LT3045 datasheet(22/32 Pages) LINER

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LT3045

3045fa

For more information www.linear.com/LT3045

Thermal Considerations

The LT3045 has internal power and thermal limiting cir-

cuits that protect the device under overload conditions.

The thermal shutdown temperature is nominally 165°C

with about 8°C of hysteresis. For continuous normal

load conditions, do not exceed the maximum junction

temperature (125°C for E- and I-grades). It is important to

consider all sources of thermal resistance from junction to

ambient.Thisincludesjunction-to-case,case-to-heatsink

interface, heat sink resistance or circuit board-to-ambient

as the application dictates. Additionally, consider all heat

sources in close proximity to the LT3045.

The undersides of the DFN and MSOP packages have

exposed metal from the lead frame to the die attachment.

Both packages allow heat to directly transfer from the die

junction to the PCB metal to limit maximum operating

junction temperature. The dual-in-line pin arrangement

allows metal to extend beyond the ends of the package

on the topside (component side) of the PCB.

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PCB and its

coppertraces.Copperboardstiffenersandplatedthrough-

holes can also be used to spread the heat generated by

the regulator.

Tables3and4listthermalresistanceasafunctionofcopper

area on 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 top/bottom planes with a total board thick-

ness 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 JESD51, notably JESD51-7 and JESD51-12.

Achieving low thermal resistance necessitates attention

to detail and careful PCB layout.

Table 3. Measured Thermal Resistance for DFN Package

COPPER AREA

BOARD AREA

THERMAL

RESISTANCE

TOP SIDE*

BOTTOM SIDE

2500mm2

34°C/W

1000mm2

2500mm2

34°C/W

225mm2

2500mm2

35°C/W

100mm2

2500mm2

36°C/W

*Device is mounted on topside

Table 4. Measured Thermal Resistance for MSOP Package

COPPER AREA

BOARD AREA

THERMAL

RESISTANCE

TOP SIDE*

BOTTOM SIDE

2500mm2

33°C/W

1000mm2

2500mm2

33°C/W

225mm2

2500mm2

34°C/W

100mm2

2500mm2

35°C/W

*Device is mounted on topside

Calculating Junction Temperature

Example:Givenanoutputvoltageof3.3Vandinputvoltage

of 5V ± 5%, output current range from 1mA to 500mA,

and a maximum ambient temperature of 85°C, what is the

maximum junction temperature?

The LT3045’s power dissipation is:

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

where:

IOUT(MAX) = 500mA

VIN(MAX) = 5.25V

IGND (at IOUT = 500mA and VIN = 5.25V) = 12.5mA

thus:

PDISS = 0.5A • (5.25V – 3.3V) + 12.5mA • 5.25V = 1W

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:

1W • 35°C/W = 35°C

The maximum junction temperature equals the maxi-

mum ambient temperature plus the maximum junction

temperature rise above ambient:

TJMAX = 85°C + 35°C = 120°C

applicaTions inForMaTion

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