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IRU1260CP

DUAL 6A AND 1A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR

International Rectifier 

IRU1260

Example # 2:

Assuming the following specifications:

VIN = 3.3V

VOUT2 = 1.5V

VOUT1 = 2.5V

IOUT2(MAX) = 1.4A

IOUT1(MAX) = 0.2A

TA = 35$C

The steps for selecting a proper heat sink to keep the

junction temperature below 135$C is given as:

1) Calculate the maximum power dissipation using:

PD = IOUT1 × (VIN - VOUT1) + IOUT2 × (VIN - VOUT2)

PD = 0.2 × (3.3 - 2.5) + 1.5 × (3.3 - 1.5) = 2.86W

2) Assuming a TO-263 surface mount package, the junc-

tion to ambient thermal resistance of the package is:

θJA = 30$C/W for 1" square pad area

3) The maximum junction temperature of the device is

calculated using the equation below:

IRU1260 in Ultra LDO, Single Output Application

The IRU1260 can also be used in single supply applica-

tions where the difference between input and output is

much lower than the standard 1.5V dropout that is ob-

tainable with standard LDO devices. The schematic in

Figure 7 shows the application of the IRU1260 in a single

supply with the second LDO being disabled.

In this application, the IRU1260 is used on the VGA

card to convert 3.3V supply to 2.7V to power the Intel

740 chip rather than the conventional LDO which due to

its 1.5V minimum dropout spec must use the 5V supply

to achieve the same result. The difference is a substan-

tial decrease in the power dissipation as shown below.

The maximum power dissipation of the I740 chip is 5.8W,

which at 2.7V results in:

Io

=

5.8

2.7

=

2.15A

a) Using standard LDO, the power dissipated in the de-

vice is:

PD = (Vin - Vo) × Io = (5 - 2.7) × 2.15 = 4.94W

TJ = TA + PD × θJA

TJ = 35 - 2.86 × 30 = 121$C

Using surface mount TO-263 package with 25$C/W

junction to air thermal resistance results in:

Since this is lower than our selected 135$C maxi-

mum junction temperature (150$C is the thermal shut-

down of the device), TO-263 package is a suitable

package for our application.

Layout Consideration

The IRU1260 like all other high speed linear regulators

need to be properly laid out to insure stable operation.

The most important component is the output capacitor,

which needs to be placed close to the output pin and

connected to this pin using a plane connection with a

low inductance path.

TJ = PD × θJA + TA = 4.94 × 25 + 25 = 148$C

This is very close to the thermal shutdown of the IC.

b) Using IRU1260, the power dissipated in the device is

drastically reduced by using 3.3V supply instead of

5V.

PD = (Vin - Vo) × Io = (3.3 - 2.7) × 2.15 = 1.3W

Using surface mount TO-263 package with 25$C/W

junction to air thermal resistance results in:

TJ = PD × θJA + TA = 1.3 × 25 + 25 = 57$C

A reduction of 91$C in junction temperature.

6

Rev. 1.9

07/03/01

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