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LT3085

3085fb

APPLICATIONS INFORMATION

Figure 8. Reducing Power Dissipation Using a Series Resistor

–

LT3085

VCONTROL

OUT

VOUT

VIN

3085 F08

SET

RSET

The power in the output transistor equals:

POUTPUT = (VIN – VOUT)(IOUT)

The total power equals:

PTOTAL = PDRIVE + POUTPUT

The current delivered to the SET pin is negligible and can

be ignored.

VCONTROL(MAX CONTINUOUS) = 3.630V (3.3V + 10%)

VIN(MAX CONTINUOUS) = 1.575V (1.5V + 5%)

VOUT = 0.9V, IOUT = 0.5A, TA = 50°C

Power dissipation under these conditions is equal to:

PDRIVE = (VCONTROL – VOUT)(ICONTROL)

CONTROL =

OUT

0.5A

= 8.3mA

PDRIVE = (3.630V – 0.9V)(8.3mA) = 23mW

POUTPUT = (VIN – VOUT)(IOUT)

POUTPUT = (1.575V – 0.9V)(0.5A) = 337mW

Total Power Dissipation = 360mW

Junction Temperature will be equal to:

TJ = TA + PTOTAL • θJA (approximated using tables)

TJ = 50°C + 360mW • 73°C/W = 76°C

In this case, the junction temperature is below the maximum

rating, ensuring reliable operation.

Reducing Power Dissipation

In some applications it may be necessary to reduce

the power dissipation in the LT3085 package without

sacriﬁcing output current capability. Two techniques are

available. The ﬁrst technique, illustrated in Figure 8, em-

ploys a resistor in series with the regulator’s input. The

voltage drop across RS decreases the LT3085’s IN-to-OUT

differential voltage and correspondingly decreases the

LT3085’s power dissipation.

As an example, assume: VIN = VCONTROL = 5V, VOUT = 3.3V

and IOUT(MAX)=0.5A.UsetheformulasfromtheCalculating

Junction Temperature section previously discussed.

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