IRU1260

4

Rev. 2.1

09/19/02

www.irf.com

Introduction

The IRU1260 is a dual adjustable Low Dropout (LDO)

regulator which can easily be programmed with the ad-

dition of two external resistors to any voltages within

the range of 1.20 to 5.5V. This voltage regulator is de-

signed specifically for applications that require two sepa-

rate regulators such as the Intel Pentium

sors requiring 1.5V and 2.5V supplies, eliminating the

need for a second regulator which results in lower over-

voltage improves by 30% which makes it ideal for appli-

cations requiring less than the standard 1.3V dropout

given in the LDO products such as IRU10XX series. The

IRU1260 also provides an accurate 1.20V voltage refer-

ence common to both regulators for programming each

output voltage. Other features of the device include: fast

response to sudden load current changes, such as GTL+

termination application for Pentium

processors. The IRU1260 also includes thermal shut-

down protection to protect the device if an overload con-

dition occurs.

Output Voltage Setting

The IRU1260 can be programmed to any voltages in the

range of 1.20V to 5.5V with the addition of R1 and R2

external resistors according to the following formula:

Figure 3 - Typical application of the IRU1260

for programming the output voltage.

(Only one output is shown here)

R2

R1

IRU1260

Gnd

R2

R1

PARASITIC LINE

RESISTANCE

IRU1260

Gnd

Where:

mA Typical

R1 and R2 as shown in Figure 3:

3

(

)

1+

pin and ground pin. By placing a resistor R1 across these

two pins a constant current flows through R1, adding to

age equal to the (1.2/R1)

added to the 1.2V to set the output voltage as shown in

the above equation. Since the input bias current of the

mA typically, it adds a very

small error to the output voltage and for most applica-

tions can be ignored. For example, in a typical 1.5V

GTL+application if R1=10.2K

V and R2=2.55KV the er-

0.004% of the nominal set point. The effective input im-

pedance seen by the feedback pins (The parallel combi-

nation of R1 and R2) must always be higher than 1.8K

V

in order for the regulator to start up properly.

Load Regulation

Since the IRU1260 does not provide a separate ground

pin for the reference voltage, it is not possible to provide

true remote sensing of the output voltage at the load.

Figure 4 shows that the best load regulation is achieved

when the bottom side of R1 resistor is connected di-

rectly to the ground pin of IRU1260 (preferably to the tab

of the device) and the top side of R2 resistor is con-

nected to the load. In fact, if R1 is connected to the load

side, the effective resistance between the regulator and

the load is gained up by the factor of (1+R2/R1), or the

3(1+R2/R1). It is

important to note that for high current applications, this

can represent a significant percentage of the overall load

regulation and one must keep the path from the regula-

tor to the load as short as possible to minimize this

effect.

Figure 4 - Schematic showing connection

for best load regulation.

(Only one output is shown here)

APPLICATION INFORMATION