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LT3083

3083f

APPLICATIONS INFORMATION

The power in the drive circuit equals:

PDRIVE = (VCONTROL – VOUT)(ICONTROL)

where ICONTROL is equal to IOUT/60. ICONTROL is a function

of output current. A curve of ICONTROL vs IOUT can be found

in the Typical Performance Characteristics curves.

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 = 3A, TA = 50°C

Power dissipation under these conditions is equal to:

PDRIVE = (VCONTROL – VOUT)(ICONTROL)

ICONTROL =

IOUT

= 50mA

PDRIVE = (3.630V – 0.9V)(50mA) = 137mW

POUTPUT = (VIN – VOUT)(IOUT)

POUTPUT = (1.575V – 0.9V)(3A) = 2.03W

Total Power Dissipation = 2.16W

Junction Temperature will be equal to:

TJ = TA + PTOTAL • θJA (using tables)

TJ = 50°C + 2.16W • 16°C/W = 84.6°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

power dissipation in the LT3083 package without sacriﬁc-

ing output current capability. Two techniques are available.

The ﬁrst technique, illustrated in Figure 7, employs a

resistor in series with the regulator’s input. The voltage

drop across RS decreases the LT3083’s input-to-output

differential voltage and correspondingly decreases the

LT3083’s power dissipation.

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

and IOUT(MAX) = 2A. Use the formulas from the Calculating

Junction Temperature section previously discussed.

Without series resistor RS, power dissipation in the LT3083

equals:

PTOTAL = 5V − 3.3V

() •

⎛

⎝

⎜

⎞

⎠

⎟ + 5V − 3.3V

() •2A

= 3.46W

If the voltage differential (VDIFF) across the NPN pass

transistor is chosen as 0.5V, then RS equals:

RS =

5V − 3.3V − 0.5V

= 0.6Ω

Power dissipation in the LT3083 now equals:

PTOTAL = 5V − 3.3V

() •

⎛

⎝

⎜

⎞

⎠

⎟ + 0.5V • 2A = 1.06W

The LT3083’s power dissipation is now only 30% compared

to no series resistor. RS dissipates 2.4W of power. Choose

appropriate wattage resistors or use multiple resistors in

parallel to handle and dissipate the power properly.

Figure 7. Reducing Power Dissipation Using a Series Resistor

–

LT3083

VCONTROL

OUT

VOUT

VIN

3083 F07

SET

RSET

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