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IRU1015CM

1.5A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR

International Rectifier 

IRU1015

Output Voltage Setting

The IRU1015 can be programmed to any voltages in the

range of 1.25V to 5.5V with the addition of R1 and R2

external resistors according to the following formula:

VOUT = VREF × o1 + RR21p + IADJ × R2

Where:

VREF = 1.25V Typically

IADJ = 50µA Typically

R1 and R2 as shown in figure 3:

Vin

Vin

Vout

IRU1015

Vout

regulator and not 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 effective resistance will be, Rp(eff)=Rp*(1+R2/

R1). It is important to note that for high current applica-

tions, this can represent a significant percentage of the

overall load regulation and one must keep the path from

the regulator to the load as short as possible to mini-

mize this effect.

Vin

Vin

Vout

PARASITIC LINE

RESISTANCE

Rp

IRU1015

Adj

Vref R1

Adj

R1

RL

IAdj = 50uA

R2

R2

1015app2-1.0

Figure 3 - Typical application of the IRU1015

for programming the output voltage.

1015app3-1.0

Figure 4 - Schematic showing connection

for best load regulation

The IRU1015 keeps a constant 1.25V between the out-

put pin and the adjust pin. By placing a resistor R1 across

these two pins a constant current flows through R1, add-

ing to the Iadj current and into the R2 resistor producing

a voltage equal to the (1.25/R1)*R2 + Iadj*R2 which will

be added to the 1.25V to set the output voltage. This is

summarized in the above equation. Since the minimum

load current requirement of the IRU1015 is 10mA, R1 is

typically selected to be 121Ω resistor so that it auto-

matically satisfies the minimum current requirement.

Notice that since Iadj is typically in the range of 50µA it

only adds a small error to the output voltage and should

only be considered when a very precise output voltage

setting is required. For example, in a typical 3.3V appli-

cation where R1=121Ω and R2=200Ω the error due to

Iadj is only 0.3% of the nominal set point.

Load Regulation

Since the IRU1015 is only a three-terminal device, 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 R2 is

connected to the load and the top side of R1 resistor is

connected directly to the case or the Vout pin of the

Stability

The IRU1015 requires the use of an output capacitor as

part of the frequency compensation in order to make the

regulator stable. Typical designs for microprocessor ap-

plications use standard electrolytic capacitors with a

typical ESR in the range of 50 to 100mΩ and an output

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

pacitance increases, the ESR decreases resulting in a

fixed RC time constant. The IRU1015 takes advantage

of this phenomena in making the overall regulator loop

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

minum electrolytic capacitor such as Sanyo MVGX se-

ries, Panasonic FA series as well as the Nichicon PL

series insures both stability and good transient response.

Thermal Design

The IRU1015 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 150!C, 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 below

4

Rev. 1.1

06/29/01

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