LTM4644 datasheet(17/32 Pages) LINER

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LTM4634

4634f

For more information www.linear.com/LTM4634

applicaTions inForMaTion

ceramic capacitor to ground at EXTVCC and ensure the

voltage at CNTL_PWR is always greater than the voltage

at EXTVCC at all times during start-up and shutdown.

Connecting VOUT3 to EXTVCC may present a convenient

way to meet the sequencing requirement. Otherwise float

EXTVCC if not used.

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”).

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 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 gives four thermal coeffi-

cients explicitly defined in JESD51-12. These coefficients

are quoted or paraphrased as follows:

θJA:Thethermalresistancefromjunctiontoambient,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 the junction to

the bottom of the product case, is determined with all

of the internal power dissipation flowing through the

bottom of the package. In a 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

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

through a portion of the board. The board temperature

is measured at a specified distance from the package.

A graphical representation of the aforementioned ther-

mal resistances is given in Figure 4; blue resistances are

contained within the μModule regulator, whereas green

resistances are external to the µModule package.

As a practical matter, it should be clear to the reader that

no individual or sub-group of the four thermal resistance

parameters defined by JESD51-12 or provided in the Pin

Configuration section replicates or conveys normal oper-

ating conditions of a μModule regulator. For example, in

normal board-mounted applications, never does 100%

of the device’s total power loss (heat) thermally con-

duct exclusively through the top or exclusively through

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