LT3080EST datasheet(14/26 Pages) LINER

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LT3080

3080fb

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 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 = 1A, TA = 50°C

Power dissipation under these conditions is equal to:

PDRIVE = (VCONTROL – VOUT)(ICONTROL)

CONTROL

OUT

PDRIVE = (3.630V – 0.9V)(17mA) = 46mW

POUTPUT = (VIN – VOUT)(IOUT)

POUTPUT = (1.575V – 0.9V)(1A) = 675mW

Total Power Dissipation = 721mW

Junction Temperature will be equal to:

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

TJ = 50°C + 721mW • 64°C/W = 96°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 LT3080 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 LT3080’s IN-to-OUT

differential voltage and correspondingly decreases the

LT3080’s power dissipation.

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

and IOUT(MAX) = 1A. Use the formulas from the Calculating

Junction Temperature section previously discussed.

Without series resistor RS, power dissipation in the LT3080

equals:

PTOTAL = 5V – 3.3V

() •

+ 5V – 3.3V

() •1A

=1.73W

If the voltage differential (VDIFF) across the NPN pass

transistor is chosen as 0.5V, then RS equals:

S =

5V – 3.3V −0.5V

=1.2Ω

Power dissipation in the LT3080 now equals:

PTOTAL = 5V – 3.3V

() •

+ 0.5V

() •1A =0.53W

The LT3080’s power dissipation is now only 30% compared

to no series resistor. RS dissipates 1.2W of power. Choose

appropriate wattage resistors to handle and dissipate the

power properly.

Figure 8. Reducing Power Dissipation Using a Series Resistor

–

LT3080

VCONTROL

OUT

VOUT

VINʹ

VIN

3080 F08

SET

RSET

APPLICATIONS INFORMATION

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