Data Sheet

LT3046

analog.com

Rev 0

34 of 47

The underside of the DFN package has exposed metal from the lead frame to the die attachment. This package

allows heat to directly transfer from the die junction to the PCB metal to limit maximum operating junction

temperature. The dual, inline pin arrangement allows metal to extend beyond the ends of the package on the top

side (component side) of the PCB.

For surface-mount devices, heat sinking is accomplished by using the heat-spreading capabilities of the PCB and its

copper traces. Copper board stiffeners and plated throughholes can also be used to spread the heat generated by

the LDO regulator.

Table 7 lists the thermal resistance as a function of the copper area on a fixed board size. All measurements were

taken in still air on a 4-layer FR4 board with 1oz solid internal planes and 2oz top and bottom planes with a total

board thickness of 1.6mm. The four layers were electrically isolated with no thermal vias present. PCB layers,

copper weight, board layout, and thermal vias affect the resultant thermal resistance. For more information on

thermal resistance and high thermal conductivity test boards, refer to JEDEC standard JESD-51, JESD51-7, and

JESD51-12. Achieving low thermal resistance necessitates careful PCB layout.

Table 7. Measured Thermal Resistance for DFN Package

COPPER AREA

BOARD AREA (mm2)

THERMAL RESISTANCE

(θJA)

TOP SIDE1 (mm2)

BOTTOM SIDE (mm2)

2500

2500

2500

34°C/W

1000

2500

2500

34°C/W

225

2500

2500

35°C/W

100

2500

2500

36°C/W

1

Device is mounted on the top side.

Calculating Junction Temperature

For example, given an output voltage of 3.3V, an input voltage of 5V ± 5%, an output current range from 1mA to

200mA, and a maximum ambient temperature of +105°C, what is the maximum junction temperature?

The power dissipation of the LT3046 is the following:

������������������������(������������������) × (������������������(������������������) − ������������������������) + ������������������������ × ������������������(������������������)

where:

IOUT(MAX) = 200mA.

VIN(MAX) = 5.25V.

IGND (at IOUT = 200mA and VIN = 5.25V) = 7.2mA.

Therefore,

������������������������������ = 0.2������ × (5.25������ − 3.3������) + 7.2������������ × 5.25������ = 0.43������

Using a DFN package, the thermal resistance is in the range of 34°C/W to 36°C/W depending on the copper area.

Therefore, the junction temperature rise above ambient approximately equals 0.43W × 35°C/W = 15°C.

The maximum junction temperature equals the maximum ambient temperature plus the maximum junction

temperature rise above ambient, which calculates as follows:

������������������������������ = 105℃ + 15℃ = 120℃