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IRU1260CM
(Rev.:2002)

DUAL 6A AND 1A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR

International Rectifier 

IRU1260

Stability

The IRU1260 requires the use of an output capacitor as

part of the frequency compensation in order to make the

regulator stable. Typical designs for the microprocessor

applications use standard electrolytic capacitors with

typical ESR in the range of 50 to 100mV and the output

capacitance of 500 to 1000mF. Fortunately as the ca-

pacitance increases, the ESR decreases resulting in a

fixed RC time constant. The IRU1260 takes advantage

of this phenomena in making the overall regulator loop

stable. For most applications a minimum of 100mF alu-

minum electrolytic capacitor with the maximum ESR of

0.3V such as Sanyo, MVGX series, Panasonic FA se-

ries as well as the Nichicon PL series insures both sta-

bility and good transient response. The IRU1260 also

requires a 1mF ceramic capacitor connected from VIN to

VCTRL and a 10V, 0.1W resistor in series with VCTRL pin

in order to further insure stability.

Thermal Design

The IRU1260 incorporates an internal thermal shutdown

that protects the device when the junction temperature

exceeds the maximum allowable junction temperature.

Although this device can operate with junction tempera-

tures in the range of 1508C, it is recommended that the

selected heat sink be chosen such that during maxi-

mum continuous load operation the junction tempera-

ture is kept below this number. The example given shows

the steps in selecting the proper regulator heat sink for

driving the Pentium IITM processor GTL+ termination

resistors and the Clock IC using the IRU1260 TO-263

package.

Example:

Assuming the following specifications:

VIN = 3.3V

VOUT1 = 2.5V

VOUT2 = 1.5V

IOUT1(MAX) = 0.2A

IOUT2(MAX) = 1.5A

TA = 358C

The steps for selecting a proper heat sink to keep the

junction temperature below 1358C is given as:

1) Calculate the maximum power dissipation using:

PD = IOUT13(VIN - VOUT1) + IOUT23(VIN - VOUT2)

PD = 0.23(3.3 - 2.5) + 1.53(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:

uJA = 308C/W for 1" square pad area

3) The maximum junction temperature of the device is

calculated using the equation below:

TJ = TA + PD3uJA

TJ = 35 + 2.86330 = 1218C

Since this is lower than our selected 1358C maxi-

mum junction temperature (1508C is the thermal shut-

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

package for our application.

Rev. 2.1

09/19/02

www.irf.com

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