IRU1015CT Datasheet IRU1015CD, IRU1015CDTR, IRU1015CM | P4-P6

IRU1015

Rev. 1.3

08/20/02

www.irf.com

VOUT

VIN

VREF

IADJ = 50uA

IRU1015

Adj

VOUT

VIN

PARASITIC LINE

RESISTANCE

IRU1015

Adj

OUT

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:

Where:

VREF = 1.25V Typically

IADJ = 50mA Typically

R1 and R2 as shown in Figure 3:

Figure 3 - Typical application of the IRU1015

for programming the output voltage.

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)3R2 + IADJ3R2 which

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

is summarized in the above equation. Since the mini-

mum load current requirement of the IRU1015 is 10mA,

R1 is typically selected to be 121V resistor so that it

automatically satisfies the minimum current requirement.

Notice that since IADJ is typically in the range of 50mA 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=121V and R2=200V 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

regulator and not to the load. In fact, if R1 is connected

VOUT = VREF3 1+ +IADJ3R2

( )

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)=RP3(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 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 100mV and an output

capacitance of 500 to 1000mF. 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 100mF 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 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 below

shows the steps in selecting the proper regulator heat

sink for an AMD 486DX4-120 MHz processor.

IRU1015

Rev. 1.3

08/20/02

www.irf.com

VIN = 5V

VOUT = 3.45V

IOUT(MAX) = 1.2A

TA = 358C

Note: For further information regarding the above com-

panies and their latest product offerings and application

support contact your local representative or the num-

bers listed below:

AAVID Thermalloy...............PH# (603) 528-3400

Designing for Microprocessor Applications

As it was mentioned before the IRU1015 is designed

specifically to provide power for the new generation of

the low voltage processors requiring voltages in the range

of 2.5V to 3.6V generated by stepping down the 5V sup-

ply. These processors demand a fast regulator that sup-

ports their large load current changes. The worst case

current step seen by the regulator is anywhere in the

range of 1 to 7A with the slew rate of 300 to 500ns which

could happen when the processor transitions from “Stop

Clock” mode to the “Full Active” mode. The load current

step at the processor is actually much faster, in the or-

der of 15 to 20ns, however, the de-coupling capacitors

placed in the cavity of the processor socket handle this

transition until the regulator responds to the load current

levels. Because of this requirement, the selection of high

frequency low ESR and low ESL output capacitor is im-

perative in the design of these regulator circuits.

Figure 5 shows the effects of a fast transient on the

output voltage of the regulator. As shown in this figure,

the ESR of the output capacitor produces an instanta-

neous drop equal to the (∆VESR=ESR3∆I) and the ESL

effect will be equal to the rate of change of the output

current times the inductance of the capacitor (∆VESL

=L3∆I/∆t). The output capacitance effect is a droop in

the output voltage proportional to the time it takes for

the regulator to respond to the change in the current

(∆VC=∆t3∆I/C ) where ∆t is the response time of the

regulator.

Air Flow (LFM)

0 100

Thermalloy 6041PB No HS Required

AAVID 574602 No HS Required

Assuming the following specifications:

The steps for selecting a proper heat sink to keep the

junction temperature below 135°C is given as:

1) Calculate the maximum power dissipation using:

2) Select a package from the regulator data sheet and

record its junction to case (or Tab) thermal resistance.

Selecting TO-220 package gives us:

3) Assuming that the heat sink is black anodized, cal-

culate the maximum Heat sink temperature allowed:

Assume, ucs=0.05°C/W (heat-sink-to-case thermal

resistance for black anodized)

4) With the maximum heat sink temperature calculated

in the previous step, the heat-sink-to-air thermal re-

sistance (uSA) is calculated by first calculating the

temperature rise above the ambient as follows:

5) Next, a heat sink with lower θSA than the one calcu-

lated in step 4 must be selected. One way to do this

is to simply look at the graphs of the "Heat Sink Temp

Rise Above the Ambient" vs. the "Power Dissipation"

and select a heat sink that results in lower tempera-

ture rise than the one calculated in the previous step.

The following heat sinks from AAVID and Thermalloy

meet this criteria.

PD = IOUT3(VIN - VOUT)

PD = 1.23(5 - 3.45) = 1.86W

uJC = 2.78C/W

TS = TJ - PD3(uJC + uCS)

TS = 135 - 1.863(2.7 + 0.05) = 1298C

uSA = = = 508C/W

1.86

DT = TS - TA = 129 - 35 = 948C

DT = Temperature Rise Above Ambient

IRU1015

Rev. 1.3

08/20/02

www.irf.com

ESR

ESL

LOAD

CURRENT

LOAD CURRENT RISE TIME

1015plt1-1.0

CIN = = 400mF

1.2 3 50

0.15

ESR = = 0.167V

(5 - 3.45 - 1.2 - 0.15)

1.2

ESR =

(VIN - VOUT - DV - VDROP)

DVc = = = 1.6mV

Dt 3 DI 2 3 1.2

C 1500

ESR [ = 45mV

1.2

Type of VOUT IMAX Max Allowed

Processor Nominal Output Tolerance

AMD 486DX4 3.45 V 1.2 A ±150 mV

2) With the output capacitance being 1500mF:

Where:

Dt = 2ms is the regulator response time

To set the output voltage, we need to select R1 and R2:

3) Assuming R1=121V, 1%

Select R2 = 215V, 1%

Selecting both R1 and R2 resistors to be 1% toler-

ance results in the least amount of error introduced

by the resistor dividers leaving a ≈ ±2.5% error bud-

get for the IRU1015 reference which is well within the

initial accuracy of the device.

Finally, the input capacitor is selected as follows:

4) Assuming that the input voltage can drop 150mV be-

fore the main power supply responds, and that the

main power supply response time is ≈ 50ms, then

the minimum input capacitance for a 1.2A load step

is given by:

The ESR should be less than:

Where:

VDROP L Input voltage drop allowed in step 4

DV L Maximum regulator dropout voltage

DI L Load current step

Select a single 1500mF the same type as the output

capacitors exceeds our requirements. Figure 6 shows

the completed schematic for our example.

Figure 5 - Typical regulator response

to the fast load current step.

An example of a regulator design to meet the AMD speci-

fication for 486DX4-120MHz is given below.

Assume the specification for the processor as shown in

Table 1:

Table 1 - GTL+ specification for Pentium Pro

The first step is to select the voltage step allowed in the

output due to the output capacitor’s ESR:

1) Assuming the regulator’s initial accuracy plus the re-

sistor divider tolerance is ≈ ±86mV (±2.5% of 3.45V

nominal), then the total step allowed for the ESR and

the ESL, is -64mV.

Assuming that the ESL drop is -10mV, the remaining

ESR step will be -54mV. Therefore the output ca-

pacitor ESR must be:

The Sanyo MVGX series is a good choice to achieve

both price and performance goals. The 6MV1500GX,

1500mF, 6.3V has an ESR of less than 36mV typi-

cal. Selecting a single capacitor achieves our design

goal.

The next step is to calculate the drop due to the ca-

pacitance discharge and make sure that this drop in

voltage is less than the selected ESL drop in the

previous step.

R2 = 3R1 = 3121 = 213V

VOUT

VREF

-1

( )

3.45

1.25

-1

( )

P1-P3 P4-P6 P7-P9 P10-P11

IRU1015CT

Mfr. #:

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IRU1015CT

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