LTM8045 datasheet(35/54 Pages) AD

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LTM4655

Rev. 0

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Radiated EMI Noise

The generation of radiated EMI noise is an inherent

disadvantage of switching regulators. Fast switching

turn-on and turn-off of the power MOSFETs—necessary

for achieving high efficiency—create high-frequency

(~30MHz+) ∆l/∆t changes within DC/DC converters. This

activity tends to be the dominant source of high-frequency

EMI radiation in such systems. The high level of device

integration within LTM4655—including optimized gate-

driver and critical front-end � filter inductor—delivers

low radiated EMI noise performance. Figure 7 through

Figure 9 show typical examples of LTM4655 meeting the

radiated emission limits established by EN55022 Class B.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section are consistent with those parameters 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 pro-

viding these thermal coefficients is found in JESD51-12

(“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 predict the

µModule regulator’s thermal performance in their appli-

cation 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 manner that yields insight and guid-

ance 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 JESD51-12; these coefficients

are quoted or paraphrased below:

APPLICATIONS INFORMATION

1. θJA, the thermal resistance from junction-to-ambient,

is the natural convection junction-to-ambient air ther-

mal 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 JESD51-9 defined test board, which

does not reflect an actual application or viable operat-

ing condition.

2. 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.

3. 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 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 condi-

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

4. θJB, the thermal resistance from junction to the printed

circuit board, is the junction-to-board thermal resis-

tance where almost all of the heat flows through the

bottom of the µModule regulator 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 two sided, two layer board. This board

is described in JESD51-9.

A graphical representation of the aforementioned thermal

resistances is given in Figure 10; blue resistances are

contained within the µModule regulator, whereas green

resistances are external to the µModule package.

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