Data Sheet

LTM4640

analog.com

Rev. 0

25 of 38

The Pin Configurations and Function Descriptions section gives three thermal coefficients explicitly defined in

JESD5112; these coefficients are quoted or paraphrased as follows.

1. θJA, the thermal resistance from junction to ambient, is the natural convection junction-to-ambient air

thermal resistance measured in 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 95mm × 76mm PCB with four layers.

2. θJCbottom, the thermal resistance from the junction to the bottom of the product case, is determined with all

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 package, 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 the junction to the top of the product case, is determined with nearly all

the component’s 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 conditions don’t generally match the user’s

application.

A graphical representation of the thermal resistances is given in Figure 25; blue resistances are contained within the

μModule regulator, whereas green resistances are external to the µModule package.

Figure 25. Graphical Representation of JESD51–12 Thermal Coefficients

As a practical matter, it should be clear to the user that no individual or sub-group of the three thermal resistance

parameters defined by JESD5112 or provided in the Pin Configurations and Function Descriptions section replicates

or conveys normal operating conditions of a μModule regulator. For example, in normal board-mounted

applications, never does 100% of the device’s total power loss (heat) thermally conducts exclusively through the

top or exclusively through the bottom of the µModule package—as the standard defines for θJCtop and θJCbottom,

respectively. In practice, power loss is thermally dissipated in both directions away from the package—granted, in

the absence of a heat sink and airflow, most of the heat flow is into the board.

Airflow and Heat Sinking

Within the LTM4640, be aware that there are multiple power devices and components dissipating power with a

consequence that the thermal resistances relative to different junctions of components or dies are not exactly

linear with respect to the total package power loss. To reconcile this complication without sacrificing modeling

µModule DEVICE

θJCtop JUNCTION-TO-CASE

(TOP) RESISTANCE

θJB JUNCTION-TO-BOARD RESISTANCE

θJA JUNCTION-TO-AMBIENT RESISTANCE

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

θJCbot JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE