IRU1010CS datasheet(4/10 Pages) IRF

4 / 10 page

IRU1010

Rev. 1.7

02/03/03

www.irf.com

Output Voltage Setting

The IRU1010 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:

Where:

VREF = 1.25V Typically

IADJ = 50

mA Typically

R1 and R2 as shown in Figure 3:

Figure 3 - Typical application of the IRU1010

for programming the output voltage.

The IRU1010 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)

3R2+IADJ3R2 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 IRU1010 is 10mA, R1 is

typically selected to be 121

V resistor so that it auto-

matically satisfies the minimum current requirement.

Notice that since IADJ is typically in the range of 50

mA 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=121

V and R2=200V the error due to

IADJ is only 0.3% of the nominal set point.

Load Regulation

Since the IRU1010 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

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

3(1+

R2/R1). It is important to note that for high current appli-

cations, 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

minimize this effect.

Figure 4 - Schematic showing connection

for best load regulation.

Stability

The IRU1010 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 IRU1010 takes advantage

of this phenomena 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 IRU1010 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

selected heat sink be chosen such that during maxi-

mum continuous load operation the junction tempera-

ture is kept below this number. The next example for a

SCSI terminator application shows the steps in sellecting

the proper regulator in a surface-mount package. (See

IRU1015 for non-surface-mount packages)

Vout

VIN

VREF

IADJ = 50uA

IRU1010

Adj

VOUT

VIN

PARASITIC LINE

RESISTANCE

IRU1010

Adj

VOUT

VIN

VOUT = VREF

3 1+

+ IADJ

3R2

(

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