8

LT3014B

3014bf

The LT3014B regulator has internal thermal limiting de-

signed to protect the device during overload conditions.

For continuous normal conditions the maximum junction

temperature rating of 125

°C must not be exceeded. It is

important to give careful consideration to all sources of

thermal resistance from junction to ambient. Additional

heat sources mounted 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. Copper board stiffeners and plated

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

ated by power devices.

The following table lists thermal resistance for several

different board sizes and copper areas. All measurements

were taken in still air on 3/32" FR-4 board with one ounce

copper.

Table 1. SOT-23 Measured Thermal Resistance

COPPER AREA

THERMAL RESISTANCE

TOPSIDE

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500 sq mm

2500 sq mm

2500 sq mm

125

°C/W

1000 sq mm

2500 sq mm

2500 sq mm

125

°C/W

225 sq mm

2500 sq mm

2500 sq mm

130

°C/W

100 sq mm

2500 sq mm

2500 sq mm

135

°C/W

50 sq mm

2500 sq mm

2500 sq mm

150

°C/W

Table 2. DFN Measured Thermal Resistance

COPPER AREA

THERMAL RESISTANCE

TOPSIDE

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500 sq mm

2500 sq mm

2500 sq mm

40

°C/W

1000 sq mm

2500 sq mm

2500 sq mm

45

°C/W

225 sq mm

2500 sq mm

2500 sq mm

50

°C/W

100 sq mm

2500 sq mm

2500 sq mm

62

°C/W

For the DFN package, the thermal resistance junction-to-

case (

die, is 16

°C/W.

Continuous operation at large input/output voltage differ-

entials and maximum load current is not practical due to

thermal limitations. Transient operation at high input/

output differentials is possible. The approximate thermal

time constant for a 2500sq mm 3/32" FR-4 board with

maximum topside and backside area for one ounce copper

is 3 seconds. This time constant will increase as more

thermal mass is added (i.e. vias, larger board, and other

components).

For an application with transient high power peaks, aver-

age power dissipation can be used for junction tempera-

ture calculations as long as the pulse period is significantly

less than the thermal time constant of the device and

board.

Calculating Junction Temperature

Example 1: Given an output voltage of 5V, an input voltage

range of 24V to 30V, an output current range of 0mA to

20mA, and a maximum ambient temperature of 50

°C,

what will the maximum junction temperature be?

The power dissipated by the device will be equal to:

where:

So:

P = 20mA • (30V – 5V) + (0.55mA • 30V) = 0.52W

The thermal resistance for the DFN package will be in the

range of 40

°C/Wto62°C/Wdependingonthecopperarea.

So the junction temperature rise above ambient will be

approximately equal to:

0.52W • 50

°C/W = 26°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

Example 2: Given an output voltage of 5V, an input voltage

of 48V that rises to 72V for 5ms(max) out of every 100ms,

and a 5mA load that steps to 20mA for 50ms out of every

250ms, what is the junction temperature rise above ambi-

ent? Using a 500ms period (well under the time constant

of the board), power dissipation is as follows:

P1(48V in, 5mA load) = 5mA • (48V – 5V)

+ (100

µA • 48V) = 0.22W

APPLICATIO S I FOR ATIO