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LTM4611

4611f

applicaTions inForMaTion

θJB is the junction-to-board thermal resistance where

almost all of the heat flows through the bottom of the

µModule 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 por-

tion 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 JESD

51-9.

Given these definitions, it should now be apparent that

none of these thermal coefficients reflects an actual

physical operating condition of a µModule. Thus, none

of them can be individually used to accurately predict the

thermal performance of the product. Likewise, it would

be inappropriate to attempt to use any one coefficient to

correlate to the junction temperature versus load graphs

given in the product’s data sheet. The only appropriate

way to use the coefficients is to run a detailed thermal

analysis, such as FEA, which considers all of the thermal

resistances simultaneously.

A graphical representation of these thermal resistances

is given in Figure 5.

The blue resistances are contained within the µModule,

and the green are outside.

The die temperature of the LTM4611 must be lower than

the maximum rating of 125°C, so care should be taken in

the layout of the circuit to ensure good heat sinking of the

LTM4611. The bulk of the heat flow out of the LTM4611 is

through the bottom of the module and the LGA pads into

the printed circuit board. Consequently, a poor printed

circuit board design can cause excessive heating, result-

ing in impaired performance or reliability. Please refer to

the PCB Layout section for printed circuit board design

suggestions

The 1.2V, 2.5V and 3.3V power loss curves in Figures 7

and 8 can be used in coordination with the load current

derating curves in Figures 9 to 16 for calculating an

approximate

θJA thermal resistance for the LTM4611

with various heat sinking and air flow conditions, as

evaluated on the aforementioned 4-layer FR4 PCB. The

power loss curves are taken at room temperature, and

are increased with multiplicative factors with ambient

temperature. These approximate factors are: 1 up to 50°C;

1.1 for 60°C; 1.15 for 70°C; 1.2 for 80°C; 1.25 for

90°C; 1.3 for 100°C; 1.35 for 110°C and 1.4 for 120°C.

The derating curves are plotted with the output current

starting at 15A and the ambient temperature at 55°C. The

output voltages are 1.2V, 2.5V and 3.3V. These are chosen

to include the lower and higher output voltage ranges for

correlating the thermal resistance. Thermal models are

derived from several temperature measurements in a

controlled temperature chamber along with thermal mod-

eling analysis. The junction temperatures are monitored

while ambient temperature is increased with and without

air flow, and with and without a heat sink attached with

thermally conductive adhesive tape. The BGA heat sinks

evaluated in Table 5 yield very comparable performance

in laminar airflow despite being visibly different in con-

struction and form factor. The power loss increase with

4611 F06

µMODULE

JUNCTION-TO-CASE (TOP)

RESISTANCE

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

CASE (BOTTOM)-TO-BOARD

RESISTANCE

Figure 6

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