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LT3089

3089f

For more information www.linear.com/LT3089

APPLICATIONS INFORMATION

For further information on thermal resistance and using

thermal information, refer to JEDEC standard JESD51,

notably JESD51-12.

PCB layers, copper weight, board layout and thermal vias

affect the resultant thermal resistance. Tables 3 through 5

provide thermal resistance numbers for best-case 4-layer

boards with 1oz internal and 2oz external copper. Modern,

multilayer PCBs may not be able to achieve quite the same

level performance as found in these tables. Demo circuit

2318A’s board layout using multiple inner VOUT planes

and multiple thermal vias achieves 17°C/W performance

for the DF package.

Calculating Junction Temperature

Example: Given an output voltage of 0.9V, an IN voltage

of 2.5V ±5%, output current range from 10mA to 0.8A

and a maximum ambient temperature of 50°C, what is

the maximum junction temperature for the DD-Pak on a

2500mm2 board with topside copper of 1000mm2?

The power in the circuit equals:

PTOTAL = (VIN – VOUT)(IOUT)

The current delivered to the SET pin is negligible and can

be ignored.

VIN(MAX_CONTINUOUS) = 2.625V (2.5V + 5%)

VOUT = 0.9V, IOUT = 0.8A, TA = 50°C

Power dissipation under these conditions equals:

PTOTAL = (VIN – VOUT)(IOUT)

PTOTAL = (2.625V – 0.9V)(0.8A) = 1.38W

Junction Temperature equals:

TJ = TA + PTOTAL • θJA (using tables)

TJ = 50°C + 1.38W • 14°C/W = 69.3°C

In this case, the junction temperature is below the maxi-

mum rating, ensuring reliable operation.

Reducing Power Dissipation

In some applications it may be necessary to reduce the

powerdissipationintheLT3089packagewithoutsacrificing

outputcurrentcapability.Twotechniquesareavailable.The

first technique, illustrated in Figure 12, employs a resis-

tor in series with the regulator’s input. The voltage drop

across RS decreases the LT3089’s IN-to-OUT differential

voltage and correspondingly decreases the LT3089’s

power dissipation.

Asanexample,assume:VIN=7V,VOUT=3.3VandIOUT(MAX)

= 0.8A. Use the formulas from the Calculating Junction

Temperature section previously discussed.

Without series resistor RS, power dissipation in the

LT3089 equals:

PTOTAL = (7V – 3.3V) • 0.8A = 2.96W

If the voltage differential (VDIFF) across the LT3089 is

chosen as 1.5V, then RS equals:

RS =

7V – 3.3V – 1.5V

0.8A

= 2.8Ω

Power dissipation in the LT3089 now equals:

PTOTAL = 1.5V • 0.8A = 1.2W

TheLT3089’spowerdissipationisnowonly40%compared

to no series resistor. RSdissipates1.8Wofpower.Choose

appropriate wattage resistors or use multiple resistors in

parallel to handle and dissipate the power properly.

3089 F12

VIN′

SET

OUT

–

LT3089

50µA

RSET

VOUT

VIN

Figure 12. Reducing Power Dissipation Using a Series Resistor

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