IRU1117

4

Rev. 1.8

02/04/03

www.irf.com

Output Voltage Setting

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

external resistors according to the following formula:

Where:

VREF = 1.25V Typically

mA Typically

R1 and R2 as shown in Figure 3:

Figure 3 - Typical application of the IRU1117

for programming the output voltage.

The IRU1117 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-

a voltage equal to the (1.25/R1)

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 IRU1117 is

10mA, R1 is typically selected to be 121

V resistor so

that it automatically satisfies the minimum current re-

of 50

mA it only adds a small error to the output voltage

and should only be considered when a very precise out-

put voltage setting is required. For example, in a typical

3.3V application where R1=121

V and R2=200V the er-

Load Regulation

Since the IRU1117 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

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 multiplied by the factor of (1+R2/

3(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.

Figure 4 - Schematic showing connection

for best load regulation.

Stability

The IRU1117 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 100m

V and an output

capacitance of 500 to 1000

mF. Fortunately as the ca-

pacitance increases, the ESR decreases resulting in a

fixed RC time constant. The IRU1117 takes advantage

of this phenomenon in making the overall regulator loop

stable. For most applications a minimum of 100

mF 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 IRU1117 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

8C, it is recommended that the

heat sink be selected such that during maximum con-

tinuous load operation the junction temperature is kept

below this number. The example below for a SCSI termi-

nator application shows the steps in selecting the proper

regulator in a surface-mount package. (See IRU1015 for

non-surface-mount packages)

R1

R2

IRU1117

Adj

R1

R2

PARASITICLINE

RESISTANCE

IRU1117

Adj

3 1+

3R2

R2

R1

(

)