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LT3050 Series

3050fa

APPLICATIONS INFORMATION

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. The following tables list thermal resis-

tance as a function of copper area in a ﬁxed board size.

All measurements were taken in still air on a four-layer

FR-4 board with one ounce solid internal planes and two

ounce 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 1. MSOP Measured Thermal Resistance

COPPER AREA

BOARD

AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE

BACKSIDE

2500 sq mm 2500 sq mm 2500 sq mm

40°C/W

1000 sq mm 2500 sq mm 2500 sq mm

41°C/W

225 sq mm 2500 sq mm 2500 sq mm

43°C/W

100 sq mm 2500 sq mm 2500 sq mm

45°C/W

Table 2. DFN Measured Thermal Resistance

COPPER AREA

TOPSIDE

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

2500 sq mm

44°C/W

1000 sq mm

2500 sq mm

45°C/W

225 sq mm

2500 sq mm

47°C/W

100 sq mm

2500 sq mm

49°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

will the maximum junction temperature be?

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) = 1.5mA

So,

P = 75mA • (12.6V - 5V) + 1.5mA • 12.6V = 0.589W

Using a DFN package, the thermal resistance ranges from

44°C/W to 49°C/W depending on the copper area. So the

junction temperature rise above ambient approximately

equals:

0.589W • 49°C/W = 28.86°C

The maximum junction temperature equals the maximum

ambient temperature plus the maximum junction tempera-

ture rise above ambient or:

TJMAX = 85°C + 28.86°C = 113.86°C

Protection Features

The LT3050 incorporates several protection features that

make it ideal for use in battery-powered circuits. In ad-

dition to the normal protection features associated with

monolithic regulators, such as current limiting and thermal

limiting, the device also protects against reverse-input

voltages, reverse-output voltages and reverse output-to-

input voltages.

Current limit protection and thermal overload protection

protect the device against current overload conditions at

the output of the device. For normal operation, do not

exceed a junction temperature of 125°C.

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