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LT3085

3085fa

from junction to ambient. This includes junction-to-case,

case-to-heat sink interface, heat sink resistance or circuit

board-to-ambient as the application dictates. Additional

heat sources nearby must also be considered.

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Surface mount heat sinks and

plated through-holes can also be used to spread the heat

generated by power devices. Boards speciﬁed in thermal

resistance tables have no vias on plated through-holes

from topside to backside.

Junction-to-case thermal resistance is speciﬁed from

the IC junction to the bottom of the case directly below

the die. This is the lowest resistance path for heat ﬂow.

Proper mounting is required to ensure the best possible

thermal ﬂow from this area of the package to the heat

sinking material. Note that the Exposed Pad is electrically

connected to the output.

The following tables list thermal resistance for several

different copper areas given a ﬁxed board size. All mea-

surements were taken in still air on two-sided 1/16” FR-4

board with one ounce copper.

PCB layers, copper weight, board layout and thermal vias

affect the resultant thermal resistance. Although Tables

2 and 3 provide thermal resistance numbers for 2-layer

board with 1 ounce copper, modern multi-layer PCBs

provide better performance than found in these tables.

For example, a 4-layer, 1 ounce copper PCB board with

5 thermal vias from the DFN or MSOP exposed backside

pad to inner layers (connected to VOUT) achieves 40°C/W

thermal resistance. Demo circuit 1401A’s board layout

achieves this 40°C/W performance. This is approximately

a 45% improvement over the numbers shown in Tables

2 and 3.

APPLICATIONS INFORMATION

Table 2. MSE Package, 8-Lead MSOP

COPPER AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

BOARD AREA

2500mm2

55°C/W

1000mm2

2500mm2

57°C/W

225mm2

2500mm2

60°C/W

100mm2

2500mm2

65°C/W

*Device is mounted on topside

Table 3. DCB Package, 6-Lead DFN

COPPER AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

BOARD AREA

2500mm2

68°C/W

1000mm2

2500mm2

70°C/W

225mm2

2500mm2

73°C/W

100mm2

2500mm2

78°C/W

*Device is mounted on topside

For future information on the thermal resistance and using thermal

information, refer to JEDEC standard JESD51, notably JESD51-12.

Calculating Junction Temperature

Example: Given an output voltage of 0.9V, a VCONTROL

voltage of 3.3V ±10%, an IN voltage of 1.5V ±5%, output

current range from 1mA to 0.5A and a maximum ambi-

ent temperature of 50°C, what will the maximum junction

temperature be for the DFN package on a 2500mm2 board

with topside copper area of 500mm2?

The power in the drive circuit equals:

PDRIVE = (VCONTROL – VOUT)(ICONTROL)

where ICONTROL is equal to IOUT/60. ICONTROL is a function

of output current. A curve of ICONTROL vs IOUT can be found

in the Typical Performance Characteristics curves.

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