LTM4613 datasheet(18/30 Pages) LINER

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LTM4649

4649fc

For more information www.linear.com/LTM4649

APPLICATIONS INFORMATION

1st Example: Figure 7 for 27°C, or 300K the diode voltage

is 0.598V, thus, 300K = –(1200mV – 598mV)/–2.0mV/K).

2nd Example: Figure 7 for 75°C, or 350K the diode voltage

is 0.5V, thus, 350K = –(1200mV – 500mV)/–2.0mV/K).

Converting the Kelvin scale to Celsius is simply taking the

Kelvin temperature and subtracting 273 from it.

Measure the forward voltage at 27°C to establish a refer-

ence point. Then using the above expression while mea-

suring the forward voltage over temperature will provide a

general temperature monitor. Connect a resistor between

TEMP and VIN to set the current to 100µA. See Figure 21

for an example.

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 JESD51-12 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 a

µModule package mounted to a hardware test board.

The motivation for providing these thermal coefficients is

found in JESD51-12 (“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 provid-

ing guidance of thermal performance; instead, the derat-

ing curves provided in this 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 JESD51-12; these coef-

ficients 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 four 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

bottom of the package. In the typical µModule regulator,

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

component power dissipation flowing through the top

of the package. As the electrical connections of the

typical µModule 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

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4649 F07

125

ID = 100µA

Figure 7. Diode Voltage VD vs Temperature T(°C)

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