Voltage VD vs Temperature T(°C)

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4650 F08

125

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