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LT3088

3088fb

For more information www.linear.com/LT3088

level performance as found in these tables. Demo circuit

DC2279A’s board layout using multiple inner VOUT planes

and multiple thermal vias achieves 18°C/W performance

for the DD 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 0.5A

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 = 0.5A, TA = 50°C

Power dissipation under these conditions equals:

PTOTAL = (VIN – VOUT)(IOUT)

PTOTAL = (2.625V – 0.9V)(0.5A) = 0.87W

Junction Temperature equals:

TJ = TA + PTOTAL • θJA (using tables)

TJ = 50°C + 0.87W • 14°C/W = 62°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

powerdissipationintheLT3088packagewithoutsacrificing

outputcurrentcapability.Twotechniquesareavailable.The

first technique, illustrated in Figure 14, employs a resis-

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

across RS decreases the LT3088’s IN-to-OUT differential

voltage and correspondingly decreases the LT3088’s

power dissipation.

As an example, assume: VIN = 7V, VOUT = 3.3V and

IOUT(MAX) = 0.8A. Use the formulas from the Calculating

Junction Temperature section previously discussed.

Without series resistor RS, power dissipation in the

LT3088 equals:

PTOTAL = (7V – 3.3V) • 0.8A = 2.96W

If the voltage differential (VDIFF) across the LT3088 is

chosen as 1.5V, then RS equals:

RS =

7V – 3.3V – 1.5V

0.8A

= 2.75Ω

Power dissipation in the LT3088 now equals:

PTOTAL = 1.5V • 0.8A = 1.2W

TheLT3088’spowerdissipationisnowonly40%compared

tonoseriesresistor.RSdissipates1.75Wofpower.Choose

appropriate wattage resistors or use multiple resistors in

parallel to handle and dissipate the power properly.

APPLICATIONS INFORMATION

Figure 14. Reducing Power Dissipation Using a Series Resistor

3088 F14

VIN′

SET

OUT

–

LT3088

50µA

RSET

VOUT

VIN

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