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LT3065 Series

3065fc

For more information www.linear.com/LT3065

applicaTions inForMaTion

Thermal Considerations

The LT3065’s maximum rated junction temperature of

125°C (E-, I-grades) or 150°C (MP-, H-grades) limits its

power handling capability. Two components comprise the

power dissipated by the device:

1. Output current multiplied by the input/output voltage

differential:

IOUT • (VIN – VOUT),

and

2. GND pin current multiplied by the input voltage:

IGND • VIN

GND pin current is determined using the GND Pin Current

curves in the Typical Performance Characteristics section.

Power dissipation equals the sum of the two components

listed above.

The LT3065 regulator has internal thermal limiting

that protects the device during overload conditions.

For continuous normal conditions, do not exceed the

maximum junction temperature of 125°C (E-, I-grades)

or 150°C (MP-, H-grades). Carefully consider all sources

of thermal resistance from junction-to-ambient including

other heat sources mounted in proximity to the LT3065.

TheundersidesoftheLT3065DFNandMSEpackageshave

exposed metal from the lead frame to the die attachment.

These packages allow heat to directly transfer from the

die junction to the printed circuit board metal to control

maximum operating junction temperature. The dual-inline

pin arrangement allows metal to extend beyond the ends

of the package on the topside (component side) of a PCB.

Connect this metal to GND on the PCB. The multiple IN

and OUT pins of the LT3065 also assist in spreading heat

to the PCB.

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Copper board stiffeners and plated

through-holes also can spread the heat generated by

power devices.

Tables4and5listthermalresistanceasafunctionofcopper

area in a fixed board size. All measurements were taken

in still air on a 4-layer FR-4 board with 1oz solid internal

planes, and 2oz external trace planes with a total board

thickness of 1.6mm. For further information on thermal

resistance and using thermal information, refer to JEDEC

standard JESD51, notably JESD51-12.

Table 4. MSOP Measured Thermal Resistance

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE

BACKSIDE

2500 sq mm 2500 sq mm 2500 sq mm

28°C/W

1000 sq mm 2500 sq mm 2500 sq mm

31°C/W

225 sq mm 2500 sq mm 2500 sq mm

32°C/W

100 sq mm 2500 sq mm 2500 sq mm

33°C/W

Table 5. DFN Measured Thermal Resistance

COPPER AREA

TOPSIDE

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

2500 sq mm

31°C/W

1000 sq mm

2500 sq mm

32°C/W

225 sq mm

2500 sq mm

34°C/W

100 sq mm

2500 sq mm

35°C/W

Calculating Junction Temperature

Example: Given an output voltage of 5V, an input voltage

range of 12V ±5%, a maximum output current range of

75mA and a maximum ambient temperature of 85°C, what

is the maximum junction temperature?

The power dissipated by the device equals:

IOUT(MAX) • (VIN(MAX) – VOUT) + IGND • VIN(MAX)

where:

IOUT(MAX) = 75mA

VIN(MAX) = 12.6V

IGND at (IOUT = 75mA, VIN = 12V) = 3.5mA

So:

P = 75mA • (12.6V – 5V) + 3.5mA • 12.6V = 0.614W

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