IRU1176 データシートの表示（PDF） - International Rectifier

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IRU1176

7.5A ULTRA LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR WITH SHUTDOWN INPUT

International Rectifier 

IRU1176

The IRU1176 keeps a constant 1.25V between the VSENSE

pin and the VADJ pin. By placing a resistor R1 across

these two pins and connecting the VSENSE and VOUT pin

together, a constant current flows through R1, adding to

the IADJ current and into the R2 resistor producing a volt-

age equal to the (1.25/R1)3R2 + IADJ3R2. This voltage

is then 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 IRU1176 is

10mA, R1 is typically selected to be a 121V resistor so

that it automatically satisfies this condition. Notice that

since the Iadj is typically in the range of 50mA it adds a

small error to the output voltage and should be consid-

ered when very precise output voltage setting is required.

Load Regulation

Since the IRU1176 has separate pins for the output (VOUT)

and the sense (VSENSE), it is ideal for providing true re-

mote sensing of the output voltage at the load. This

means that the voltage drops due to parasitic resistance

such as PCB traces between the regulator and the load

are compensated for using remote sensing. Figure 4

shows a typical application of the IRU1176 with remote

sensing.

Panasonic FA series as well as the Nichicon PL series

insures both stability and good transient response.

Shutdown Operation

The IRU1176 can be disabled by pulling the SD pin low

using an open collector device such as a low cost 2N3904

general purpose transistor as shown in the application

circuit. The current sink of the pin is equal to:

ISINK

=

(VCTRL -

R

1.4)

Where: R = 50KV Typically

Thermal Design

The IRU1176 incorporates an internal thermal shutdown

that protects the device when the junction temperature

exceeds the allowable maximum 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

shows the steps in selecting the proper surface mount

package.

Assuming, the following conditions:

VI N

V IN

VOUT

IRU1176

VC T R L

V CTRL

V SENSE

Adj

R1

VOUT = 2.7V

VIN = 3.3V

VCTRL = 5V

IOUT = 2A (DC Avg)

RL Calculate the maximum power dissipation using the fol-

lowing equation:

R2

PD

=

IOUT

3

(VIN

-

VOUT)

+o

IOUT

60

p3

(VCTRL

-

VOUT)

Figure 4 - Schematic showing connection

for best load regulation.

Stability

The IRU1176 requires the use of an output capacitor as

part of the frequency compensation in order to make the

regulator stable. Typical designs for the microproces-

sor applications use standard electrolytic capacitors with

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 IRU1176 takes advantage

of this phenomena in making the overall regulator loop

stable.

For most applications a minimum of 100mF aluminum

electrolytic capacitor such as Sanyo, MVGX series,

2

PD = 2 3 (3.3 - 2.7) +o60 p3 (5 - 2.7) = 1.28W

Using table below select the proper package and the

amount of copper board needed.

Pkg Copper θJA(°C/W)

Area

M or P 1.4"X1.4" 25

M or P 1.0"X1.0" 30

M or P 0.7"X0.7" 35

M or P Pad Size 45

Max Pd

(TA=25°C)

4.4W

3.7W

3.1W

2.4W

Max Pd

(TA=45°C)

3.6W

3.0W

2.6W

2.0W

Note: Above table is based on the maximum junction

temperature of 1358C.

As shown in the above table, any of the two packages

will do the job. For lower cost applications the Ultra Thin-

Pak package is recommended.

Rev. 1.4

03/18/02

www.irf.com

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